EP1841820A1 - Tyre running thread - Google Patents

Abstract

The inventive tyre running thread comprises a rubber composition based on at least one type of diene elastomer, a non-black reinforcement filler, in particular inorganic, a coupling agent for bonding said dien elastomer and the filler, wherein said composition is characterised in that it also comprises a pro-photo oxidant agent and optionally a carbon black at a rate of less than 5 pce (parts by weight per hundred parts of elastomer). Said pro-photo oxidant agent is mainly embodied in the form of a radical or cationic photoinitiator activated or not by the presence of a photo-sensitiser. Tyres provided with said photo oxide (exposed to UV-visible radiation) running thread exhibit a substentially increased adherence on a wet road.

Description

 BANDEDE ROULEMENTPOURPNEUMATIQUE

The present invention relates to treads of tires comprising rubber compositions reinforced by a non-black filler, in particular an inorganic filler such as silica.

It relates more particularly to treads reinforced mainly with a non-black charge and optionally comprising carbon black, a minority.

A tire tread must obey, as is known, a large number of technical, often antinomic, requirements including low rolling resistance, high wear resistance, as well as high adhesion, in particular on wet or wet road.

These property compromises, especially from the point of view of rolling resistance and wear resistance, have been improved in recent years on "green tires" with low energy consumption (or "energy-saving"). Green Tires "), intended in particular for passenger vehicles, thanks to the use of new low hysteretic rubber compositions having the characteristic of being reinforced mainly with specific reinforcing inorganic fillers such as highly dispersible silicas (called" HDS "for" Highly Dispersible Silica "), capable of competing, from the point of view of reinforcing power, with conventional pneumatic grade carbon blacks.

Thus, today, these reinforcing inorganic fillers gradually replace the conventional carbon blacks in the treads of tires, especially since they have another known virtue, that of increasing the adhesion of the tires to the tires. wet, snowy or icy road. However, we continue to use low-grade carbon black, typically in a range of 5 to 10 or 20 phr; in the intervals indicated, it is known that the black coloring agents (black pigmenting agent) and the anti-UV properties of the carbon blacks can be used, without in addition penalizing the typical performances provided by the reinforcing inorganic filler, namely low hysteresis and adhesion. improved on wet, snowy or icy ground. Illustrative of this state of the art are, for example, patents or patent applications FR 2797636 (or GB 2354246), EP 0890606, EP 1092565, US20020151640, WO 01/96442, WO 02/10269, WO 02/22728, WO 02/31041, WO 02/053634, WO 02/066519, WO 02/083782, WO 03/002648 and WO 03/002649. The increase in the adhesion properties of tires, even for treads reinforced with a reinforcing inorganic filler, however, remains a constant concern for tire designers.

Surprisingly, the Applicants have discovered that wet or wet adhesion can be further improved, without compromising on the other properties mentioned above, by incorporating a pro-photo-oxidizing agent into the rubber compositions of the strips. bearing reinforced with a non-black load, particularly inorganic.

Accordingly, a first object of the invention relates to a tread comprising a rubber composition based on at least one diene elastomer, a non-black reinforcing filler, a coupling agent ensuring the bond between the diene elastomer and said filler, optionally carbon black, said tread being characterized in that said composition further comprises a pro-photo-oxidizing agent and in that its carbon black content is zero or less than 5 phr (parts in weight per hundred parts of elastomer).

The invention also relates to the use of such a tread for the manufacture of new tires or the retreading of used tires.

The tread according to the invention is particularly suitable for tires intended to equip tourism-type motor vehicles, SUV ("Sport Utility Vehicles"), two wheels (including bicycles, scooters, motorcycles), planes, such as vehicles. industrial chosen for example from vans or "heavy vehicles" (that is to say, metro, bus, road transport equipment such as trucks, tractors, trailers).

The invention also relates to these tires themselves when they comprise a tread according to the invention.

The subject of the invention is also a process for preparing a tire tread capable, after photooxidation of its surface, of having an improved adhesion on a wet surface, this process being characterized in that it is incorporated by mixing with at least one diene elastomer, at least one non-black filler as a reinforcing filler, a coupling agent for bonding between the diene elastomer and said filler, optionally carbon black at a rate of zero or less than 5; pce, a pro-photo-oxidizing agent, a crosslinking system, and what is extruded in the form of a tread. The invention as well as its advantages will be readily understood in the light of the description and the following exemplary embodiments.

I. MEASUREMENTS AND TESTS USED

The treads and rubber compositions constituting these treads are characterized, before and after cooking, as indicated below.

I- 1. Mooney Plasticity

An oscillating consistometer is used as described in the French standard NF T 43-005 (November 1980). The Mooney plasticity measurement is carried out according to the following principle: the raw rubber composition (ie, before firing) is molded in a cylindrical chamber heated to 100 ^° C. After a minute of preheating, the rotor rotates within of the test piece 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).

1-2. Traction tests

These tensile 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 second elongation is measured (ie, after an accommodation cycle at the extension rate provided for the measurement itself). nominal secants (or apparent stresses, in MPa) at 10% elongation (denoted MlO), 100% elongation (denoted MlOO) and 300% elongation (denoted M300).

The breaking stresses (in MPa) and the elongations at break (in%) are also measured. 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).

1-3. Tire testing ( ^" wet braking ^" ) - AT -

The tires are mounted on a motor vehicle equipped with an ABS braking system and the distance required to go from 50 km / h to 10 km / h is measured during a sudden braking on a wet ground (asphalt concrete). A value greater than that of the control, arbitrarily set at 100, indicates an improved adhesion, ie a shorter braking distance.

II. DETAILED DESCRIPTION OF ¹ INfVENTION

The treads according to the invention are formed, at least for their surface portion intended to come into contact with the road, with a rubber composition based on at least: (i) a (at least one) diene elastomer ; (ii) a (at least one) non-black filler, in particular an inorganic filler, as a reinforcing filler; (iii) a (at least one) coupling agent providing the bond between said reinforcing filler and the diene elastomer; (iv) a (at least one) pro-oxidant agent; optionally carbon black at a rate of less than 5 phr, preferably less than 3 phr.

Of course, the term "composition based" means a composition comprising the mixture and / or the reaction product in situ of the various constituents used, some of these basic constituents (for example the coupling agent and the reinforcing filler) being capable of, or intended to react with each other, at least in part, during the different phases of manufacture of the treads, in particular during their vulcanization (firing).

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

II- 1. Diene elastomer

By "diene" elastomer (or synonym rubber), is generally meant an elastomer derived at least in part (i.e. a homopolymer or a copolymer) of monomers dienes (monomers bearing two carbon-carbon double bonds, conjugated or not).

The diene elastomers, in known manner, can be classified into two categories: those said to be "essentially unsaturated" and those termed "essentially saturated". By diene elastomer

"essentially unsaturated" means a diene elastomer derived at least in part from conjugated diene monomers, having a level of units or units of diene origin (dienes conjugates) which is greater than 15% (mol%). Thus, for example, diene elastomers such as butyl rubbers or copolymers of dienes and alpha-olefins of the EPDM type do not fall within this definition and may instead be referred to as "essentially saturated" diene elastomers. "(low or very low diene origin, always less than 15%). In the category of "essentially unsaturated" diene elastomers, the term "highly unsaturated" diene elastomer is particularly understood to mean a diene elastomer having a content of units of diene origin (conjugated dienes) which is greater than 50%.

These general definitions being given, those skilled in the tire will understand that the present invention is first implemented with highly unsaturated diene elastomers, in particular with:

(a) any homopolymer obtained by polymerization of a conjugated diene monomer having from 4 to

12 carbon atoms; (b) any copolymer obtained by copolymerization of one or more conjugated dienes with each other or with one or more vinyl aromatic compounds having from 8 to 20 carbon atoms.

By way of conjugated dienes 1,3-butadiene, 2-methyl-1,3-butadiene and 2,3-di-(C ₁ -C ₅ ) alkyl-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl- 1,3-butadiene, aryl-1,3-butadiene, 1,3-pentadiene, 2,4-hexadiene. Suitable vinyl aromatic compounds are, for example, styrene, ortho-, meta-, para-methylstyrene, the "vinyl-toluene" commercial mixture, para-tert-butylstyrene, methoxystyrenes, chlorostyrenes, vinylmesitylene and divinylbenzene. vinyl naphthalene.

The copolymers may contain between 99% and 20% by weight of diene units and between 1% and 80% by weight of vinyl aromatic units. The elastomers may have any microstructure which is a function of the polymerization conditions used, in particular the presence or absence of a modifying and / or randomizing agent and the amounts of modifying and / or randomizing agent used. The elastomers can be for example block, statistical, sequence, microsequential, and be prepared in dispersion or in solution; they can be coupled and / or star or functionalized with a coupling agent and / or starring or functionalization.

Polybutadienes, and in particular those having a 1,2-unit content of between 4% and 80%, or those having a cis-1,4 content of greater than 80%, polyisoprenes and copolymers of butadiene- styrene and in particular those with a styrene content of between 5% and 50% by weight and more particularly between 20% and 40%, a 1,2-butadiene content of the butadiene part of between 4% and 65%, a trans-1,4 bond content of between 20% and 80%, butadiene-isoprene copolymers and in particular those having an isoprene content of between 5% and 90% by weight and a glass transition temperature ("Tg" - measured according to ASTM D3418-82) of - 40 ^° C. at -80 ^° C., the isoprene-styrene copolymers and in particular those having a styrene content of between 5% and 50% by weight and a Tg of between -25 ° C. and -50 ^° C. In the case of copolymers of butadiene-styrene-isoprene are especially suitable those having a styrene content of between 5% and 50% by weight and more particularly between 10% and 40%, an isoprene content of between 15% and 60% by weight and more especially between 20% and 50%, a butadiene content of between 5% and 50% by weight and more particularly preferably between 20% and 40%, a content of -1,2 units of the butadiene part of between 4% and 85%, a content of trans-1,4 units of the butadiene part of between 6% and 80%, a content in -1,2 units plus -3,4 of the isoprenic part of between 5% and 70% and a trans-1,4 content of the isoprenic part of between 10% and 50%, and more generally any butadiene-styrene-isoprene copolymer having a Tg of between -20 ^° C. and -70 ^° C.

In summary, particularly preferably, the diene elastomer of the composition according to the invention is chosen from the group of diene elastomers (highly unsaturated) consisting of polybutadienes (BR), synthetic polyisoprenes (IR), rubber natural (NR), butadiene copolymers, isoprene copolymers and mixtures of these elastomers. Such copolymers are more preferably selected from the group consisting of butadiene-styrene copolymers (SBR), isoprene-butadiene copolymers (BIR), isoprene-styrene copolymers (SIR), isoprene-copolymers of butadiene-styrene (SBIR) and mixtures of such copolymers.

The tread according to the invention is preferably intended for a passenger car tire. In such a case, the diene elastomer is preferably an SBR copolymer, whether it is prepared in solution (SSBR) or in emulsion (ESBR), preferably used in admixture with a polybutadiene; more preferably, the SBR has a styrene content of between 20% and 30% by weight, a vinyl content of the butadiene part of between 15% and 65%, a trans-1,4 content of between 15% and 75% and a Tg of between -20 ^° C. and -55 ^° C., and the polybutadiene has more than 90% of cis-1,4 bonds.

In the case of a truck tire, the diene elastomer is preferably an isoprene elastomer, ie a homopolymer or copolymer of isoprene, in other words a diene elastomer chosen from the group made up of natural rubber (NR), the synthetic polyisoprenes (IR), the various isoprene copolymers or a mixture of these elastomers. Among the isoprene copolymers, mention will in particular be made of copolymers of isobutene-isoprene (butyl rubber - IIR), isoprene-styrene (SIR), isoprene-butadiene (BIR) or isoprene-butadiene-styrene (SBIR). This isoprene elastomer is preferably natural rubber or synthetic cis-1,4 polyisoprene; of 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. For such a truck tire, the diene elastomer may also consist, in whole or in part, of another highly unsaturated elastomer such as, for example, an SBR elastomer.

The compositions of the treads of the invention may contain a single diene elastomer or a mixture of several diene elastomers, the diene elastomer or elastomers that may be used in combination with any type of synthetic elastomer other than diene, or even with other polymers. as elastomers, for example thermoplastic polymers.

II-2. Non-black reinforcing filler

By "non-black reinforcing filler" must be understood here any inorganic or organic filler other than carbon black, regardless of its color and origin (natural or synthetic), sometimes called "mineral" filler, "white" filler "or" clear "charge as opposed to carbon black, this non-black charge being able to reinforce on its own, with no other means than an intermediate coupling agent, a rubber composition for the manufacture of a strip of tire bearing, in other words able to replace, in its reinforcing function, a conventional tire grade carbon black for tread; such a filler is generally characterized, in known manner, by the presence of functional groups, in particular hydroxyl groups, at its surface, thereby requiring the use of a coupling agent intended to ensure a stable chemical bond between the diene elastomer and said charge.

Preferably, the non-black reinforcing filler is an inorganic filler, more particularly a filler of the siliceous or aluminous type, or a mixture of these two types of filler.

The silica (SiO ₂ ) 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 area both less than 450 m ² / g, preferably 30 to 400 m ² / g. Silicas highly dispersible precipitates (so-called "HDS") are preferred, in particular when the invention is implemented for the manufacture of tires having a low rolling resistance; examples of such silicas include Ultrasil 7000 silicas from Degussa, Zeissil 1165 MP, 1135 MP and 1115 MP from Rhodia, Hi-SiI EZ150G silica from PPG, Zeopol 8715 silicas, 8745 or 8755 of the Huber Company, the silicas as described in the application WO 03/016387.

The reinforcing alumina (Al 2 O 3) preferably used is a highly dispersible alumina having a BET surface area ranging from 30 to 400 m ² / g, more preferably from 60 to 250 m ² / g, an average particle size of at most 500 nm. more preferably at most equal to 200 nm. As non-limiting examples of such reinforcing aluminas, there may be mentioned the aluminas "Baikalox A125" or "CR125" (Baikowski company), "APA-100RDX" (Condea), "Aluminoxid C" (Degussa) or "AKP-G015" (Sumitomo Chemicals).

By way of other examples of inorganic filler suitable for use in the rubber compositions of the invention, mention may also be made of aluminum (oxide) hydroxides, aluminosilicates, titanium oxides, carbides or nitrides of silicon, all of the reinforcing type as described in WO 99/28376, WO 00/73372, WO 02/053634, WO 2004/003067, WO 2004/056915.

When the treads of the invention are intended for tires with low rolling resistance, the non-black reinforcing filler used, in particular if it is a reinforcing inorganic filler such as silica, preferably has a BET surface area between 60 and 350 m2 / g. An advantageous embodiment of the invention consists in using a non-black filler, in particular a reinforcing inorganic filler such as silica, having a high BET specific surface area, in a range of 130 to 300 m ² / g, because of of the high reinforcing power recognized such charges. According to another preferred embodiment of the invention, it is possible to use a non-black filler having a BET specific surface area of less than 130 m ² / g, preferably in such a case between 60 and 130 m ² / g (see examples WO03 / 002648 and WO03 / 002649 applications cited above).

The physical state in which the non-black (especially inorganic) reinforcing filler is present is indifferent whether in the form of powder, microbeads, granules, beads or any other suitable densified form. Of course, a non-black reinforcing filler is also understood to mean mixtures of different non-black reinforcing fillers, in particular inorganic fillers such as highly dispersible siliceous and / or aluminous fillers as described above. The person skilled in the art will be able to adapt the rate of non-black reinforcing filler according to the nature of the load used and according to the type of tire concerned, for example a tire for a motorcycle, for a passenger vehicle or for a commercial vehicle such as a van or weight. heavy. Preferably, this level of non-black reinforcing filler, in particular inorganic, will be selected between 20 and 200 phr, more preferably between 30 and 150 phr. More preferably still, in particular when the composition of the invention is intended for a tire tread tourism, the rate of this reinforcing filler is chosen greater than 50 phr, for example between 60 and 140 phr, especially in a area from 70 to 130 pce.

In the present disclosure, the BET surface area is determined in a known manner by gas adsorption using the Brunauer-Emmett-Teller method described in "The Journal of the American Chemical Society" Vol. 60, page 309, February 1938, more precisely according to the French standard NF ISO 9277 of December 1996 (multipoint volumetric method (5 points) - gas: nitrogen - degassing: hour at 160 ⁰ C - relative pressure range p / po: 0.05 at 0.17). The CTAB specific surface is the external surface determined according to the French standard NF T 45-007 of November 1987 (method B).

Finally, those skilled in the art will understand that, as an equivalent load of the non-black reinforcing filler described in this paragraph, carbon black could be used if it were covered with a non-black layer, in particular inorganic such as silica, having on its surface functional sites (especially hydroxyl) in known manner requiring the use of a coupling agent to establish the bond between the filler (the surface of its particles) and the diene elastomer.

H-3. Coupling agent

It is recalled that a coupling agent, by definition at least bifunctional, can be represented by the simplified general formula "Y-T-X", in which:

Y represents a functional group ("Y" function) capable of binding to the non-black charge, in particular an inorganic filler, such a bond possibly being established, for example, between a silicon atom of the coupling agent and the hydroxyl groups (OH ) surface of the inorganic filler (for example silanols surface when it is silica); X represents a functional group ("X" function) capable of binding to the diene elastomer, for example via a sulfur atom; T represents a divalent group making it possible to connect Y and X. In particular, the coupling agents must not be confused with simple non-black, in particular inorganic, charge-covering agents which, in known manner, may comprise the active function Y with respect to said charge but lack the function X active with respect to the elastomer.

Coupling agents (silica / diene elastomer), of variable efficiency, have been described in a very large number of documents and are well known to those skilled in the art. Any coupling agent capable of effectively ensuring, in the diene rubber compositions which can be used for the manufacture of tire treads, the bonding between an inorganic reinforcing filler such as silica and a diene elastomer, in particular organosilanes or polyfunctional polyorganosiloxanes carrying X and Y functions.

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

Particularly suitable for carrying out the invention, without the following definition being limiting, so-called "symmetrical" polysulfide silanes having the following general formula (I):

(I) 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 in

C ₁ -C ₁₈ or C ₆ -C ₁₂ arylene groups, more particularly C ₁ -C ₁₀ alkylenes, in particular C ₁ -C ₄ alkylenes, in particular propylene); Z responds to one of the following formulas:

,

in which:

the radicals R ¹ , substituted or unsubstituted, which are identical to or different from each other, represent a C ₁ -C ₈ alkyl, C ₅ -C ₁₈ cycloalkyl or C ₆ -C ₁₈ aryl group; preferably C ₁ -C ₆ alkyl, cyclohexyl or phenyl, especially C ₁ -C ₄ alkyl groups, more particularly methyl and / or ethyl);

the radicals R ² , substituted or unsubstituted, which are identical to or different from one another, represent a hydroxyl, C ₁ -C ₁₈ alkoxyl or C ₅ -C ₈ cycloalkoxyl group (preferably a group chosen from hydroxyl, C ₅ -C ₁₈ alkoxyls); C ₈ and cycloalkoxyls in

C ₅ -C ₈ , more preferably still a group selected from hydroxyl and alkoxyls in

C ₁ -C ₄ , in particular from hydroxyl, methoxyl and ethoxyl).

In the case of a mixture of polysulphurized silanes corresponding to formula (I) above, in particular common commercially available mixtures, the average value of "n" is a fractional number preferably between 2 and 5, more preferably close to However, the invention can also be advantageously used for example with disulfide silanes (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 (Ci-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 .

TESPD is marketed for example by the company Degussa under the name Si75 (as a mixture of disulfide - 75% by weight - and polysulfides), or by the company Witco under the name Silquest A1589. TESPT is marketed, for example, by Degussa under the name Si69 (or X50S when it is supported at 50% by weight on carbon black), or by Osi Specialties under the name Silquest A 1289 (in the US). two cases, commercial mixture of polysulfides with a mean value for n which is close to 4).

Examples of advantageous coupling agents are polysulfides (especially disulfides, trisulphides or tetrasulfides) of bis- (monoalkoxyl (C ₁ -C ₄ ) -dialkyl (C 1 -C ₄ ) silylpropyl), more particularly the tetrasulfide or disulfide of bis-monoethoxy-dimethylsilylpropyl as described in the aforementioned WO02 / 083782 application.

By way of examples of coupling agents other than the above-mentioned polysulphide alkoxysilanes, mention may be made in particular of bifunctional polyorganosiloxanes or polysulfides. hydroxysilane (R ² = OH in formula I above) as described in WO 02/30939 and WO 02/31041.

In the treads according to the invention, the content of coupling agent is preferably between 2 and 15 phr, more preferably between 2 and 12 phr (for example between 4 and 8 phr). But it is generally desirable to use as little as possible. With respect to the weight of non-black reinforcing filler, the level of coupling agent is typically between 0.5 and 15% by weight relative to the amount of non-black filler. In the case for example tire treads for passenger vehicles, the coupling agent is used at a preferential rate of less than 12%, or even less than 10% by weight relative to this amount of filler.

The coupling agent could be previously grafted (via function "X") on the diene elastomer of the composition of the invention, the elastomer thus functionalized or "pre-coupled" then having the function "Y" free for the load reinforcing inorganic. The coupling agent could also be grafted beforehand (via the "Y" function) on the non-black, in particular inorganic, reinforcing filler, the thus "pre-coupled" filler then being bonded to the diene elastomer via the free function "X". However, it is preferred, especially for reasons of better implementation of the compositions in the green state, to use the coupling agent either grafted onto the filler or in the free (i.e., ungrafted) state.

II-4. Carbon black

The rubber composition of the tread of the invention may or may not include carbon black. If the latter is present, it is at a rate as low as possible, typically less than 5 phr, preferably less than 3 phr, in order to prevent the carbon black can play, because of its anti UV-known, an antagonistic role therefore undesirable with respect to the action of the pro-photo-oxidizing agent. For this reason, the carbon black content is more preferably less than 2 phr.

According to one particular embodiment of the invention, carbon black is used in a very small amount, between 0.05 and 2.0 phr, in particular between 0.05 and 1.5 phr, a narrow concentration range in which the black retains its function of black coloring agent of the composition but no longer fulfills that of anti-UV agent.

Suitable carbon blacks are all carbon blacks capable of bringing a black color to the rubber compositions, in particular blacks of the HAF, ISAF and SAF known to those skilled in the art and conventionally used in tires. Among these are the reinforcing carbon blacks of the series (ASTM grades) 100, 200 or 300 used in the treads of these tires (eg Nl 15, Nl 34, N234, N326, N330, N339, N347 , N375), but also those of the non-reinforcing type (because less structured) of the higher series 400 to 700 (for example the blacks N660, N683, N772). Non-reinforcing blacks called "ink blacks" used as black pigments in printing inks and paints could also be used as examples.

The carbon blacks can be used in the isolated state, as commercially available, or in any other form, for example as a carrier for some of the rubber additives used.

II-5. Pro-photo oxidant agent

The heart of the present invention lies in the use of a pro-photo-oxidizing agent in the rubber composition forming at least the surface or extreme surface of the tread of the invention. This agent is intended to promote, accelerate the surface photooxidation process of the tread when the latter is subjected to exposure to UV-visible radiation.

In a known manner, the pro-photo-oxidizing agents are stable compounds able to release, when exposed to light of appropriate wavelength, radical or ionic species which will promote and accelerate the photo-oxidation processes. .

These pro-photo-oxidizing agents preferably consist of photoinitiators (also called photoinitiators) of the radical or cationic type. Such compounds are commonly used in the fields of photopolymerization of multifunctional monomers, surface treatment of materials with protective coatings, graphic arts and electronics for the production of microcircuits. In all these applications, the main objective is to rapidly obtain highly crosslinked polymers having high resistance to chemical agents and the required mechanical properties (see, for example, JW Crivello and K. Dietliker, "Photoinitiators for free radical, cationic and Anionic Photopolymerization, "Vol III, Chemistry and Technology of UV and EB Formulation or Coatings, Inks and Paint, John Wiley and Sons, SITA Technology Limited, ^2nd Edition, London 1998 - E. Chesneau and JP Fouassier," Polymerization-induced visible laser irradiation ", Die Angewandte Makromolekulare Chemie, 135, 1985, pp. 41-64). Examples of cationic photoinitiators that may be mentioned are onium salts, more particularly sulphonium salts such as triarylsulphonium salts or iodonium salts such as diaryliodonium salts.

A photoinitiator of the radical type, more preferentially chosen from aromatic ketones, is preferably used, in particular those chosen from the group consisting of benzal ketals (especially diketals), benzoins (in particular benzoin ethers) and α, α-dialkoxy. acetophenones, α-hydroxyalkylphenones, α-aminoaromatic ketones (or α-aminoalkylphenones), acylphosphine oxides, benzophenones or thioxanthones in combination with a hydrogen donor (eg, a tertiary amine), and mixtures of such compounds.

Examples of such preferred radical photoinitiators can be cited in particular:

benzyl ketals and benzoin ethers: diphenylethanones and their halogenated derivatives (eg, 2-ethoxy-1,2-diphenylethanone, 2-isopropoxy-1,2-diphenylethanone, 2-isobutoxy-1; 2-diphenylethanone, 2-butoxy-1,2-diphenylethanone, 2,2-dimethoxy-1,2-diphenylethanone, 2,2-diethoxy-1,2-diphenylethanone); among the α, α-dialkoxy-acetophenones: α, α-diethoxy-acetophenone and α, α-di- (n-butoxy) -acetophenone); among the α-hydroxy-alkylphenones: 1-hydroxy-cyclohexyl-phenyl-ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, 1 - [4- (2-hydroxyethoxy) phenyl] -2 -hydroxy-2-methylpropan-1-one); - among the α-amino aromatic ketones: 2-methyl-1- [4- (methylthio) phenyl] -2-morpholino-propan-1-one, 2-benzyl-2-dimethylamino-1- (4-morpholino-phenyl) ) -butan 1-one); among the oxides of acylphosphines: 2,4,6-trimethylbenzoyl-diphenylphosphine oxide,

2,4,6-trimethylbenzoylphenylphosphinic acid ethyl ester, bis (2,4,6-trimethylbenzoyl) phenylphosphine oxide); Of the benzophenones: 2,4,6-trimethylbenzophenone, 4-chloro-benzophenone, 4-methyl-benzophenone, [4- (4-methylphenylthio) phenyl] phenylmethanone, 3,3'-dimethyl-4-methoxybenzophenone benzoyl, 4-phenyl-benzophenone methyl, benzoyl, 4,4'-bis (dimethylamino) benzophenone, 4,4'-bis (diethylamino) benzophenone, benzil benzophenone, 4-phenylbenzophenone; among the thioxanthones: 2-isopropyl-thioxanthone, 4-isopropyl-thioxanthone, 2,4-dimethylthioxanthone, 2,4-diethoxythioxanthone, 1-chloro-4-propoxy-thioxanthone; the tertiary amines used in combination with benzophenones or thioxanthones: methyl diethanol amine, triethanolamine, 4- (dimethylamino) benzoate, 4- (dimethylamino) benzoate, n-butoxyethyl, 4- (dimethylamino) isoacryl benzoate, 2- (dimethylamino) ethyl benzoate, 1- (4-dimethylaminophenyl) -ethanone, 2-ethylhexyl 4-dimethylaminobenzoate.

The above compounds are well known and commercially available, some marketed for example by the company Ciba (Ciba Specialty Chemicals Inc.) under the names "IRGACURE" or "DAROCUR".

By way of example, the bis (2,4,6-trimethylbenzoyl) -phenylphosphine oxide, "IRGACURE 819" from Ciba, has the formula:

As another example, 2,2-dimethoxy-1,2-diphenylethanone, "IRGACURE 651" from Ciba, having the formula:

The photoinitiators previously described may be used alone when exposed to UV radiation or, when exposed to visible light, in combination with photosensitizers capable of activating their own photooxidative action. Such photosensitizers are, for example, dyes such as phthalenes (eosin, erythrosine, ethyléosine, phloxine, bengal rosé) and thiazines (thionine and methylene blue). If they are used without photosensitizers, the photoinitiators will preferably be chosen in such a way that their UV absorption spectrum and the emission spectrum of the light source (whether natural or artificial) responsible for photooxidation. of the tread, have a recovery zone.

Those skilled in the art will be able to adjust the amount of pro-photo-oxidizing agent in the light of the description and the following exemplary embodiments. This amount is preferably between 0.1 and 10 phr, more preferably between 0.2 and 5 phr. Below the minima indicated, the technical effect is likely to be insufficient, while beyond the recommended maxima, costs increase significantly. A reduced amount of between 0.2 and 2 phr has been found to be suitable for a large number of cases.

II-6. Various additives

The tread rubber compositions according to the invention also comprise all or part of the usual additives normally used in elastomer compositions intended for the production of treads, for example plasticizers or extension oils. whether the latter are of aromatic or non-aromatic nature, pigments, protective agents such as anti-ozone waxes, chemical antiozonants, anti-oxidizing agents which it is preferable to keep present in the mass, agents anti-fatigue, reinforcing or plasticizing resins, acceptors (for example phenolic novolac resin) or methylene donors (for example HMT or H3M) as described, for example, in the aforementioned application WO 02/10269, a crosslinking system with either sulfur or sulfur and / or peroxide and / or bismaleimide, vulcanization accelerators, vulcanization activators ation.

Preferably, these compositions comprise, as preferential non-aromatic or very weakly aromatic plasticizing agent, at least one compound chosen from the group consisting of naphthenic, paraffinic, MES, TDAE, ester (especially trioleate) oils. glycerol, hydrocarbon plasticizing resins having a high Tg preferably greater than 30 ^° C., and mixtures of such compounds. The overall level of such a preferred plasticizer is preferably between 15 and 45 phr, more preferably between 20 and 40 phr. Among these hydrocarbon plasticizing resins (it is recalled that the designation

"resin" is reserved by definition for a solid compound), mention may be made especially of homo- or copolymer resins of alphapinene, betapinene, dipentene or polylimonene, C5-cut, by example of C5 cut copolymer / styrene, used alone or in combination with plasticizing oils such as MES or TDAE oils.

The reinforcing filler described above, that is to say the non-black (in particular inorganic) reinforcing filler plus carbon black, if appropriate, may also be added, depending on the intended application, to the inert fillers ( ie, non-reinforcing) such as clay particles, bentonite, talc, chalk, kaolin, usable for example in colored tire sidewalls or treads.

These compositions may also contain, in addition to the coupling agents, coupling activators, covering agents (comprising, for example, the only Y function) of the non-black reinforcing filler or, more generally, agents for assisting the setting-up. in a known manner, by improving the dispersion of the non-black filler in the rubber matrix and lowering the viscosity of the compositions, to improve their ability to implement in the green state, these agents being, for example, hydrolyzable silanes such as alkylalkoxysilanes (in particular alkyltriethoxysilanes), polyols, polyethers (for example polyethylene glycols), primary, secondary or tertiary amines (for example trialkanol amines), hydroxylated POSs or hydrolyzable, for example α, ω-dihydroxy-polyorganosiloxanes (in particular α, ω-dihydroxy-polydimethylsiloxanes).

II-7. Tread manufacturing

The subject of the invention is also a process for preparing a tire tread capable, after photooxidation of its surface, of having an improved adhesion on a wet surface, this process being characterized in that it is incorporated by mixing with at least one diene elastomer, at least:

one (at least one) non-black filler, in particular an inorganic filler, as a reinforcing filler; - a (at least one) coupling agent intended to ensure the connection between the diene elastomer and said filler; from 0 to less than 5 phr, preferably from 0 to less than 3 phr of carbon black;

a (at least one) pro-photooxidant agent;

a (at least one) crosslinking system,

and extruded in the form of a tread. The rubber compositions are manufactured in suitable mixers, using two successive preparation phases according to a general procedure 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 high temperature, up to a maximum temperature of between 130 ^° C. and 200 ^° C., preferably between 145 ° C. and 185 ^° C., followed by a second mechanical working phase (sometimes referred to as a "productive" phase) at lower temperature, typically below 120 ^° C., for example between 60 ^° C. and 100 ^° C., a finishing phase during which the crosslinking or vulcanization system is incorporated.

According to a preferred embodiment of the invention, all the basic constituents of the compositions of the treads according to the invention, with the exception of the crosslinking or vulcanization system, namely the non-black reinforcing filler, the coupling agent, the optional carbon black and the pro-photo-oxidizing agent are intimately incorporated, by kneading, with the diene elastomer during the so-called non-productive first phase, that is to say that is introduced into the mixer and which is thermomechanically kneaded, in one or more steps, at least these different basic constituents until the maximum temperature between 130 ⁰ C and 200 ⁰ C, preferably between 145 ⁰ C and 185 ⁰ C. The total mixing time, in this non-productive phase, is preferably between 1 and 15 min.

After cooling the mixture thus obtained during the first non-productive phase, the vulcanization system is then incorporated at low temperature, generally in an external mixer such as a roller mixer; the whole is then mixed (productive phase) for a few minutes, for example between 2 and 15 min.

The actual crosslinking system is preferably a vulcanization system based on sulfur and an accelerator. Any compound capable of acting as a vulcanization accelerator for diene elastomers in the presence of sulfur, especially those 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-benzothiazyl sulfenamide (abbreviated as "TBBS"), N-tert-butyl -2-benzothiazyl sulfenimide (abbreviated as "TBSI") and mixtures of these compounds. Preferably, a primary accelerator of the sulfenamide type is used.

To this vulcanization system may be 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), etc. The sulfur content is preferably between 0.5 and 3.0 phr, that of the primary accelerator is preferably between 0.5 and 5.0 phr.

The final composition thus obtained is then calendered, for example in the form of a sheet or a plate, in particular for a characterization in the laboratory, or else extruded in the form of a rubber profile that can be used directly as a tire tread. .

The vulcanization (or cooking) is conducted in a known manner at a temperature generally between 130 ^° C. and 200 ^° C., for a sufficient time which may vary, for example, between 5 and 90 min depending in particular on the cooking temperature, the system of vulcanization adopted and the kinetics of vulcanization of the composition under consideration.

In summary, the method according to the invention for preparing a tire tread capable of having, after photooxidation of its surface, an improved adhesion on a wet surface, preferably comprises the following steps:

to incorporate in a diene elastomer, in a mixer:

A non-black reinforcing filler, in particular an inorganic filler; • a coupling agent;

• optionally carbon black at a rate of zero or less than 5 phr; a pro-photo-oxidant agent; by thermomechanically kneading the whole, in one or more times, until a maximum temperature of between 130 ^° C. and 200 ^° C. is reached; - Cool all at a temperature below 100 ⁰ C; then incorporate: a crosslinking system; knead everything to a maximum temperature below 120 ⁰ C; extruding or calendering the rubber composition thus obtained, in the form of a tire tread.

As explained above and illustrated by the following exemplary embodiments, a final step of photooxidation of the surface of the tread gives the latter improved adhesion on wet or damp ground.

This complementary step may consist of a simple natural exposure to UV-visible radiation or preferably an accelerated artificial aging, for example using one or more lamp (s) emitting in the UV-visible (eg mercury vapor lamp, xenon lamp, fluorescent lamp, metal halide lamps), or in an accelerated photo-aging chamber that may comprise one or more several of said lamps.

In the process according to the invention, there is preferably at least one, more preferably all, of the following characteristics which is verified:

the amount of non-black reinforcing filler, in particular inorganic, is between 20 and 200 phr, more preferably between 30 and 150 phr; the quantity of coupling agent is between 2 and 15 phr; the maximum thermomechanical mixing temperature is between 145 ^° C. and

185 ° C; the non-black filler is an inorganic filler, in particular a siliceous or aluminous filler; the optional carbon black content is less than 3 phr, more preferably less than

2 pce; the level of pro-photo-oxidizing agent is between 0.1 and 10 phr; the at least bifunctional coupling agent is an organosilane or a polyorganosiloxane; the diene elastomer is a butadiene-styrene copolymer (SBR), preferably used in admixture with a polybutadiene.

More preferably, in this process, there is at least one, still more preferably all of the following characteristics which is verified:

the quantity of non-black filler, in particular inorganic filler, is greater than 50 phr, in particular between 60 and 140 phr, for example included in a range from 70 to

130 pce; the amount of coupling agent is between 4 and 12 phr, in particular between 3 and 8 phr; the reinforcing inorganic filler is silica; the carbon black content is between 0.05 and 2 phr, more preferably between -0.05 and 1.5 phr; the level of pro-photo-oxidizing agent is between 0.2 and 5 phr, in particular between 0.2 and

2 pce; the coupling agent is a polysulfide, especially disulfide or tetrasulfide,

3IkOXyI (C ₁ -C ₄ ) SiIyIPrOPyIe or bis-hydroxysilylpropyl; SBR is a SBR prepared in solution and polybutadiene has more than 90% cis-1,4 bonds.

The rubber compositions described above may constitute all or only part of the tread according to the invention.

The invention applies in particular to the cases where these rubber compositions comprising the pro-photo-oxidizing agent and the optional carbon black at zero or very low level, form only a part of the tread of the invention. , in particular only the superficial part of the latter intended to come into contact with the ground during the running of the tire, at one time or another during the life of the latter, that is to say that said tire is at in new condition or in a state of partial wear.

Thus, for example, in a tread of the composite type consisting of two radially superimposed rubber layers (so-called "cap-base" structure), both intended to come into contact with the road during the rolling of the tire, the part comprising the pro-photo-oxidizing agent and the optional low-carbon carbon black may constitute the radially outer layer of the tread in contact with the ground from the beginning of the roll of the new tire, or on the contrary its radially inner layer intended to come into contact with the ground later, in cases where it would be desirable, for example, to "delay" the technical effect provided by the invention, thus compensating for the loss of adhesion on wet ground resulting from partial wear of the tread (decrease of depth of relief of the sculpture).

The invention relates to the treads previously described as well as the tires comprising these treads, both in the green (ie, before the firing) and the fired (ie, after crosslinking or vulcanization), before as after photo-oxidation treatment.

III. EXAMPLES OF CARRYING OUT THE INVENTION

HI-I. Preparation of rubber compositions and treads

The following tests are carried out as follows: an internal mixer, filled to 70% and whose initial tank temperature is approximately 60 ^° C., is introduced successively into the reinforcing inorganic filler (silica), the black of carbon, the coupling agent, the agent optionally, the diene elastomer or the mixture of diene elastomers, as well as the various other ingredients with the exception of the vulcanization system. Theromechanical work (non-productive phase) is then carried out in one step, which lasts a total of about 3 to 4 minutes, until a maximum temperature of "fall" of 165 ° C is reached. The mixture thus obtained is recovered, cooled and then sulfur and sulfenamide accelerator is incorporated on an external mixer (homo-fînisseur) at 30 ° C., mixing all (productive phase) for a suitable time, between 5 and 12 min. According to the case.

The compositions thus obtained are then calendered either in the form of plates (thickness of 2 to 3 mm) or thin sheets of rubber for the measurement of their physical or mechanical properties, or extruded in the form of treads of passenger tires.

In the examples according to the invention which follow, according to a particularly preferred embodiment, the level of pro-photo-oxidizing agent is between 0.2 and 2 phr, more particularly between 0.5 and 1.5 phr. ; independently, the carbon black content is preferably between 0.1 and 1.0 phr, more particularly between 0.1 and 0.5 phr.

HI-2. testing

The following tests demonstrate that the introduction of a pro-photo-oxidizing agent in the tread rubber compositions of the invention, combined with a very low level of carbon black, makes it possible to obtain a significant increase the wet grip of said treads, resulting in a significant reduction in braking distances.

A) Trial 1

In this first test, three compositions based on diene elastomers SBR and BR known, reinforced with silica, are first compared.

These three compositions, intended to form tire treads for passenger vehicles, are noted here CI, C-2 and C-3 and are distinguished by the amount of carbon black and pro-photo-oxidant agent that they contain :

- C-I (control): no pro-photooxidant agent; 6 pc of carbon black;

C-2 (invention): 1 phr of pro-photo-oxidizing agent; 0.3 phr of carbon black; C-3 (control): no pro-photooxidant; 0.3 phr of carbon black. The CI composition is the reference used in green tires with low energy consumption; it conventionally uses HDS silica as reinforcing inorganic filler and silane TESPT as coupling agent.

In this composition C-I, the carbon black is used, at the indicated rate of 6 phr, both as a black pigmentation agent and as an anti-UV agent, in accordance with the teaching of the state of the art. In compositions C-2 and C-3, the carbon black at the very low level used still fulfills its function of black pigmentation agent, but no longer fulfills that of an effective anti-UV agent.

The composition C-2, thanks to the presence of the pro-photo-oxidizing agent (combined with the very low level of carbon black) has a high photo-oxidability which is supposed to be a posteriori - it is at least what seems to prove the test N ° 2 which follows - as being favorable to the adhesion on wet road. The composition C-3 constitutes another control of the test, which has a very low level of carbon black, but lacks the pro-oxidizing agent.

Tables 1 and 2 give the formulation of the various compositions (Table 1 - rate of the various products expressed in phr), their properties before and after curing (20 min at 165 ^° C.).

Examination of the different results of Table 2 reveals no significant difference between the properties of the three compositions, both from the point of view of their ability to implement in the green state (Mooney plasticity) and the mechanical properties (modules, ratio of the known M300 / M100 modulus of the reinforcement level, properties at break), which can be described as expected given the minimal differences in formulation between the three compositions.

It is in fact only during actual rolling tests conducted on tires that the unexpected result brought by the invention is revealed, as is evident from the following test.

B) Test 2

The previously described compositions CI, C-2 and C-3 are then used as treads of passenger tires with radial carcass, of dimension 195/65 Rl 5 (speed index H), conventionally manufactured and identical in all respects except the constitutive rubber composition of their tread. These tires are noted respectively PI, P-2 and P-3. Half of them were subjected to UV aging (natural exposure to ultraviolet and visible radiation) lasting 6 hours. For this UV treatment, the tires were exposed to the outside, in a static position, in a southerly orientation (sunny Mediterranean site with an average daytime temperature of 15 ^° C.), these tires being rotated about their axis by 1 ^° C. 1/3 turn every two hours, so as to completely expose their tread.

All the tires were then mounted on a passenger vehicle to be subjected to the wet braking test described in paragraph 1-3 above. The particular conditions of the test are as follows: Laguna Renault model vehicle (front and rear pressure: 2.0 bar); tested tires mounted on the front of the vehicle. The results of the braking test are reported in Table 3, in relative units, the base 100 being selected for the control tire P-I before UV aging (reminder: a value greater than 100 indicates improved performance).

It is found that only the tire of the invention P-2, and this only after exposure to UV, has a reduction in the braking distance of about 6% which is quite significant for the skilled person . The UV aging of 6 hours, on the other hand, has no effect on the performance of the control tire PI or that of the P-2, the tread of which has a very low level of carbon black, but is devoid of the protective agent. photo-oxidant.

A series of six successive brakings conducted on the same Laguna vehicle fitted with "photo-oxidized" P-2 tires demonstrated (see results in Table 4) the durability of the technical effect observed, with an average gain of 6% for each successive braking.

This behavior, which the rubber properties of the previous Trial 1 did not predict, is just as remarkable as it was unexpected. This improved adhesion could be related to an oxidized surface state of the tread, very superficial and localized, favorable to a better attachment of the tire on wet, surface condition caused by the presence of the pro-photo-oxidant agent .

This result is moreover obtained while maintaining the rolling resistance and wear resistance performances at the high levels that are now expected to be expected from rubber compositions based on reinforcing inorganic fillers such as than highly dispersible silicas. Table 1

(1) SBR (expressed in dry SBR) extended with 10% by weight (7 phr) of MES oil (in total

77 pp of extended SBR); 25% styrene, 58% 1-2 polybutadiene units and 23% trans-polybutadiene 1-4 units (Tg = -24 ° C);

(2) BR with 4.3% of 1-2; 2.7% trans; 93% cis 1-4 (Tg = -106 ⁰ C);

(3) "Zeosil 1165MP" silica from Rhodia, type "HDS" (BET and CTAB: approximately 160 m 7 g);

(4) coupling agent TESPT ("Si69" from Rhodia);

(5) bis (2,4,6-trimethylbenzoyl) -phenylphosphine oxide ("Irgacure 819" from Ciba);

(6) MES oil ("Flexon 683", Exxon Mobil company);

(7) High Tg plasticizing hydrocarbon resin (polyalphapinene "R2495" - Hercules company);

(8) N-1,3-dimethylbutyl-N-phenylparaphenylenediamine ("Santoflex 6-PPD" from Flexsys);

(9) diphenylguanidine ("Perkacit DPG" from Flexsys);

(10) carbon black N234;

(11) N-cyclohexyl-2-benzothiazyl sulfenamide ("Santocure CBS" from Flexsys). Table 2

Table 3

Performance in braking (1 ^r braking) on wet road (in relative units)

Table 4

Successive braking on wet roads (in relative units)

Claims

A tire tread comprising a rubber composition based on at least one diene elastomer, a non-black reinforcing filler, a coupling agent ensuring the bond between the diene elastomer and said filler, optionally carbon black. , said composition being characterized in that it further comprises a propox-oxidizing agent and in that its carbon black content is zero or less than 5 phr (parts by weight per hundred parts of elastomer).

2. tread according to claim 1, the pro-photo-oxidizing agent being selected from the group consisting of radical-type or cationic photoinitiators.

3. tread according to claim 2, the pro-photo-oxidizing agent being a cationic photoinitiator.

4. tread according to claim 3, the pro-photo-oxidizing agent being an onium salt.

5. Tread according to claim 2, the pro-photo-oxidizing agent being a radical photoinitiator.

6. tread according to claim 5, the radical photoinitiator being an aromatic ketone.

7. tread according to claim 6, the aromatic ketone being selected from the group consisting of benzylic ketals, benzoines, α, α-dialkoxy-acetophenones, α-hydroxy-alkylphenones, α-amino alkylphenones, acylphosphine oxides, benzophenones or thioxanthones in combination with a hydrogen donor, and mixtures of such compounds.

8. Tread according to any one of claims 1 to 7, the diene elastomer being selected from the group consisting of polybutadienes, synthetic polyisoprenes, natural rubber, butadiene copolymers, isoprene copolymers and mixtures of these elastomers.

9. A tread according to any one of claims 1 to 8, the non-black reinforcing filler being an inorganic filler.

10. Tread according to claim 9, the inorganic reinforcing filler being a siliceous or aluminous filler.

11. A tread according to any one of claims 1 to 10, the non-black reinforcing filler content being between 20 and 200 phr.

12. Tread according to claim 11, the non-black reinforcing filler content being between 30 and 150 phr.

13. Tread according to any one of claims 1 to 12, the coupling agent being a silane or an at least bifunctional polysiloxane.

14. tread according to any one of claims 1 to 13, the level of coupling agent being between 2 and 15 phr.

15. Tread according to any one of claims 1 to 14, the carbon black content being less than 3 phr.

16. Tread according to claim 15, the carbon black content being less than 2 phr.

17. Tread according to any one of claims 1 to 16, the propox oxidant agent content being between 0.1 and 10 phr.

18. tread according to claim 17, the level of pro-photo-oxidizing agent being between 0.2 and 5 phr.

19. Tread according to claim 18, the level of pro-photo-oxidizing agent being between 0.2 and 2 phr.

20. Tread according to any one of claims 1 to 19, the rubber composition further comprising a photosensitizer.

21. The tread according to claim 20, the photosensitizer being chosen from phthalein and thiazine.

22. Tread according to any one of claims 8 to 21, the diene elastomer being a butadiene-styrene copolymer (SBR).

23. Tread according to claim 22, the SBR being used in admixture with a polybutadiene.

24. Use of a tread according to any one of claims 1 to 23, for the manufacture or retreading of tires.

25. A tire comprising a tread according to any one of claims 1 to 23.

26. Process for preparing a tire tread having, after photooxidation of its surface, an improved adhesion on a wet road, characterized in that at least one non-black filler is mixed with at least one diene elastomer. as reinforcing filler, a coupling agent providing the bond between the diene elastomer and said filler, optionally carbon black at a content of less than 5 phr, a pro-oxidizing agent, a crosslinking system, and it is extruded in the form of a tread.

27. The method of claim 26, comprising the following steps:

to incorporate in the diene elastomer, in a mixer:

. the non-black reinforcing filler; • the coupling agent;

• the optional carbon black at a rate of less than 5 phr;

. the pro-photo-oxidant agent; thermomechanically kneading the whole, in one or more times, until reaching a maximum temperature of between 130 ° C and 200 ° C; - Cool all at a temperature below 100 ⁰ C; then incorporate: the crosslinking system; knead everything to a maximum temperature below 120 ⁰ C; extruding or calendering the rubber composition thus obtained, in the form of the tire tread.

28. The method according to claim 26 or 27, the pro-photo-oxidizing agent being chosen from the group consisting of radical-type or cationic photoinitiators.

29. The method of claim 28, the pro-photo-oxidizing agent being a radical photoinitiator.

30. The method of claim 29, the radical photoinitiator being an aromatic ketone.

31. The method of claim 30, the aromatic ketone being selected from the group consisting of benzylic ketals, benzoids, α, α-dialkoxy-acetophenones, α-hydroxy-alkylphenones, α-amino alkylphenones, oxides of acylphosphines, benzophenones or thioxanthones in combination with a hydrogen donor, and mixtures of such compounds.

32. Process according to any one of claims 26 to 31, the diene elastomer being chosen from the group consisting of polybutadienes, synthetic polyisoprenes, natural rubber, butadiene copolymers, isoprene copolymers and these elastomers.

33. Process according to any one of claims 26 to 32, the non-black reinforcing filler being an inorganic filler.

34. Process according to any one of claims 26 to 33, the carbon black content being less than 3 phr.

35. The method of claim 34, the carbon black content being less than 2 phr.

36. Process according to any one of claims 26 to 35, the level of pro-oxidizing agent being between 0.1 and 10 phr.

37. The method of claim 36, the level of pro-photo-oxidizing agent being between 0.2 and 5 phr.

38. The method of claim 37, the level of pro-photo-oxidizing agent being between 0.2 and 2 phr.

39. Method according to any one of claims 26 to 38, characterized in that it is completed by a step of photo-oxidation of the surface of the tread.

40. The method of claim 39, the photo-oxidation step consisting of accelerated aging.

41. The method of claim 40, the accelerated aging being conducted using one or more lamps emitting in the UV-visible.