EP2013038A1 - Tyre belt incorporating an anti-oxidant agent - Google Patents

Abstract

Tyre belt comprising a rubber composition based on at least one isoprenic elastomer, a reinforcing filler, a cross-linkage system and an anti-oxidant agent, characterised in that said anti-oxidant agent comprises a 4,4'- bis(alkylamino)-triphenylamine that fits the formula (I) in which R<SUP>1 </SUP>and R<SUP>2</SUP>, identical or different, each represent a linear or branched alkyl group, that has between 1 and 12 atoms of carbon or a cycloalkyl group that has between 5 and 8 atoms of carbon. Said anti-oxidant gives the inventive belt better long-term resistance to fatigue and to crack propagation.

Description

 _ _

PNEUMATIC BELT INCORPORATING AN ANTIOXIDANT AGENT

The present invention relates to tires and reinforcements reinforcement of the top of these tires, also called "belts".

It relates more particularly to the diene elastomer compositions used to form all or part of the rubber matrix of such reinforcements, as well as to the antioxidant agents used for the anti-aging protection of such compositions.

It will be briefly recalled that a tire with a radial carcass reinforcement comprises, in known manner, a tread, two inextensible beads, two flanks connecting the beads to the tread and a belt circumferentially disposed between the carcass reinforcement and the tread. tread, this belt consisting of various plies (or "layers") of rubber reinforced or not by reinforcing elements (or "reinforcements") such as cords or monofilaments, metal or textile type.

The tire belt generally consists of at least two superimposed layers or web plies, sometimes referred to as "working" webs or "crossed" webs, the reinforcements of which are arranged substantially parallel to each other inside a web. layer, but crossed from one layer to another, that is to say inclined, symmetrically or otherwise, with respect to the median circumferential plane, of an angle which is generally between 10 ° and 45 ° depending on the type considered. Each of these two crossed layers consists of a rubber matrix generally isoprenic based, sometimes called "calendering rubber", coating the reinforcements. The crossed layers may be supplemented by various other layers or layers of auxiliary rubber, of varying widths depending on the case, with or without reinforcements; examples of simple rubber cushions are so-called "protective" layers intended to protect the rest of the belt from external aggressions, perforations, or so-called "hooping" layers comprising reinforcements oriented substantially according to the circumferential direction (so-called "zero degree" layers), whether they are radially external or internal with respect to the crossed layers.

This tire belt must meet in a known manner to many requirements, sometimes contradictory, including:

(i) be as rigid as possible with low deformation, since it contributes substantially to stiffening the crown of the tire;

(ii) have a hysteresis as low as possible, in order firstly to minimize rolling heating of the inner zone of the crown and, on the other hand, to reduce the rolling resistance of the tire, which is synonymous with fuel economy; (iii) finally possess a high endurance, in particular vis-à-vis the phenomenon of separation or cracking of the ends of the crossed layers in the "shoulder" area of the tire, a problem known as "cleavage".

The third condition requires, in particular, rubber compositions forming tire belts, that they have a very high resistance to crack propagation and thermo-oxidation, which is attained notably by the use of antioxidant agents which provide a high degree of stability. effective anti-aging protection.

This requirement is particularly strong for truck tire casings, designed to be retreadable one or more times when their treads reach a critical degree of wear after prolonged rolling.

Antioxidants which have been used for a very long time as anti-aging agents in tire rubber compositions, especially in the belts of such tires, belong to the family of p-phenylenediamine derivatives ("PPD") such as, for example N-isopropyl-N'-phenyl-p-phenylenediamine ("I-PPD") or N, 3-dimethylbutyl-N'-phenyl-p-phenylenediamine ("6-

PPD "), both excellent antioxidants and antiozonants (see for example applications WO 2004/033548, WO 2005/063510, WO 2005/133666.

However, the Applicants have discovered in their research that the use of a different antioxidant agent, not yet used in pneumatic, can further increase the long-term resistance to fatigue and crack propagation of tire belts.

Accordingly, a first subject of the invention relates to a tire belt comprising a rubber composition based on at least one isoprene elastomer, a reinforcing filler, a crosslinking system and an antioxidant agent, characterized in that said agent antioxidant comprises a 4,4'-bis (alkylamino) -triphenylamine corresponding to formula (I):

wherein R ¹ and R ² , identical or different, each represents a linear or branched alkyl group having 1 to 12 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms.

The invention thus provides tire crown reinforcements, as well as these tires themselves, with an overall compromise of new and particularly advantageous properties.

The invention also relates to any tire comprising a belt according to the invention, in particular that this tire is of the radial or non-radial type.

The tires of the invention are particularly intended for equipping tourism-type motor vehicles, SUVs ("Sport Utility Vehicles"), two wheels (in particular motorcycles), planes, such as industrial vehicles chosen from light trucks, "heavy vehicles". - that is, metros, buses, road transport vehicles (trucks, tractors, trailers), off-the-road vehicles such as agricultural or civil engineering vehicles -, other transport or handling vehicles.

The invention also relates to the use of a belt according to the invention for the manufacture of new tires or for the retreading of used tires, particularly in the case of heavy-duty tires.

The belts according to the invention are prepared by a method which constitutes another object of the present invention, said method comprising the following steps:

o incorporating in an isoprene elastomer, in a mixer, a reinforcing filler; an antioxidant, by thermomechanically kneading the whole, in one or more times, until a maximum temperature of between ⁰ ° C and 190 ° C; o cool the assembly to a temperature below 100 ° C; o then incorporate a crosslinking system; kneading the whole up to a maximum temperature below 110 ^° C .; calender or extrude the composition thus obtained in the form of a layer of rubber; o incorporate this layer, after possible addition of textile or metal reinforcements, in the tire belt,

and characterized in that said antioxidant agent comprises a 4,4'-bis (alkylamino) triphenylamine of formula (I) above. . .

The invention also relates, in itself, to the use of a compound of formula (I) above for the anti-aging protection of a tire belt.

The invention as well as its advantages will be readily understood in the light of the description and the following exemplary embodiments, as well as schematic figures relating to these examples, which represent a radial section of a pneumatic tire with heavy radial carcass reinforcement. (Fig. 1) as well as a synthetic scheme of a compound of formula (I) suitable for the tire belt of the invention (Fig. 2).

MEASUREMENTS AND TESTS USED

The rubber compositions are characterized before and after firing, as indicated below.

A) Mooney plasticity:

An oscillating consistometer is used as described in the French standard NF T 43-005 (1991). The Mooney plasticity measurement is carried out according to the following principle: the raw composition (i.e., before firing) is molded in a cylindrical chamber heated to 100 ° C. After one minute of preheating, the rotor rotates within the test tube at 2 revolutions / minute and the useful torque is measured to maintain this movement after 4 minutes of rotation. The Mooney plasticity (ML 1 + 4) is expressed in "Mooney unit" (UM, with 1 UM = 0.83 Newton.meter).

B) Rheometry:

The measurements are carried out at 150 ° C. with an oscillating chamber rheometer according to DIN 53529 - Part 3 (June 1983). The evolution of the rheometric torque as a function of time describes the evolution of the stiffening of the composition as a result of the vulcanization reaction. The measurements are processed according to DIN 53529 - Part 2 (March 1983): t, is the induction time, that is to say the time required for the beginning of the vulcanization reaction; t _a (for example tg,,) is the time necessary to reach a conversion of α%, that is to say α% (for example 99%) of the difference between the minimum and maximum couples. The conversion rate constant denoted by K (expressed in min ^-1 ), of order 1, calculated between 30% and 80% conversion, which makes it possible to evaluate the kinetics of vulcanization, is also measured. .

C) Traction tests:

These tests make it possible to determine the elastic stress and the properties at break. Unless otherwise indicated, they are carried out in accordance with the French standard NF T 46-002 of September 1988. The secant modules known as "nominal" (or apparent stresses, in MPa) are measured in second elongation (ie after an accommodation cycle). or said "true" (in this case brought back to the actual section of the test piece) at 10% elongation (denoted respectively "MA10" and "ElO"), 100% elongation (respectively "MA100" and "ElOO"). ") and 300% elongation (respectively" MA300 "and" E300 "). All these tensile measurements are carried out under normal conditions of temperature (23 ± 2 ° C.) and hygrometry (50 ± 5% relative humidity), according to the French standard NF T 40-101 (December 1979). The breaking stresses (in MPa) and the elongations at break (in%) are also measured at a temperature of 23 ° C.

D) "MFTRA" test:

The fatigue and nick propagation resistance (with initial primer), expressed in number of cycles or in relative units (ur), is measured in a known manner on a test specimen with a 1 mm notch and subjected to repeated tractions. from low frequency to an elongation of 20%, using a Monsanto device ("MFTR" type), until the test piece is broken, according to the French standard NF T46-021.

The above test is carried out after an accelerated thermo-oxidative aging of 26 days, the sample of the composition tested being placed in a ventilated oven maintained at a temperature of 80 ° C. and at an ambient humidity of 40%.

DETAILED DESCRIPTION OF THE INVENTION

The tire belts of the invention have the essential feature of incorporating, in all or part of their rubbery matrix, at least one elastomeric composition based on at least each of the following constituents: (i) one (at least one) isoprene elastomer; (ii) one (at least one) reinforcing filler; (iii) a crosslinking system; and (iv) a (at least one) compound of formula (I) as an antioxidant.

Of course, the expression "composition based on" means a composition comprising the mixture and / or the reaction product in situ of the various constituents used, some of these basic constituents being capable of or intended to react. between them, less in part, during the various phases of manufacture of rubber compositions, belts and tires, in particular during their vulcanization.

In the present description, unless expressly indicated otherwise, all the percentages (%) indicated are% by weight.

H-I. Diene elastomer

By elastomer (or rubber, both considered synonymous) of the "diene" type, is generally meant an elastomer derived at least in part (ie, a homopolymer or a copolymer) from diene monomers, that is to say monomers carrying two carbon-carbon double bonds (conjugated or otherwise).

This general definition being given, the term "isoprenic elastomer" is understood to mean a homopolymer or copolymer of isoprene, in other words a diene elastomer chosen from the group consisting of natural rubber (NR) and synthetic polyisoprenes. (IR), the various isoprene copolymers and the mixtures of these elastomers.

Among the isoprene copolymers, mention will in particular be made of copolymers of isobutene-isoprene (butyl rubber - HR), isoprene-styrene (SIR), isoprene-butadiene (BIR) or isoprene-butadiene-styrene (SBIR).

The isoprene elastomer is preferably natural rubber or synthetic polyisoprene of the cis-1,4 type. Among these synthetic polyisoprenes, polyisoprenes having a content (mol%) of cis-1,4 bonds greater than 90%, more preferably still greater than 98%, are preferably used.

In blending (i.e., blending) with the above isoprene elastomer, the compositions of the invention may contain diene elastomers other than isoprenic, preferably minor (i.e., less than 50 phr). The isoprene elastomer is more preferably 75 to 100% by weight of the total of diene elastomers, ie 75 to 100 phr (parts by weight per hundred parts of elastomer).

As such other non-isoprenic diene elastomers, mention will be made in particular of any diene elastomer of the unsaturated type chosen in particular from the group consisting of polybutadienes (BR), in particular cis-1,4 or 1,2-syndiotactic polybutadienes, and those having a 1,2-unit content of between 4% and 80%, and butadiene copolymers, especially styrene-butadiene copolymers (SBR), and in particular those having a styrene content of between 5 and 50% by weight and more particularly between 20% and 40% by weight, a 1,2-butadiene content of the butadiene part of between 4% and 65%, a . .

trans-1,4 bond content of between 30% and 80%, styrene-butadiene-isoprene copolymers (SBIR), and mixtures of these different elastomers (BR, SBR and SBIR).

By way of example, when the belt of the invention is intended for a tire of the tourism type, if such a cutting is used, it is preferentially a mixture of SBR and BR that is used in cutting with the natural rubber, preferably up to less than 25% by weight (ie less than 25 phr) of SBR and BR mixture.

The belt of the invention is particularly intended for a truck tire, whether it is a new or used tire (in the case of a retreading). In such a case, the isoprene elastomer is preferably used alone, that is to say without cutting with another diene or polymer elastomer. More preferably still, this isoprene elastomer is exclusively natural rubber.

H-2. Reinforcing charge

Any type of reinforcing filler known for its capacity to reinforce a rubber composition that can be used for manufacturing tires, for example an organic filler such as carbon black, or a reinforcing inorganic filler such as silica to which must be associated with a coupling agent.

Carbon blacks are suitable for all carbon blacks, in particular blacks of the HAF, ISAF, SAF type conventionally used in tires (so-called pneumatic grade blacks). Among these, the reinforcing carbon blacks of the 100, 200 or 300 series (ASTM grades), for example blacks Nl 15, Nl 34, Nt34, N326, N330, N339, N347 or N375, or more particularly , depending on the intended applications, blacks of higher series (for example N660, N683, N772). The carbon blacks could for example already be incorporated into the isoprene elastomer in the form of a masterbatch (see for example WO 97/36724 or WO 99/16600).

By "reinforcing inorganic filler" is meant in this application, by definition, any inorganic or mineral filler (regardless of its color and origin (natural or synthetic), also called "white" filler, "clear" filler or "non-blackfiller" as opposed to carbon black, capable of reinforcing on its own, without any other means than an intermediate coupling agent, a rubber composition intended for the manufacture of tires, in other words capable of replacing, in its reinforcing function, a conventional carbon black of pneumatic grade, such a charge is generally characterized, in known manner, by the presence of hydroxyl groups (-OH) on its surface. Suitable reinforcing inorganic fillers are, in particular, mineral fillers of the siliceous type, in particular silica (SiC 2), or aluminous type, in particular alumina (Al 2 O 3). The silica used may be any reinforcing silica known to those skilled in the art, in particular any precipitated or fumed silica having a BET surface and a CTAB specific surface both less than 450 m ² / g, preferably from 30 to 400 m ² / g. As highly dispersible precipitated silicas (referred to as "HD"), mention may be made, for example, of Ultrasil 7000 and Ultrasil 7005 silicas from Degussa, Zeissil 1165MP, 1135MP and 1135MP silicas from Rhodia, Hi-SiI silica EZ150G. from PPG, the Zeopol 8715, 8745 and 8755 silicas of the Huber Company, the high surface area silicas as described in the application WO 03/16837.

When the compositions of the invention are intended for tire treads with a low rolling resistance, the reinforcing inorganic filler used, in particular if it is silica, preferably has a BET surface area of between 45 and 400. m ² / g, more preferably between 60 and 300 m ² / g.

Preferably, the total reinforcing filler content (carbon black, reinforcing inorganic filler or mixture of the two types of filler) is between 20 and 200 phr, more preferably between 30 and 150 phr, the optimum being in a different known manner. according to the particular applications concerned.

In order to couple the reinforcing inorganic filler to the diene elastomer, an at least bifunctional coupling agent (or bonding agent) is used in known manner to ensure a sufficient chemical and / or physical connection between the inorganic filler ( surface of its particles) and the diene elastomer, in particular organosilanes or bifunctional polyorganosiloxanes.

In particular, polysulfurized silanes, called "symmetrical" or "asymmetrical" silanes according to their particular structure, are used, as described for example in the applications WO03 / 002648 and WO03 / 002649.

Particularly suitable, without the following definition being limiting, so-called "symmetrical" polysulfide silanes having the following general formula:

Z - A - S _n - A - Z, wherein:

n is an integer of 2 to 8 (preferably 2 to 5);

- A is a divalent hydrocarbon radical (preferably alkylene groups Ci-Ci ₈ or arylene groups C ₂ -C _6, particularly alkylene Ci-Ci _0, in particular Ci-C _4, in particular propylene ); Z is one of the following formulas:

R1 'RV R2 ¹

- Si- R1 ^' ; - Si- R2 ¹ ; - Si- R2 ¹ , R2 ¹ R2 ¹ _R 2 '

in which:

- R.1 radicals, substituted or unsubstituted, identical or different, represent an alkyl group -C g cycloalkyl, C ₅ -C ₈ aryl or C ₆ -C ₈ (preferably alkyl C ₁ -C ₆ alkyl, cyclohexyl or phenyl, especially C ₁ -C ₄ alkyl groups, more particularly methyl and / or ethyl). - the R.2 radicals, substituted or unsubstituted, identical or different, represent an alkoxy group or Ci-Ci ₈ cycloalkoxy, C ₅ -C ₈ (preferably a group selected from alkoxyls C ₈ and C ₅ -C ₈ cycloalkoxyls, more preferably still a group selected from C ₁ -C ₄ alkoxyls, in particular methoxyl and ethoxyl).

In the case of a mixture of polysulfurized alkoxysilanes corresponding to the above formula, especially common commercially available mixtures, the average value of "n" is a fractional number preferably between 2 and 5, more preferably close to 4. However, the invention can also be advantageously used for example with disulfide alkoxysilanes (n = 2).

As examples of silane polysulfides, are more particularly the bis (mono, trisulfide or tetrasulfide) of bis (alkoxyl (Ci-C ₄₎ alkyl (Ci-C ₄₎ alkyl silyl (Cj-C ₄ )), such as polysulfides of bis (3-trimethoxysilylpropyl) or bis (3-triethoxysilylpropyl). Among these compounds, bis (3-triethoxysilylpropyl) tetrasulfide, abbreviated to TESPT, of formula [(C ₂ H ₅ O) ₃ Si (CH ₂ ) ₃ S ₂ ] ₂ or bis-disulfide ( triethoxysilylpropyl), abbreviated TESPD, of formula [(C ₂ H ₅ O) ₃ Si (CH ₂ ) ₃ S] ₂ . Mention may also be made, by way of preferred examples, of polysulfides (in particular disulfides, trisulphides or tetrasulfides) of bis- (monoalkoxyl (Ci-C ₄ ) -dialkyl (Ci-C ₄ ) silylpropyl), more particularly bis-monoethoxydimethylsilylpropyl tetrasulfide. as described in patent application WO 02/083782.

As coupling agent other than polysulfurized alkoxysilane, there may be mentioned in particular bifunctional POS (polyorganosiloxanes) or polysulfides of hydroxysilane (R ² = OH in the formula above) as described in the patent applications WO 02/30939 and WO 02/31041. . _

In the rubber compositions according to the invention, the content of coupling agent is preferably between 4 and 12 phr, more preferably between 3 and 8 phr.

The coupling agent could be grafted beforehand on the diene elastomer or on the reinforcing inorganic filler. However, it is preferred, particularly for reasons of better implementation of the compositions in the green state, to use the coupling agent is grafted on the reinforcing inorganic filler, or in the free state (Le., Ungrafted).

Finally, one skilled in the art will understand that, as equivalent filler of the reinforcing inorganic filler described in this paragraph, it would be possible to use a reinforcing filler of another nature, in particular organic, since this reinforcing filler would be covered with an inorganic layer such as silica, or would comprise on its surface functional sites, in particular hydroxyl sites, requiring the use of a coupling agent to establish the bond between the filler and the elastomer.

II-3. Antioxidant agent

The tire belts of the invention have the essential characteristic of comprising, as an antioxidant, a 4,4'-bis (alkylamino) -triphenylamine corresponding to formula (I):

wherein R ¹ and R ² , identical or different, each represents a linear or branched alkyl group having 1 to 12 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms.

Preferably, R ¹ and R ² , which are identical or different, each represent an alkyl having from 2 to 8 carbon atoms, preferably chosen from the group consisting of ethyl, propyl (ie, n-propyl, iso-propyl), butyl ( ie, n-butyl, sec-butyl and tert-butyl), pentyl, hexyl, heptyl and octyl, or a cycloalkyl group having 5 to 8 carbon atoms (cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl), more preferably a cyclohexyl group .

Among the compounds of formula (I) above, use is more preferably made of a compound whose R ¹ and R ^{2 groups} are branched, of formula (II) below:

in which R ³ , R ⁴ , R ⁵ and R ⁶ , which are identical or different, each represent an alkyl group whose number of carbon atoms is in accordance with the preferred definitions given above for R ¹ and R ² .

Compounds corresponding to the generic formulas (I) and (II) above are known as antiozonants or antioxidants for rubber, some have for example been described, as well as their synthesis, in US Pat. No. 3,277,174. No application in the compositions of US Pat. isoprenic elastomer for tires, especially in the belts of said tires, is envisaged.

By way of more preferred examples of branched radicals R ¹ and R ² , mention may be made in particular of isopropyl (a), 1,3-dimethylbutyl (b) and 1,4-dimethylpentyl (c) of the formulas below. below:

Thus, according to a particularly preferred embodiment of the invention, the tire belt of the invention uses as antioxidant one of the following compounds:

o 4,4'-bis (isopropylamino)] - triphenylamine corresponding to the specific formula:

(III-a) 4,4'-bis (1,3-dimethylbutylamino) -triphenylamine corresponding to the formula:

(III-b)

4,4'-bis (1,4-dimethylpentylamino) triphenylamine corresponding to the formula:

(III-c)

In the isoprene rubber composition forming part of the tire belt according to the invention, the level of antioxidant of formulas (I), (II) or (III) is preferably between 1 and 10 phr. Below the minimum indicated, the endurance may be insufficient while beyond the maximum recommended, there is a risk of degradation of the mechanical properties due to a parasitic plasticizing effect.

For all these reasons, this level of antioxidant is more preferably between 2 and 8 phr, in particular between 2 and 6 phr.

II-4. Various additives

The rubber matrices of the belts according to the invention may also comprise all or part of the usual additives used in rubber compositions intended for the manufacture of tire belts, such as, for example, extension oils, plasticizers, anti-aging protection other than those mentioned above, such as anti-ozone, other anti-ozonants chemical or anti-oxidants, anti-fatigue agents, acceptors and donors of methylene, bismaleimides or other reinforcing resins, a crosslinking system based sulfur, or sulfur and / or peroxide donors, vulcanization accelerators, vulcanization activators or retarders, rubber-to-metal adhesion promoter systems such as example salts or metal complexes (for example cobalt, boron, phosphorus, or even lanthanide salts as described in WO 2005/133666 cited above).

The isoprenic matrices may also contain, in addition to any coupling agents for reinforcing inorganic fillers, agents for recovering these inorganic fillers, or more generally, agents for assisting the processing which can in known manner, thanks to a improving the dispersion of the filler in the rubber matrix and lowering the viscosity of the compositions, to improve their processability.

5H. Belts and tires according to the invention

The isoprenic compositions previously described are intended to form all or part of the rubber matrix of the belt of a tire, in particular of a truck tire for heavy vehicles or tourism.

They can be used, for example, as a calendering gum with a layer or a cabled fabric web ply, whether it is a "crossed" layer, a protective layer or a hooping layer (with zero degrees), or intended to form a simple cushion, strip or strip of rubber, lacking reinforcements, arranged radially above or below the different belt layers mentioned above, or even inserted between the latter, for example to form an underlayer of the tread, or placed (e) at the lateral ends of these belt layers, in the "shoulder" areas of the tire, for example to form decoupling gums.

By way of example, the appended FIG. 1 schematically represents a radial section of a heavy-vehicle tire 1 with a radial carcass reinforcement that may or may not conform to the invention, in this general representation. This tire 1 comprises an apex 2, two flanks 3, two beads 4, a radial carcass reinforcement 7 extending from one bead to the other. The top 2, surmounted by a tread (not shown in this very schematic figure, for simplification) is in known manner reinforced by a belt 6 consisting of at least two "crossed" top layers, covered with at least one protective top layer, all of these layers being reinforced by carbon steel wire cords. The carcass reinforcement 7 is wound around the two rods 5 in each bead 4, the upturn 8 of this armature 7 being for example disposed towards the outside of the tire 1 which is shown here mounted on its rim 9. The armature of carcass 7 consists of at least one ply or layer reinforced by so-called "radial" metal cables, that is to say that these cables are arranged substantially parallel to each other and extend from a bead to the other so as to form an angle of between 80 ° and 90 ° with the median circumferential plane (plane perpendicular to the axis of rotation of the tire which is located midway between the two beads 4 and passes through the middle of the belt 6).

The tire according to the invention of the above example has the essential feature of having in its top 2 a belt 6 according to the invention, the isoprene composition based on the compound of formula (I) constituting the calendering rubber belt layers 6 (two crossed layers and one protective layer, in this example).

In the case of a tire comprising for example one or more "zero degree" layers, it is preferable that the calendering gum of the woven fabric, that it is in the form of a layer of a certain width, the width of the crossed layers, narrower strips or even a unit wire wrapped in rubber, is also based on an isoprene composition comprising the compound of formula (I).

According to a preferred embodiment of the invention, the rubber composition based on the isoprene elastomer, the reinforcing filler and the antioxidant of formula (I) has, in the vulcanized state (ie, after firing). a secant modulus in extension (ElO) which is greater than 5 MPa, more preferably between 7 and 20 MPa. It is in the module areas indicated above that the best endurance tradeoff has been recorded.

H-6. Preparation of rubber compositions

The rubber compositions are manufactured in suitable mixers, using two successive preparation phases well known to those skilled in the art: a first phase of work or thermomechanical mixing (sometimes referred to as a "non-productive" phase) at a high temperature. temperature, up to a maximum temperature (denoted T _max ) of between 110 ° C. and 190 ° C., preferably between 130 ° C. and 180 ° C., followed by a second mechanical working phase (sometimes referred to as a "productive" phase). ") at a lower temperature, typically less than 110 ° C, for example between 40 ° C and 100 ° C, finishing phase during which is incorporated the crosslinking system or vulcanization.

The method of manufacturing the tire belts according to the invention is characterized in that at least the reinforcing filler and the compound of formula (I) are incorporated by kneading with the isoprene elastomer during the so-called non-productive first phase. , that is to say that is introduced into the mixer and that is kneaded thermomechanically, in one or more steps, at least these different basic constituents until a maximum temperature of between 110 ° C and 190 ° C, preferably between 130 ° C and 180 ° C. By way of example, the first (non-productive) phase is carried out in a single thermomechanical step during which, in a suitable mixer such as a conventional internal mixer, all the necessary basic constituents are first introduced. (Isoprene elastomer, reinforcing filler and compound of formula (I), then in a second step, for example after one to two minutes of mixing, any additional processing agents and other various additives, with the exception of crosslinking or vulcanization The total mixing time, in this non-productive phase, is preferably between 2 and 10 minutes.

After cooling the mixture thus obtained, the vulcanization system is then incorporated at low temperature, generally in an external mixer such as a roll mill; the whole is then mixed (productive phase) for a few minutes, for example between 5 and 15 minutes.

The final composition thus obtained is then calendered, for example in the form of plates (thickness of 2 to 3 mm) or thin rubber sheets for the measurement of its physical or mechanical properties, in particular for a characterization in the laboratory, or extruded to form a rubber profile directly usable, after cutting or assembly to the target dimensions, and after adding the desired metal or textile reinforcements, as a belt layer.

In summary, the method according to the invention, for preparing a tire belt according to the invention comprising a rubber composition based on at least one isoprene elastomer, a reinforcing filler, a crosslinking system and an antioxidant , comprises the following steps:

o incorporating in an isoprene elastomer, in a mixer, a reinforcing filler; an antioxidant, by thermomechanically kneading the whole, in one or more times, until a maximum temperature of between 110 ° C. and 190 ° C. is reached; o cool the assembly to a temperature below 100 ⁰ C; o then incorporate a crosslinking system; kneading the whole up to a maximum temperature below 110 ⁰ C; calender or extrude the composition thus obtained in the form of a layer of rubber; o incorporate this layer, after possible addition of textile or metal reinforcements, in the tire belt, and it is characterized in that said antioxidant agent comprises a 4,4'-bis (alkylamino) triphenylamine corresponding to the formula (I) above.

The vulcanization or cooking is carried out in a known manner at a temperature preferably between 130 ^° C. and 200 ° C., under pressure, for a sufficient time which may vary, for example, between 5 and 90 min depending, in particular, on the cooking temperature, of the vulcanization system adopted, the kinetics of vulcanization and the size of the tire in question.

The actual crosslinking system is preferably based on sulfur and a primary vulcanization accelerator, in particular a sulfenamide type accelerator. To this vulcanization system are added, incorporated during the first non-productive phase and / or during the productive phase, various known secondary accelerators or vulcanization activators such as zinc oxide, stearic acid, guanidine derivatives ( especially diphenylguanidine), vulcanization retarders, etc. Sulfur is used at a preferential rate of between 1 and 10 phr, more preferably between 2 and 8 phr, especially when the invention is applied to a truck-type tire. The primary vulcanization accelerator is used at a preferential rate of between 0.5 and 5 phr, more preferably between 0.5 and 2 phr.

It is possible to use as accelerator (primary or secondary) any compound capable of acting as a vulcanization accelerator for diene elastomers in the presence of sulfur, especially thiazole type accelerators and their derivatives, thiuram type accelerators, zinc dithiocarbamates. These accelerators are more preferably selected from the group consisting of 2-mercaptobenzothiazyl disulfide (abbreviated "MBTS"), N-cyclohexyl-2-benzothiazyl sulfenamide (abbreviated "CBS"), N, N-dicyclohexyl-2-benzothiazyl sulfenamide (abbreviated "DCBS"), N-tert-butyl-2-benzothiazylsulfenamide (abbreviated "TBBS"), N-tert-butyl-2-benzothiazylsulfenimide (abbreviated "TBSI"), zinc dibenzyldithiocarbamate (in abbreviated "ZBEC") and mixtures of these compounds. Preferably, a primary accelerator of the sulfenamide type is used.

It goes without saying that the invention relates to belts and tires previously described both in the said state "raw" (ie, before cooking) in the state said "cooked" or vulcanized (ie, after crosslinking or vulcanization) .

III. EXAMPLES OF REALIZATION

HI- 1. Synthesis of 4,4'-bis (1,3-dimethylbutylamino) -triphenylamine The compound of formula (III) was prepared according to the synthesis scheme shown in FIG. 2, based in particular on the known synthetic process described in the aforementioned US Pat. No. 3,277,174 (Example 7).

In more detail, three steps (A, B and C) were carried out as follows.

A) Preparation of 4,4'-dinitrotriphenylamine:

In a 2 liter tricolor equipped with a thermometer, a stirrer and a "Dean Stark" trap filled with benzene, 121 g (1 mole) of formanilide, 314 g (2.5 moles) of nitrochlorobenzene are added. 220 g (1.6 moles) of K ₂ CO ₃ in 115 ml of DMF and 5 ml of benzene. The medium is heated at 165-175 ° C for 21 hours with permanent extraction of the formed water. The reaction is cooled and stopped by 1 liter of water. The precipitate is filtered, suspended with dilute hydrochloric acid, filtered and resuspended in hot ethanol. The suspension thus obtained is filtered to obtain 242 g (yield of 72%) of 4,4'-dinitrotriphenylamine.

B) Preparation of 4,4'-diaminotriphenylamine

In order to obtain 4,4'-diaminotriphenylamine, a reduction of the above compound is then carried out by hydrogenation in isopropanol at 100 ° C. under approximately 20 bars of pressure H ₂ and in the presence of a palladium catalyst (on coal).

C) Synthesis of 4,4'-bis (1,3-dimethylbutylamino) triphenylamine:

In a reactor are then placed 85 g (ie 0.3 mol) of 4,4'-diaminotriphenylamine above, 400 ml (or 3 moles) of 4-methylpentan-2-one (or "MIBK") in the presence of 2 g of Pd (5% by weight on charcoal), at about 25 bar of H ₂ pressure, for 45 min at 100-130 ° C and then for 2 h 30 min at 130-135 ° C. The catalyst is filtered off and the excess MIBK is removed by distillation. The residue (yield equal to about 92%) is purified by chromatography on alumina. The product is recrystallized from ethanol. The compound of formula (III) is thus obtained with a total yield of about 66%.

IH-2. Preparation of rubber compositions

The following tests are carried out as follows: the isoprene elastomer, the reinforcing filler (carbon black) is introduced into an internal mixer, filled to 70% and whose initial tank temperature is approximately 60 ° C. ), then, after one to two minutes of mixing, the various other ingredients, including the compound of formula (III), at the exception of the vulcanization system. Thermomechanical work (non-productive phase) is then carried out in one or two stages (total mixing time, for example, equal to about 7 minutes), until a maximum "falling" temperature of about 165 ° -170 ^° C. is reached. The mixture thus obtained is recovered, cooled and then the vulcanization system (sulfur and primary sulfenamide accelerator) is added to an external mixer (homo-finisher) at 30 ° C., mixing the whole (productive phase), for example during 3 to 10 min.

The compositions thus obtained are then either extruded in the form of plates (thickness of 2 to 3 mm) for the measurement of their physical or mechanical properties, or calendered for producing a metallic cabled fabric forming a belt layer ("layer" of work ") of a truck tire.

H-3. Characterization tests

The purpose of this test is to demonstrate the improved endurance performance of an isoprene tire belt composition when it has an antioxidant according to formula (I), compared with a control rubber composition using a conventional antioxidant (6). PPD).

For this purpose, two compositions based on natural rubber are prepared:

composition noted C-I (control); composition noted C-2 (in accordance with the invention).

These two compositions, of rigorously identical formulations apart from the nature and the weight concentration of antioxidant, are intended to constitute the "calendering gum" of working layers of a heavy-duty tire belt.

Tables 1 and 2 give the formulation of the two compositions (Table 1 - rate of the different products expressed in phr), their properties before and after cooking (60 min.

140 ° C). The vulcanization system consists of sulfur and sulfenamide. In these compositions CI and C-2, the two antioxidants are used at a substantially isomolar level, that is to say that, regardless of the composition tested, the same number of moles of active functions (secondary amines) are used. ); this explains the difference in weight of product (expressed in phr).

It is recalled here that the compound 6-PPD, reference antioxidant in tire rubber compositions, especially in the belts thereof, has the structural formula:

(IV)

For comparison, it is possible to recall below formula (III) of 4,4'-bis (1,3-dimethylbutylamino) -triphenylamine used in composition C-2:

(III)

It can be noted immediately that the second molecule has a much larger steric hindrance and therefore a lower ability to diffuse, which for the skilled person is a priori unfavorable to effective anti-fatigue protection.

On examination of the results of Table 2, it is firstly noted that both compositions tested have, prior to aging, identical rubber properties, both as regards the properties before firing (Mooney viscosity and rheology properties) and after cooking (mechanical properties in extension). This is already the indicator that the antioxidant of the composition of the invention is, before aging of the composition, as effective as the reference antioxidant of the control composition.

Only after accelerated thermal aging, through the measurement of "MFTRA" (base 100 retained for the control composition CI), was it observed, unexpectedly, a very significantly improved endurance (gain of nearly 50%) on the composition according to the invention, an improvement which can only be attributed to the use of the compound of formula (III).

This result suggests to the skilled person a high endurance belts and tires according to the invention, vis-à-vis in particular the problem of separation of the ends of the crown plies ("cleavage") mentioned above. Table 1

(1) natural rubber;

(2) N330 carbon black (ASTM grade);

(3) N-1,3-dimethylbutyl-N-phenyl-para-phenylenediamine ("Santoflex 6-PPD" from Flexsys);

(4) 4,4'-bis (1,3-dimethylbutylamino) triphenylamine;

(5) zinc oxide (industrial grade - Umicore company);

(6) stearin ("Pristerene 4931" - Uniqema company);

(7) N-dicyclohexyl-2-benzothiazyl sulfenamide ("Santocure DCBS" from Flexsys).

Table 2

Claims

A tire belt comprising a rubber composition based on at least one isoprene elastomer, a reinforcing filler, a crosslinking system and an antioxidant, characterized in that said antioxidant agent comprises a 4,4'-bis ( alkylamino) -triphenylamine having the formula (I):

wherein R ¹ and R ² , identical or different, each represents a linear or branched alkyl group having 1 to 12 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms.

Tire belt according to claim 1, wherein R ¹ and R ² , which may be identical or different, each represent an alkyl having from 2 to 8 carbon atoms or a cyclohexyl group.

Tire belt according to Claim 2, in which the radicals R ¹ and R ² , which may be identical or different, each represent an alkyl group chosen from the group consisting of isopropyl, 1,3-dimethylbutyl and 1,4-dimethylbutyl. -diméthylpentyle.

Tire belt according to Claim 3, the antioxidant being 4,4'-bis (isopropylamino) -triphenylamine corresponding to the formula (III-a):

(III-a)

A tire belt according to claim 3, the antioxidant being 4,4'-bis (1,3-dimethylbutylamino) triphenylamine having the formula (III-b):

(III-b)

Tire belt according to claim 3, the antioxidant being 4,4'-bis (1,4-dimethylpentylamino) triphenylamine corresponding to formula (III-c):

(III-c)

7. tire belt according to any one of claims 1 to 6, the level of antioxidant being between 1 and 10 phr.

8. tire belt according to claim 7, the level of antioxidant being between 2 and 8 phr.

Tire belt according to any one of claims 1 to 8, the isoprene elastomer being selected from the group consisting of natural rubber, synthetic cis-1,4 polyisoprenes and mixtures of these elastomers.

Tire belt according to claim 9, the isoprene elastomer being natural rubber.

11. Use, for the anti-aging protection of a tire belt, of an antioxidant agent comprising a 4,4'-bis (alkylamino) -triphenylamine corresponding to formula (I):

wherein R and R, identical or different, each represents a linear or branched alkyl group having 1 to 12 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms.

12. Use, for the manufacture or retreading of a tire, of a tire belt according to any one of claims 1 to 10.

13. A tire comprising a belt according to any one of claims 1 to 10.

14. A tire according to claim 13 consisting of a truck tire.

A method for preparing a tire belt comprising a rubber composition based on at least one isoprene elastomer, a reinforcing filler, a crosslinking system and an antioxidant, comprising the steps of:

o incorporating in an isoprene elastomer, in a mixer, a reinforcing filler; an antioxidant, thermomechanically kneading the whole, one or more times, until a maximum temperature of between 110 ° C and 190 ° C; o cool the assembly to a temperature below 100 ° C; o then incorporate a crosslinking system; o mix everything up to a maximum temperature below 110 ° C; calender or extrude the composition thus obtained in the form of a layer of rubber; o incorporate this layer, after possible addition of textile or metal reinforcements, in the tire belt,

said method being characterized in that said antioxidant agent comprises a 4,4'-bis (alkylamino) -triphenylamine of formula (I):

wherein R ¹ and R ² , identical or different, each represents a linear or branched alkyl group having 1 to 12 carbon atoms or a cycloalkyl group having 5 to 8 carbon atoms.