FR2979076A1 - TIRE FOR VEHICLE WITH TREAD BAND COMPRISING THERMO-EXPANDABLE RUBBER COMPOSITION - Google Patents

Abstract

A tire for a vehicle, particularly a winter tire, the tread of which comprises a vulcanized expansible heat-expandable rubber composition; said composition comprises a diene elastomer such as natural rubber and / or polybutadiene, more than 50 phr of a reinforcing filler such as silica and / or carbon black, more than 25 phr of a liquid plasticizer at 23 ° C. and, as blowing agent, between 2 and 15 phr of a sulphonyl hydrazide compound such as, for example, 4,4'-oxybis (benzene sulphonyl hydrazide). The use of such a composition makes it possible to greatly improve the adhesion on melting ice of the tire.

Description

FIELD OF THE INVENTION The invention relates to rubber compositions used as treads of tires for vehicles, in particular tires "winter" able to roll on floors covered with ice or ice without being provided with nails (also called "studless" tires). It is more particularly relative to treads of winter tires specifically adapted for running under so-called "melting ice" conditions encountered in a temperature range typically between -5 ° C and 0 ° C. It is recalled that, in such a field, the pressure of the tires at the passage of a vehicle causes a superficial melting of the ice which is covered with a thin film of water detrimental to the adhesion of these tires. 2. STATE OF THE ART In order to avoid the harmful effects of nails, in particular their strong abrasive action on the floor covering itself and a markedly degraded road behavior on dry ground, the tire manufacturers have proposed various solutions of modifying the formulation. rubber compositions themselves. Thus, it has been proposed first of all to incorporate solid particles of high hardness, such as, for example, silicon carbide (see, for example, US Pat. No. 3,878,147), some of which are flush with the surface of the tread. as it wears, and thus come into contact with the ice. Such particles, able to act finally as micro-nails on hard ice, thanks to a well known "claw" effect, remain relatively aggressive vis-à-vis the ground; they are not well adapted to driving conditions on melting ice.

Other solutions have therefore been proposed, consisting in particular of incorporating water-soluble powders into the constitutive composition of the tread. Such powders solubilize more or less in contact with snow or melted ice, which allows on the one hand the creation on the surface of the tread of porosities likely to improve the attachment of the tread on the ground and on the other hand the creation of grooves acting as evacuation channels of the liquid film created between the tire and the ground. Examples of such water-soluble powders are, for example, the use of cellulose powder, vinyl alcohol or starch, or guar gum or gum powders. xanthan (see, for example, patent applications JP 3-159803, JP 2002-211203, EP 940,435, WO 2008/080750, WO 2008/080751).

It has also been proposed to use powder particles which are neither of high hardness nor water-soluble, still capable of generating an effective surface micro-roughness (see in particular patent applications WO 2009/083125 and WO 2009/112220).

Finally, to improve the ice adhesion performance of a tread, it is also well known to use a layer of diene elastomer foam rubber, a blowing agent ("blowing agent"). ) and various other additives such as in particular an expansion promoter. These expansion agents, such as, for example, nitro, sulfonyl or azo compounds, are capable of liberating, during a thermal activation, for example during the vulcanization of the tire, a large amount of gas, in particular nitrogen, and thus lead to the formation of bubbles within a sufficiently soft material such as a rubber composition comprising such expansion agents. Such winter tire foam rubber formulations have been described, for example, in patent documents JP 2003-183434, JP 2004-091747, JP 2006-299031, JP 2007-039499, JP 2007-314683, JP 2008-001826, JP 2008. -150413, EP 826,522, US 5,147,477, US 6,336,487, WO 2011/064128. 3. BRIEF DESCRIPTION OF THE INVENTION In their research on the above foam rubber technology, the Applicants have found a specific formulation based on a particular blowing agent and a rate of blowing. high liquid plasticizer, which greatly improves the adhesion on melting ice treads.

Accordingly, according to a first object, the present invention relates to a tire whose tread comprises, in the uncured state, a heat-expandable rubber composition, comprising at least one diene elastomer, more than 50 phr of a reinforcing filler, more than 25 phr of a liquid plasticizer at 23 ° C and, as blowing agent, between 2 and 15 phr of a sulfonyl hydrazide compound. The invention also relates to a tire in the vulcanized state obtained after firing (vulcanization) of the green tire according to the invention as described above. P10-2686 - 3 - The tires of the invention are particularly intended to equip tourism-type motor vehicles, including 4x4 vehicles (four-wheel drive) and SUV vehicles ("Sport Utility Vehicles"), two-wheeled vehicles. (especially motorcycles) as industrial vehicles chosen in particular from vans and "heavy goods vehicles" (eg metro, buses, road transport vehicles such as trucks, tractors). The invention as well as its advantages will be readily understood in the light of the description and the following exemplary embodiments. 4. DETAILED DESCRIPTION OF THE INVENTION In the present description, unless expressly indicated otherwise, all the percentages (%) indicated are% by weight. The abbreviation "pce" means parts by weight per hundred parts of elastomer (of the total elastomers if several elastomers are present). On the other hand, any range of values designated by the expression "between a and b" represents the range of values greater than "a" and less than "b" (i.e., terminals a and b excluded). while any range of values designated by the term "from a to b" means the range of values from "a" to "b" (i.e. including the strict limits a and b). The tire of the invention therefore has the essential characteristic that its tread in the uncured state, at least for the portion (thus radially outermost) of this tread which comes directly into contact with the surface of the tread. the road comprises a heat-expandable rubber composition comprising at least one (at least one) diene elastomer; more than 50 phr of one (at least one) reinforcing filler; more than 25 phr of a (at least one) liquid plasticizer (at 23 ° C); As an expanding agent, between 2 and 15 phr of a (at least one) sulfonyl hydrazide compound. The various components above are described in detail below. 4.1. Diene Elastomer By elastomer (or rubber, the two terms being synonymous) of the "diene" type, it is recalled that must be understood an elastomer derived at least in part (ie a homopolymer or a copolymer) from monomers dienes (monomers carrying two double carbon-carbon bonds, conjugated or otherwise). The diene elastomers can be classified in known manner into two categories: those known as "essentially unsaturated" and those known as "essentially saturated". Butyl rubbers, as well as, for example, copolymers of dienes and alpha-olefins of the EPDM type, fall into the category of essentially saturated diene elastomers, having a level of diene origin units which is low or very low, always less than 15% (mole%). By contrast, essentially unsaturated diene elastomer is understood to mean a diene elastomer derived at least in part from conjugated diene monomers having a proportion of units or units of diene origin (conjugated dienes) which is greater than 15% (mol%). . 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%. It is preferred to use at least one diene elastomer of the highly unsaturated type, in particular a diene elastomer chosen from the group consisting of natural rubber (NR), synthetic polyisoprenes (IR), polybutadienes (BR) and butadiene copolymers, copolymers of isoprene 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 elastomers can be for example block, statistical, sequenced, microsequenced, and be prepared in dispersion or in solution; they may be coupled and / or starred or functionalized with a coupling agent and / or starring or functionalization. For coupling with carbon black, there may be mentioned for example functional groups comprising a C-Sn bond or amino functional groups such as benzophenone for example; for coupling to a reinforcing inorganic filler such as silica, mention may be made, for example, of silanol or polysiloxane functional groups having a silanol end (as described, for example, in US Pat. No. 6,013,718), alkoxysilane groups (as described, for example, in US 5,977,238), carboxylic groups (as described, for example, in US 6,815,473 or US 2006/0089445) or polyether groups (as described for example in US 6,503,973). By way of other examples of such functionalized elastomers, mention may also be made of elastomers (such as SBR, BR, NR or IR) of the epoxidized type.

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 having 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-trans-1,4 content between 20% and 80%, butadiene-isoprene copolymers and in particular those having an isoprene content of between 5% and 90% by weight and a transition temperature glass ("Tg" - measured according to ASTM D3418-82) from -40 ° C to -80 ° C, isoprene-styrene copolymers and especially those having a styrene content of between 5% and 50% by weight and a Tg included between -25 ° C and -50 ° C. In the case of butadiene-styrene-isoprene copolymers are especially suitable those having a styrene content of between 5% and 50% by weight and more particularly of between 10% and 40%, an isoprene content of between 15% and 60%. by weight and more particularly between 20% and 50%, a butadiene content of between 5% and 50% by weight and more particularly between 20% and 40%, a content of -1,2 units of the butadiene part between 4% and 85%, a content of trans-1,4 units of the butadiene part of between 6% and 80%, a content of -1,2 units plus -3,4 of the isoprenic part of between 5% and 70% and a trans-1,4-unit content of the isoprene portion of between 10% and 50%, and more generally any butadiene-styrene-isoprene copolymer having a Tg of between -20 ° C and -70 ° C. According to a particularly preferred embodiment of the invention, the diene elastomer is chosen from the group consisting of natural rubber, synthetic polyisoprenes and polybutadienes having a cis-1,4 bond ratio of greater than 90%, copolymers of butadiene-styrene and mixtures of these elastomers. According to a more particular and preferred embodiment, the heat-expandable rubber composition comprises 50 to 100 phr of natural rubber or synthetic polyisoprene, said synthetic rubber or synthetic polyisoprene can be used in particular in blending (mixing) with at most 50 phr of a polybutadiene having a cis-1,4 bond ratio greater than 90%. According to another particular and preferred embodiment, the heat-expandable rubber composition comprises 50 to 100 phr of a polybutadiene having a cis-1,4 bond ratio of greater than 90%, said polybutadiene being able to be used in particular in cutting with not more than 50 phr of natural rubber or synthetic polyisoprene. The above diene elastomers could be associated, in a minor amount, with synthetic elastomers other than dienes, or even polymers other than elastomers, for example thermoplastic polymers. 4.2. Load P10-2686 - 6 - Any load known for its ability to reinforce (ie increase its modulus in tension) a rubber composition is usable, for example an organic load such as carbon black, a load inorganic such as silica to which is associated in a known manner a coupling agent, or a mixture of these two types of filler.

Such a reinforcing filler preferably consists of nanoparticles whose average size (in mass) is less than one micrometer, generally less than 500 nm, most often between 20 and 200 nm, in particular and more preferably between 20 and 150 min, this mass average size being measured in a manner well known to those skilled in the art (for example, according to the application WO 2009/083160). Preferably, the content of total reinforcing filler (in particular silica or carbon black or a mixture of silica and carbon black) is between 50 and 150 phr. A content equal to or greater than 50 phr is favorable for good mechanical strength; beyond 150 phr, there is a risk of excessive rigidity of the rubber layer. For these reasons, the total reinforcing filler content is more preferably within a range of 70 to 120 phr. Suitable carbon blacks are, for example, all carbon blacks which are conventionally used in tires (so-called pneumatic grade blacks) such as blacks of the 100, 200, 300 series (ASTM grades), for example blacks N115, N134. , N234, N326, N330, N339, N347, N375. The carbon blacks could for example already be incorporated into the diene elastomer, in particular isoprenic elastomer, in the form of a masterbatch (see, for example, applications WO 97/36724 or WO 99/16600).

As examples of organic fillers other than carbon blacks, mention may be made of functionalized polyvinyl organic fillers as described in applications WO-A2006 / 069792, WO-A-2006/069793, WO-A-2008/003434 and WO -A-2008/003435.

"Reinforcing inorganic filler" means any inorganic or mineral filler, irrespective of its color and origin (natural or synthetic), also called "white" filler, "clear" filler or even "non-black filler" "As opposed to carbon black, capable of reinforcing on its own, with no other means than an intermediate coupling agent, a rubber composition for the manufacture of tires, in other words able to replace, in its function of reinforcement, a conventional carbon black of pneumatic grade; such a filler is generally characterized, in known manner, by the presence of hydroxyl groups (-OH) on its surface. Suitable reinforcing inorganic fillers are mineral fillers of the siliceous type, in particular silica (SiO 2). The silica used may be any reinforcing silica known to those skilled in the art, especially 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 m2 / g, especially between 60 and 300 m2 / g. As highly dispersible precipitated silicas (called "HDS"), mention may be made, for example, of the "Ultrasil" 7000 and "Ultrasil" silicones 7005 from the Evonik company, the "Zeosil" 1165MP, 1135MP and 1115MP silicas from the Rhodia company, the silica "Hi-Sil" EZ150G from the company PPG, the silicas "Zeopol" 8715, 8745 and 8755 of the Huber Company. According to another particularly preferred embodiment, the majority filler used is a reinforcing inorganic filler, in particular silica, at a level within a range of 70 to 120 phr, reinforcing inorganic filler to which advantageously black of carbon at a minority rate at most equal to 15 phr, in particular in a range of 1 to 10 phr.

In order to couple the reinforcing inorganic filler to the diene elastomer, an at least bifunctional coupling agent (or bonding agent) is used in a well-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 at least bifunctional polyorganosiloxanes are used.

In particular, polysulfide silanes, called "symmetrical" or "asymmetrical" silanes according to their particular structure, are used, as described, for example, in claims WO 03/002648 (or US 2005/016651) and WO 00/002649 (or US 2005/016650).

Particularly suitable, but not limited to, polysulfide silanes having the following general formula (I): (I) Z - A - Sx - A - Z, wherein: - x is an integer of 2 at 8 (preferably from 2 to 5); the symbols A, which may be identical or different, represent a divalent hydrocarbon radical (preferably a C1-C18 alkylene group or a C6-C12 arylene group, more particularly a C1-C10 alkylene, especially a C1-C4 alkylene, in particular propylene); the symbols Z, which may be identical or different, correspond to one of the following three formulas: ## STR1 ## in which: ## STR1 ## in which where: - the radicals R1, substituted or unsubstituted, identical or different from each other, represent a C1-C18 alkyl, C5-C18 cycloalkyl or C6-C18 aryl (preferably C1-C6 alkyl, cyclohexyl) groups; or phenyl, especially C1-C4 alkyl groups, more particularly methyl and / or ethyl). the radicals R2, substituted or unsubstituted, which are identical to or different from one another, represent a C1-C18 alkoxyl or C5-C18 cycloalkoxyl group (preferably a group chosen from C1-C8 alkoxyls and C5-C8 cycloalkoxyls, plus still more preferably a group selected from C1-C4 alkoxyls, in particular methoxyl and ethoxyl). In the case of a mixture of polysulfurized alkoxysilanes corresponding to the formula (I) above, in particular common commercially available mixtures, the average value of "x" is a fractional number preferably between 2 and 5, more preferably close to 4. But the invention can also be advantageously used for example with disulfide alkoxysilanes (x = 2). By way of examples of polysulphide silanes, mention may be made more particularly of bis (C 1 -C 4 alkoxy-C 1 -C 4 alkylsilyl-C 1 -C 4 alkyl) polysulfides (especially disulfides, trisulphides or tetrasulfides), as for example polysulfides of bis (3-trimethoxysilylpropyl) or bis (3-triethoxysilylpropyl). Among these compounds, bis (3-triethoxysilylpropyl) tetrasulfide, abbreviated TESPT, of formula [(C2H50) 3Si (CH2) 3S1 or bis (triethoxysilylpropyl) disulfide, abbreviated TESPD, of formula (C2H50) 3Si (CH2) 3SL. Mention may also be made, by way of preferred examples, of polysulfides (in particular disulphides, trisulphides or tetrasulfides) of bis- (C 1 -C 4 monoalkoxyl) -dialkyl (C 1 -C 4) silylpropyl), more particularly bis-monoethoxydimethylsilylpropyl tetrasulfide, as described above. in the aforementioned patent application WO 02/083782 (or US 7,217,751). By way of example of coupling agents other than a polysulphurized alkoxysilane, mention may be made in particular of bifunctional POS (polyorganosiloxanes) or hydroxysilane polysulfides (R 2 = OH in formula I above) as described, for example in patent applications WO 02/30939 (or US Pat. No. 6,774,255), WO 02/31041 (or US 2004/051210), and WO 2007/061550, or else silanes or POS bearing functional azo-dicarbonyl groups, such as as described for example in patent applications WO 2006/125532, WO 2006/125533, WO 2006/125534. As examples of other sulphurized silanes, mention may be made, for example, of silanes carrying at least one thiol function (-SH) (called mercaptosilanes) and / or of at least one blocked thiol function, as described for example in patents or patent applications US 6,849,754, WO 99/09036, WO 2006/023815, WO 2007/098080. Of course, it would also be possible to use mixtures of the coupling agents described above, as described in particular in the aforementioned application WO 2006/125534.

When they are reinforced with an inorganic filler such as silica, the rubber compositions preferably comprise between 2 and 15 phr, more preferably between 3 and 12 phr of coupling agent. Those skilled in the art will understand that, as the 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 an organic filler, since this reinforcing filler would be covered with a filler. inorganic layer such as silica, or would comprise on its surface functional sites, especially hydroxyl, requiring the use of a coupling agent to establish the bond between the filler and the elastomer. 4.3. Liquid Plasticizer The thermo-expandable rubber composition also comprises a liquid plasticizer whose function is to soften the matrix by diluting the diene elastomer and the reinforcing filler; its Tg (glass transition temperature, measured according to ASTM D3418-1999) is preferably below -20 ° C, more preferably below -40 ° C. For optimum performance of the tread of the tire of the invention, this liquid plasticizer is used at a level greater than 25 phr (especially between 25 and 60 phr), preferably greater than 30 phr (in particular between 30 and 50 phr). phr).

Preferably, the weight ratio reinforcing filler on liquid plasticizer is less than 3.0, more preferably less than 2.5. Any extender oil, whether aromatic or non-aromatic, any liquid plasticizer known for its plasticizing properties vis-à-vis diene elastomers, is usable. At room temperature (23 ° C.), these plasticizers or these oils, more or less viscous, are liquids (that is to say, as a reminder, substances having the capacity to eventually take on the shape of their container) , in contrast to, in particular, hydrocarbon plasticizing resins which are inherently solid at room temperature.

According to a particular embodiment of the invention, the liquid plasticizer is in particular a petroleum oil, preferably a non-aromatic oil. A liquid plasticizer is described as non-aromatic if it has a content of polycyclic aromatic compounds, determined with the extract in DMSO according to the IP 346 method, of less than 3% by weight, relative to the total weight of the plasticizer. . P10-2686 - 1 0 - Particularly suitable liquid plasticizers selected from the group consisting of naphthenic oils (low or high viscosity, including hydrogenated or not), paraffinic oils, oils MES (Medium Extracted Solvates), oils DAE (Distillate Aromatic Extracts), Treated Distillate Aromatic Extracts (TDAE) oils, Residual Aromatic Extract oils (RAE), Treated Residual Aromatic Extract (TREE) oils, Safety Residual Aromatic Extract oils (SRAE), mineral oils , vegetable oils, ethers plasticizers, ester plasticizers, phosphate plasticizers, sulphonate plasticizers and mixtures of these compounds. According to a more preferred embodiment, the liquid plasticizer is selected from the group consisting of MES oils, TDAE oils, naphthenic oils, vegetable oils and mixtures of these oils. As phosphate plasticizers, for example, mention may be made of those containing from 12 to 30 carbon atoms, for example trioctyl phosphate. By way of examples of ester plasticizers, mention may be made in particular of compounds selected from the group consisting of trimellitates, pyromellitates, phthalates, 1,2-cyclohexane dicarboxylates, adipates, azelates, sebacates, tri-esters and glycerol and mixtures of these compounds. Among the triesters above, mention may be made in particular of glycerol triesters, preferably consisting mainly (for more than 50%, more preferably for more than 80% by weight) of a C18 unsaturated fatty acid, that is, that is to say chosen from the group consisting of oleic acid, linoleic acid, linolenic acid and mixtures of these acids. More preferably, whether of synthetic or natural origin (for example vegetable oils of sunflower or rapeseed), the fatty acid used is more than 50% by weight, more preferably still more than 80% by weight. % by weight of oleic acid. Such high oleic acid triesters (trioleates) are well known and have been described, for example, in application WO 02/088238, as plasticizers in tire treads. 4.4. Expansion agent In known manner, a blowing agent ("blowing agent" in English) is a thermally decomposable compound, intended to release during a thermal activation, for example during the vulcanization of the tire, an amount of important gas and thus lead to the formation of bubbles. The release of gas in the rubber composition therefore comes from this thermal decomposition of the blowing agent; in most cases, the gas formed is nitrogen. There are physical or chemical expansion agents, of the endothermic or exothermic type; chemical expansion agents, of the exothermic type, are most often used. P10-2686 In the context of the present invention, the blowing agent used is a compound of the sulfonyl hydrazide type. Among the sulphonyl hydrazide compounds, mention may be made preferably of those selected from the group consisting of benzene sulphonyl hydrazide, toluene sulphonyl hydrazide, 4,4'-oxybis (benzene sulphonyl) hydrazide, and mixtures thereof. More preferably the blowing agent used is 4,4'-oxy-bis- (benzene sulfonyl) hydrazide (abbreviated as "OBSH"). The amount of this blowing agent is between 2 and 15 phr. If it is less than 2 phr, the expansion appears to be insufficient whereas if it is greater than 15 phr, the expansion proved to be too important for obtaining mechanical properties (notably modules and compressibility) compatible with the requirements of use in tread. For all these reasons, the amount of blowing agent is preferably between 3 and 12 phr, more preferably between 4 and 10 phr. 4.5. Various additives The heat-expandable rubber composition may also comprise all or part of the usual additives normally used in tire tread rubber compositions, such as, for example, protective agents such as antiozone waxes, chemical antiozonants, oxidizing agents, plasticizing agents other than the liquid plasticizer previously described, a crosslinking system based on either sulfur, or sulfur and / or peroxide and / or bismaleimide donors, vulcanization accelerators, vulcanization activators, vulcanization retarders.

In particular, it is advantageous to use hydrocarbon plasticizing resins with a high Tg, preferably greater than 20 ° C., more preferably greater than 30 ° C. (measured according to ASTM D3418-1999), in order to complete the plasticizing effect of the liquid plasticizer.

Hydrocarbon resins (it is recalled that the term "resin" is reserved by definition for a solid compound at 23 ° C.) are polymers well known to those skilled in the art, which can be used in particular as plasticizers or tackifying agents in matrices. polymer. They can be aliphatic, aromatic, aliphatic / aromatic type that is to say based on aliphatic and / or aromatic monomers, hydrogenated or not. They may be natural or synthetic, whether based on petroleum or not (if so, also known as petroleum resins). They are preferably exclusively hydrocarbon-based, that is to say they contain only carbon and hydrogen atoms. Preferably, their number-average molecular weight (Mn) is between 400 and 2000 g / mol, in particular between 500 and 1500 g / mol; their polymolecularity index (Ip) is preferentially less than 3, especially less than 2 (booster: Ip = Mw / Mn with Mw weight average molecular weight). The macrostructure (Mw, Mn and Ip) of the hydrocarbon resin is determined by steric exclusion chromatography ("SEC"): solvent tetrahydrofuran; temperature 35 ° C; concentration 1 g / 1; flow rate 1 ml / min; filtered solution on 0.45 μm porosity filter before injection; Moore calibration with polystyrene standards; set of 3 "WATERS" columns in series ("STYRAGEL" HR4E, HR1 and HR0.5); differential refractometer detection ("WATERS 2410") and its associated operating software ("WATERS EMPOWER").

By way of examples of above-mentioned hydrocarbon plasticizing resins, mention may be made in particular of homopolymer resins or copolymers of cyclopentadiene or dicyclopentadiene, resins of homopolymers or terpene copolymers (eg alpha-pinene, beta-pinene, dipentene or polylimonene) , homopolymer resins or copolymers of C5 or C9 cut, for example of C5 / styrene cut copolymer or of C5 / C9 cut copolymer.

The content of hydrocarbon resin is preferably between 2 and 50 phr, in particular between 3 and 30 phr, more preferably still in a range of 5 to 20 phr. In the case where it is desired to increase the rigidity of the tread once expanded, without reducing the content of liquid plasticizer above, it will be advantageous to incorporate reinforcing resins (eg acceptors and donors of methylene) such as described for example in WO 02/10269 or US 7,199,175. The heat-expandable rubber composition may also contain coupling enhancers when a coupling agent is used, inorganic filler recovery agents when an inorganic filler is used, or more generally, bleaching agents. implementation (processability) likely in a known manner, through an improvement in the dispersion of the load in the rubber matrix and a lowering of the viscosity of the compositions, to improve their processability in the green state; these agents are for example hydroxysilanes or hydrolysable silanes such as alkyl-alkoxysilanes, polyols, polyethers, amines, hydroxylated or hydrolysable polyorganosiloxanes. 4.6. Manufacture of the Compositions The rubber compositions are manufactured in appropriate mixers, for example using two successive preparation phases according to a general procedure known to those skilled in the art: a first thermomechanical working or mixing phase (sometimes referred to as a "no" phase). -productive ") at high temperature, up to a maximum temperature of between 130 ° C and 200 ° C, preferably between 145 ° C and 185 ° C. Finally, in a second phase of mechanical work (sometimes called a "productive" phase) at low temperature, typically less than 120 ° C, for example between 60 ° C and 100 ° C, finishing phase during of which are incorporated the blowing agent and the crosslinking or vulcanization system. A process which can be used for the manufacture of such rubber compositions comprises, for example, and preferably the following steps: - incorporating in a mixer, at the elastomer or in the mixture of elastomers, at least the charge by thermomechanically kneading the whole, in a or several times, until reaching a maximum temperature of between 130 ° C and 200 ° C; - cool all at a temperature below 100 ° C; - Then incorporate the blowing agent to the mixture thus obtained and cooled, kneading thermomechanically all until a maximum temperature below 100 ° C; - Then incorporate a crosslinking system; - mix everything up to a maximum temperature below 100 ° C; - Extrude or calender the rubber composition thus obtained, especially in the form of a heat-expandable tread. By way of example, during the first non-productive phase, all the necessary constituents, any additional coating or processing agents and other various additives, are introduced into a suitable mixer such as a conventional internal mixer. with the exception of the blowing agent and the crosslinking system. After thermomechanical work, fall and cooling of the mixture thus obtained, then incorporating the blowing agent and then the low temperature crosslinking system, usually in an external mixer such as a roller mixer; the whole is then mixed (productive phase) for a few minutes, for example between 5 and 15 min. 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 (in particular diphenylguanidine), or else vulcanization retarders. The sulfur content is preferably between 0.5 and 5 phr, that of the primary accelerator is preferably between 0.5 and 8 phr. As accelerator (primary or secondary) can be used any compound capable of acting as an accelerator of vulcanization of diene elastomers in the presence of sulfur, in particular thiazole-type accelerators and their derivatives, accelerators of thiuram type, zinc dithiocarbamates. These accelerators are for example selected from the group consisting of 2-mercaptobenzothiazyl disulfide (abbreviated "MBTS"), tetrabenzylthiuram disulfide ("TBZTD"), N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"). N, Ndicyclohexyl-2-benzothiazylsulfenamide ("DCBS"), N-tert-butyl-2-benzothiazylsulfenamide ("TBBS"), N-tert-butyl-2-benzothiazylsulfenimide ("TBSI"), dibenzyldithiocarbamate zinc ("ZBEC") and mixtures of these compounds. It is possible to use as a vulcanization retarder (preferably at a level of between 0.5 and 10 phr, more preferably between 1 and 5 phr), any compound capable of increasing the induction delay (that is to say, if necessary). say the time required at the start of the vulcanization reaction) during the baking of the composition, and thus to provide the rubber composition with the time necessary for complete expansion prior to vulcanization. By way of examples, mention may be made of the following compounds marketed by the company Lanxess: N-cyclohexylthiophthalimide under the name "Vulkalent G", N (trichloromethylthio) benzenesulfonamide under the name "Vulkalent E / C", or else phthalic anhydride under the name "Vulkalent B / C"; N-cyclohexylthiophthalimide (abbreviated as "CTP") is preferably used. 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 calendered or extruded in the form of a heat-expandable tread. In the green (that is to say unvulcanized) and therefore unexpanded state, the density or density denoted D1 of the heat-expandable rubber composition is preferably between 1,100 and 1,400 g / cm 3, more preferably within a range of 1,150 to 1,350 g / cm3. 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. It is during this vulcanization step that the blowing agent will release a significant amount of gas, lead to bubble formation in the foam rubber composition and eventually expand. In the cured (vulcanized) state, the rubber composition once expanded (i.e., in the foam rubber state) has a density denoted D2 which is preferably between 0.650 and 1,000 g / cm3. more preferably in a range of 0.680 to 0.960 g / cm3. Its volume expansion ratio TE (expressed in%) is preferably between 20% and 75%, more preferably in a range of 25 to 50%, this TE expansion ratio being calculated from known manner from densities D1 and D2 above, as follows: TE = [(D1 / D2) - 1] X 100. 5. EXAMPLES OF EMBODIMENT OF THE INVENTION The thermosorbable rubber composition described above is advantageously can be used in winter tire treads for any type of vehicle, particularly in passenger car tires, as demonstrated in the following tests.

For the purposes of these tests, two rubber compositions (denoted C-0 and C-1) were prepared whose formulation is given in Table 1 (rate of the different products expressed in phr). The composition C-0 is the control composition; the composition C-1 is that according to the invention, it additionally comprises the blowing agent. The liquid plasticizer was adjusted (reduced by one-third) in composition C-1 in order to maintain rigidity after curing to a level close to that of control composition C-0 (Shore A hardness equal to about 53 ± 2, measured according to ASTM D 2240-86). For the manufacture of these compositions, the following procedure was carried out: the reinforcing filler, the diene elastomer (NR and BR), as well as the various other ingredients with the exception of the vulcanization system and the blowing agent (OBSH) for the composition C-1; the mixer was thus filled to about 70% (% by volume). Thermomechanical work (non-productive phase) was then carried out in a step of about 2 to 4 minutes, until a maximum "falling" temperature of about 150 ° C. was reached. The mixture thus obtained was recovered, cooled to about 50 ° C and then the blowing agent, sulfenamide accelerator and sulfur were incorporated on an external mixer (homo-finisher) at 30 ° C. mixing everything (productive phase) for a few minutes. The compositions C-0 and C-1 thus prepared were then used as treads for winter tire tires with radial carcass, denoted respectively P-0 (control tires) and P-1 (tires according to the invention), of dimensions 205/65 R16, conventionally manufactured and identical in all respects, except the rubber compositions constitutive of their tread. Table 2 shows the properties measured before and after baking: the tread of the tire according to the invention has, after baking, once in the state of foam rubber (ie, expanded), a density significantly reduced corresponding to a particularly high volume expansion rate of about 35%.

The P-0 and P-1 tires are then mounted, under nominal inflation pressure, to the front and rear of a motor vehicle ("Honda Civic") equipped with an anti-lock braking system (ABS system). ) and an anti-slip acceleration system (TCS system for Traction Control System). The distance necessary to increase from 20 to 5 km / h is measured during a brutal longitudinal braking (activated ABS) on an ice-covered runway maintained at a temperature of -2 ° C (so-called "melting ice" conditions), as well as on an ice-covered runway maintained at a temperature of -6 ° C (so-called "cold ice" conditions). After this first series of tests on new tires, these tires are run on a circuit of about 10,000 km, on dry ground, for early wear (break-in). Then the partially worn tires are again subjected to the ice adhesion tests as described above. The overall results of the rolling tests are reported in Table 3, in relative units, the base 100 being selected for the P-0 control tires. A value greater than that of the control, arbitrarily set at 100, indicates an improved result, that is to say a shorter braking distance. It is noted that if an improvement (5%) is already observed on a new tire, the braking distance on melting ice is increased remarkably and unexpectedly by more than 40% compared to the control tires, after a rolling of 10,000 km. . It is further noted that, after running in, the cold ice adhesion is also improved (by 20%) on the tires of the invention.

This result illustrates very well the ability of the foam rubber composition, once vulcanized (expanded), to generate throughout the use of the tire of the invention, a particularly effective and significant surface microroughness.

Table 1 Composition No .: C-0 C-1 BR (1) 60 60 NR (2) 40 40 silica (3) 80 80 coupling agent (4) 6.4 6.4 carbon black (5) Expansion agent (6) liquid plasticizer (7) 60 40 plasticizing resin (8) 10 10 DPG (9) 1.5 1.5 ZnO 1.2 1.2 stearic acid 1.0 1.0 anti-ozone wax 1.5 1.5 antioxidant (10) 2.0 2.0 sulfur 2.0 2.0 accelerator (11) 1.7 1.7 (1) polybutadiene with 0.3% of 1-2; 2.7% trans; 97% cis 1-4 (Tg = -104 ° C); (2) natural rubber (peptized); (3) silica "Ultrasil 7000" from Evonik, type "HDS" (BET and CTAB: about 160 m2 / g); (4) TESPT coupling agent ("Si69" from Evonik); (5) ASTM grade N234 (Cabot Company); (6) 4,4'-oxybis (benzenesulfonylhydrazide) (Cellmic S, Sankyo Kasei Company); (7) MES oil ("Catenex SNR" from Shell); (8) C5 / C9 resin ("Escorez ECR-373" from Exxon Mobil) (9) diphenylguanidine ("Perkacit DPG" from Flexsys); (10) N-1,3-dimethylbutyl-N-phenylparaphenylenediamine ("Santoflex 6-PPD" from Flexsys); (11) N-tert-butyl-2-benzothiazol sulfenamide ("Santocure CBS" from Flexsys).

Table 2 Composition: C-0 C-1 Shore A hardness 55 51 Pre-bake tire density 1.14 1.21 Post-bake tire density 1.14 0.82 Volume expansion ratio (%) 0 35 15 Table 3 Pneumatics: P-0 P-1 Ice braking (-2 ° C) (1) 100 105 Ice braking (-2 ° C) (2) 100 143 Ice braking (-6 ° C) (1) 100 100 Ice braking (-6 ° C) (2) 100 120 (1) tire in new condition (2) tire after break-in P10-2686

Claims (15)

REVENDICATIONS1. A tire whose tread comprises, in the unvulcanized state, a heat-expandable rubber composition comprising at least one diene elastomer, more than 50 phr of a reinforcing filler, more than 25 phr of a liquid plasticizing agent. 23 ° C and, as blowing agent, between 2 and 15 phr of a sulfonyl hydrazide compound. The tire of claim 1, wherein the diene elastomer is selected from the group consisting of natural rubber, synthetic polyisoprenes, polybutadienes, butadiene copolymers, isoprene copolymers and mixtures of these elastomers. 3. A tire according to claim 2, wherein said rubber composition comprises 50 to 100 phr of natural rubber or synthetic polyisoprene. 4. A tire according to claim 3, wherein the synthetic rubber or the synthetic polyisoprene is used in a blend with at most 50 phr of a polybutadiene having a cis-1,4 bond ratio greater than 90%. 5. A tire according to claim 2, wherein said composition comprises 50 to 100 phr of a polybutadiene having a cis-1,4 bond ratio greater than 90%. 6. A tire according to claim 5, wherein the polybutadiene is used in a blend with at most 50 phr of natural rubber or synthetic polyisoprene. A tire according to any one of claims 1 to 6, wherein the reinforcing filler comprises an inorganic filler, carbon black or a mixture of inorganic filler and carbon black. 8. A tire according to any one of claims 1 to 7, wherein the reinforcing filler content is between 50 and 150 phr, preferably within a range of 70 to 120 phr. 9. A tire according to any one of claims 1 to 8, wherein the weight ratio reinforcing filler on liquid plasticizer is less than 3.0, preferably less than 2.5. P10-2686- 20 - 10. A tire according to any one of claims 1 to 9, wherein the level of sulfonyl hydrazide compound is between 3 and 12 phr, preferably between 4 and 10 phr. Tire according to any of claims 1 to 10, wherein the sulfonyl hydrazide compound is 4,4'-oxybis (benzenesulfonylhydrazide). Tire according to any one of claims 1 to 11, wherein the density of the heat-expandable rubber composition is between 1,100 and 1,400 g / cm3, preferably in the range of 1,150 to 1,350 g / cm3. 13. Vulcanized tire, obtained after firing a tire according to any one of claims 1 to 12. Tire according to claim 13, wherein the density of the expanded rubber composition is between 0.650 and 1,000 g / cm3, preferably in the range of 0.680 to 0.960 g / cm3. A tire according to claims 13 or 14, wherein the volume expansion ratio of the expanded rubber composition is between 20 and 75%, preferably in a range of 25 to 50%. P10-2686