WO2015014576A1 - Heat-expandable rubber composition and tyre comprising such a composition - Google Patents

Abstract

Heat-expandable rubber composition, that can be used in particular as a tyre tread, comprising at least: - a diene elastomer; - a reinforcing filler; - between 2 and 25 phr of a blowing agent; - between 2 and 25 phr of a carboxylic acid, the melting point of which is between 60°C and 220°C; - a crosslinking system based on sulphur and on 5 to 15 phr of a vulcanization accelerator; - the (carboxylic acid/accelerator) weight ratio being less than 2.0. Process for obtaining such a composition and tyre comprising same. This composition makes it possible to reduce the rolling noise of tyres without degrading the rolling resistance thereof.

Description

 THERMO-EXPANDABLE AND PNEUMATIC RUBBER COMPOSITION COMPRISING SUCH A COMPOSITION

1. DOMAIN OF THE INVENTION

The invention relates to motor vehicle tires and thermo-expandable rubber compositions usable for the manufacture of such tires.

It relates more particularly to tires having, in the uncured state, such heat-expandable compositions and, in the vulcanized state, foam rubber compositions intended to reduce the noise emitted by these tires during the running of the vehicles.

2. STATE OF THE ART

It is known (see, for example, patent application WO 2011/051203) that the noise emitted by a rolling tire originates, inter alia, from the vibrations of its structure consecutive to the contact of the tire with the irregularities of the roadway, also causing a generation various acoustic waves. The whole thing finally comes in the form of noise, both inside and outside the vehicle. The amplitude of these different manifestations is dependent on the modes of vibration of the tire itself but also on the nature of the coating on which the vehicle moves. The frequency range corresponding to the noise generated by the tires typically ranges from about 20 to about 4000 Hz.

With regard to the perceived noise inside the vehicle, two modes of sound propagation coexist: - the vibrations are transmitted by the wheel center, the suspension system, the transmission to finally generate noise in the passenger compartment; this is called solid-state transmission, which is generally dominant at low frequencies of the spectrum (up to about 400 Hz); the so-called "cavity" noise, in particular, refers to the annoyance due to the resonance of the inflation cavity of the tire casing;

the acoustic waves emitted by the tire are directly propagated by air inside the vehicle, the latter acting as a filter; this is referred to as airborne transmission, which generally dominates in the high frequencies (about 600 Hz and beyond). The so-called "road noise" refers rather to the perceived overall level in the vehicle and in a frequency range up to 2000 Hz.

Concerning the noise emitted outside the vehicle, are relevant the various interactions between the tire and the road surface, the tire and the air, which will cause an inconvenience to the residents of the vehicle when it rolls on a road. floor. There are also in this case several sources of noise such as the so-called "indentation" noise due to the impact of roughness of the road in the contact area, the noise called "friction" essentially generated in the output of the 'contact area, the noise' says of sculpture 'due to the arrangement of the elements of sculpture and to the resonance in the various furrows. The frequency range concerned here typically corresponds to a range of about 300 to 3000 Hz.

To reduce the rolling noise of tires, many solutions have been proposed, including the use in their structure, for example in their tread or their inflation cavity, of a diene elastomer-based foam rubber, a blowing agent and various other additives such as, in particular, an expansion promoter. In a well-known manner, 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 quantity 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.

Foam tire rubber formulations, which may be expanded (cured) when reduced to reduce rolling noise, have been described, for example, in EP 337,787 or US 5,176,765, EP 885,925 or US Pat. No. 6,427,738. 1 800 911 or US 2007/0065821, JP 3-167008, WO 2009/003577 or US 2010/0133, WO 2011/051203.

The more recent application WO 2012/164001, filed by the Applicants, has described a heat-expandable rubber composition which, incorporated in particular to the tread of tires, has sound barrier properties effective in a frequency range between 300 and 2000 Hz, and is therefore able to reduce the noise emitted both inside and outside the vehicles when rolling their tires. This specific heat-expandable rubber composition is based on sodium or potassium carbonate or hydrogencarbonate as an expansion agent, to which must be associated, as expansion activator, a carboxylic acid such as the acid. citric melting temperature of between 60 ° C and 220 ° C. Such an expansion agent releases, very advantageously, only carbon dioxide and water during its decomposition; it is therefore particularly favorable to the environment.

P10-3103 PCT However, experience shows that the use of a carboxylic acid as an expansion promoter has the disadvantage of significantly degrading (increasing) the hysteresis of the rubber compositions once vulcanized (in the state of foam) and therefore of ultimately penalize the rolling resistance of tires.

3. BRIEF DESCRIPTION OF THE INVENTION In continuing their research on the above technology relating to the use of foam rubber in tires, the Applicants have discovered an improved carboxylic acid formulation which makes it possible to overcome the problem set forth above. above, that is to say to reduce rolling noise through the use of a sufficiently expanded foam, without penalizing the rolling resistance of the tires.

Consequently, the present invention relates to a thermoexpansible rubber composition, usable especially for the manufacture of tires, comprising at least: a diene elastomer;

 a reinforcing filler;

 between 2 and 25 phr of an expanding agent;

 between 2 and 25 phr of a carboxylic acid whose melting temperature is between 60 ° C and 220 ° C;

 a sulfur-based crosslinking system and 5 to 15 phr of a vulcanization accelerator;

 the weight ratio (carboxylic acid / accelerator) being less than 2.0.

The invention also relates to a process for preparing such a heat-expandable composition, said process comprising at least the following steps: in a mixer, incorporating at least one reinforcing filler and the acid into the diene elastomer or the mixture of diene elastomers; carboxylic, thermomechanically kneading the whole, in one or more times, until a maximum temperature of between 130 ° C and 200 ° C;

 - cool all at a temperature below 100 ° C;

 then incorporating the blowing agent into the mixture thus obtained and cooled, kneading thermomechanically all until a maximum temperature below 100 C;

P10-3103 PCT then incorporating a sulfur-based crosslinking system and 5 to 15 phr of the vulcanization accelerator, the weight ratio carboxylic acid / accelerator being less than 2.0;

 mix everything up to a maximum temperature below 120 ° C; - Extrude or calender the rubber composition thus obtained.

The invention also relates to any tire, in the uncured state, comprising such a heat-expandable composition. The tires of the invention are particularly intended, once vulcanized, 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 light of the detailed description which follows.

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 thermosorbable rubber composition of the invention, which can be used especially for the manufacture of tires and capable of reducing rolling noise, therefore has the essential characteristic of comprising, in the uncured state, at least: at least one, that is, one or more diene elastomers;

 - one (at least one, that is to say one or more) reinforcing filler;

 between 2 and 25 phr of a (at least one, ie one or more) blowing agent; between 2 and 25 phr of a (at least one, that is to say one or more) carboxylic acid whose melting temperature is between 60 ° C and 220 ° C;

P10-3103 PCT a sulfur-based crosslinking system and from 5 to 15 phr of a (at least one, that is to say one or more) vulcanization accelerator,

 the weight ratio (carboxylic acid / accelerator) being less than 2.0.

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) of monomers dienes (monomers carrying two double bonds carbon - carbon, conjugated or not).

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, and for example copolymers of dienes and alpha-olefins of the EPDM type, fall into the category of essentially saturated diene elastomers, with a level of diene origin units which is low or very low, still 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.

More preferably, the heat-expandable rubber composition of the invention comprises 50 to 100 phr of a copolymer based on styrene and butadiene, that is to say a copolymer of at least one styrene monomer and at least one butadiene monomer; in other words, said copolymer based on styrene and butadiene has by definition at least units derived from styrene and units derived from butadiene.

P10-3103 PCT As butadiene monomers, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di (C 1 -C ₅ alkyl) -1,3-butadienes, such as for example 2, are particularly suitable. 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, an aryl-1,3-butadiene. As styrene monomers are especially suitable styrene, methylstyrenes, para-tert-butylstyrene, methoxystyrenes, chloro styrenes.

Said preferential copolymer based on styrene and butadiene 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. . It can be for example block, statistical, sequenced, microsequenced, and be prepared in dispersion or in solution; it may be coupled and / or starred or functionalized with a coupling agent and / or starring or functionalization. Preferably, the styrene-butadiene-based copolymer is selected from the group consisting of styrene-butadiene copolymers (abbreviated to SBR), styrene-butadiene-isoprene copolymers (abbreviated to SBIR) and mixtures of such copolymers.

Among the SBIR copolymers, mention may in particular be made of 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 (mol%) in -1,2 units of the butadiene part of between 4% and 85%, a content (mol%) in trans-1,4 units of the butadiene part of between 6% and 80%, a content (mol%) in -1,2 units more - 3.4 of the isoprenic part comprised between 5% and 70% and a content (mol%) in trans units -1.4 of the isoprene part of between 10% and 50%.

More preferably, an SBR copolymer is used. Among the SBR copolymers, there may be mentioned especially those having a styrene content of between 5% and 60% by weight and more particularly between 20% and 50%, a content (mol%) in -1,2 bonds of the butadiene part. between 4% and 75%, a content (mol%) of trans-1,4 bonds of between 10% and 80%. The Tg (glass transition temperature) of the copolymer based on styrene and butadiene is preferably greater than -40 ° C., more preferably greater than -35 ° C., in particular between -30 ° C. and + 30 ° C. ( more particularly in a range of -25 ° C to + 25 ° C). The Tg of the elastomers described here is measured in a conventional manner, well known to those skilled in the art, on an elastomer in the dry state (ie, without extension oil) and by DSC (for example according to ASTM D3418-1999). .

P10-3103 PCT The person skilled in the art knows how to modify the microstructure of a copolymer based on styrene and butadiene, in particular on an SBR, to increase and adjust its Tg, in particular by modifying the styrene contents in -1-bonds. 2 or in trans-1,4 bonds of the butadiene part. It is more preferable to use an SBR (solution or emulsion) having a styrene content (mol%) which is greater than 35%, more preferably between 35% and 60%), in particular in a range of 38% to 50%> . High Tg SBRs are well known to those skilled in the art, they have been used primarily in tire treads to improve some of their wear properties.

To the preferred copolymer based on styrene and butadiene described above, may be associated with at least one other (or second) diene elastomer, optional and different from said copolymer (that is to say not having units derived from styrene and butadiene), said second diene elastomer being present at a weight ratio which is therefore at most equal to 50 phr.

This second optional diene elastomer is preferably selected from the group consisting of natural rubbers (NR), synthetic polyisoprenes (IR), polybutadienes (BR), isoprene copolymers and mixtures of these elastomers. Such copolymers are more preferably selected from the group consisting of isoprene-butadiene copolymers (BIR) and isoprene-styrene copolymers (SIR).

Among these, polybutadiene homopolymers (BR) and in particular those having a content (mol%) in units of 1,2,2 between 4% and 80% or those having a content (mol%) of cis-1, are particularly suitable, 4 greater than 80%>; polyisoprene homopolymers (IR); butadiene-isoprene copolymers (BIR) and in particular those having an isoprene content of between 5% and 90% by weight and a Tg of -40 ° C to -80 ° C .; isoprene-styrene copolymers (SIR) 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.

According to a more preferred embodiment, the second diene elastomer is an isoprene elastomer, more preferably natural rubber or a synthetic polyisoprene of cis-1,4 type; 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.

According to another more preferred embodiment, the second diene elastomer is a polybutadiene, preferably a polybutadiene having a cis-1,4 bond ratio greater than 90%.

P10-3103 PCT According to another more preferred embodiment, the second diene elastomer is a polybutadiene mixture as described above with an isoprene elastomer (natural rubber or synthetic polyisoprene). 4.2. Reinforcing charge

Any known filler for its ability to reinforce a rubber composition is usable, for example an organic filler such as carbon black, or an inorganic filler such as silica with which a coupling agent is associated in known manner, or still a mixture of these two types of load.

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 nm.

Preferably, the content of total reinforcing filler (in particular silica or carbon black or a mixture of silica and carbon black) is greater than 40 phr, more preferably greater than 50 phr, in particular between 50 and 150 pce. A content greater than 40 phr is favorable for good mechanical strength, particularly in the case of use in tread; above 150 phr, there is a risk of excessive rigidity of the rubber composition. For these reasons, the total reinforcing filler content is more preferably within a range of 60 to 120 phr. Suitable carbon blacks are, for example, all carbon blacks which are conventionally used in tires (so-called tire-grade blacks) such as blacks of the series 100, 200, 300 (ASTM grades), for example blacks NI 15, 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-A-2006/069792 and 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 intended for the manufacture of tires, in other words

P10-3103 PCT terms capable of replacing, in its reinforcing function, a conventional tire grade carbon black; 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, in particular any precipitated or fumed silica having a BET surface and a CTAB specific surface both less than 450 m ² / g, preferably from 30 to 400 m ² / g, especially between 60 and 300 m ² / g. As highly dispersible precipitated silicas (referred to as "HDS"), mention may be made, for example, of the "Ultrasil" 7000 and "Ultrasil" silicones 7005 from the company Evonik, the "Zeosil" silicas 1 165MP, 1 135MP and 1 115MP of the company Rhodia, the silica "Hi-Sil" EZ150G from the company PPG, the silicas "Zeopol" 8715, 8745 and 8755 from the Huber Company. According to a particular and preferred embodiment, in particular for use of the composition of the invention in tread, the majority filler is a reinforcing inorganic filler, in particular silica, at a rate in a range of 70 to 120 phr, reinforcing inorganic filler to which carbon black may be advantageously added at a minority rate at most equal to 15 phr, in particular in a range from 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 the applications WO03 / 002648 (or US 2005/016651) and WO03 / 002649 (or US 2005/016650).

Particularly suitable, without the definition below being limiting, are polysulphide silanes having the general formula (I): (I) Z - A - S _x - A - Z wherein:

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

- the symbols A, which are identical or different, represent a divalent hydrocarbon group (preferably an alkylene Ci-Cig or an arylene group C ₆ -C ₂

P10-3103 PCT more particularly C ₁ -C ₁₀ alkylene, especially C ₁ -C ₄ alkylene, in particular propylene);

 the symbols Z, identical or different, correspond to one of the three formulas below:

-

in which:

the radicals R ¹ , which may be substituted or unsubstituted, which are identical to or different from one another, represent a Ci-C18 alkyl, C ₅ -C ₈ cycloalkyl or C ₆ -C ₁₈ aryl group (preferably C ₁ -C ₈ alkyl groups); C ₆ , cyclohexyl or phenyl, especially C ₁ -C ₄ alkyl groups, more particularly methyl and / or ethyl).

- the radicals R ^2, substituted or unsubstituted, identical or different, represent an alkoxy group or Ci-Ci ₈ cycloalkoxy, C ₅ -C ₈ (preferably a group selected from alkoxyls Cg and C cycloalkoxyls ₅ -C ₈ , more preferably still a group selected from C ₁ -C ₄ alkoxyls, in particular methoxyl and ethoxyl).

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

By way of examples of polysulphurized silanes, mention may be made more particularly of bis (C ₁ -C ₄ alkoxy) -alkyl (C ₁ -C ₄ ) silylalkyl (C ₁ -C ₄ ) polysulfides (especially disulfides, trisulphides or tetrasulfides). ), such as polysulfides of bis (3-trimethoxysilylpropyl) or bis (3-triethoxysilylpropyl). Among these compounds, bis (3-triethoxysilylpropyl) tetrasulfide, abbreviated to TESPT, of formula [(C ₂ H ₅ O) ₃ Si (CH ₂ ) ₃ S ₂ ] ₂ or bis-disulfide ( triethoxysilylpropyl), abbreviated TESPD, of formula [(C ₂ H ₅ O) ₃ Si (CH ₂ ) ₃ S] ₂ . Mention may also be made, by way of preferred examples, of bis (monoalkoxyl (C ₁ -C ₄ ) -dialkyl (C ₁ -C ₄ ) silylpropyl) polysulfides (especially disulfides, trisulphides or tetrasulfides), more particularly bis-monoethoxydimethylsilylpropyl tetrasulfide, such as described in the aforementioned patent application WO 02/083782 (or US Pat. No. 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 by US Pat. example in

P10-3103 PCT patent applications WO 02/30939 (or US 6,774,255), WO 02/31041 (or US 2004/051210), and WO 2007/061550, or silanes or POS bearing azodicarbonyl functional groups, as described for example in the 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. 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 bandage, a significant amount of gas (for example nitrogen or carbon dioxide) 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.

There are physical or chemical expansion agents, of the endothermic or exothermic type. Preferably, chemical expansion agents are used, more preferably exothermic chemical expansion agents, for example diazo, dinitroso, hydrazide, carbazide, semi-carbazide, tetrazole, carbonate or citrate compounds, as they have been used. described especially in the aforementioned application WO 2011/064128.

P10-3103 PCT The blowing agent preferably used is a carbonate or hydrogencarbonate, in particular a carbonate or hydrogencarbonate (also called bicarbonate) of sodium (Na), potassium (K) or ammonium (NH ₄ ). More preferably, it is a carbonate selected from the group consisting of sodium carbonate, sodium hydrogencarbonate, potassium carbonate, potassium hydrogencarbonate, and mixtures of such carbonates (including, of course, their hydrated forms) . Such an expansion agent has the advantage of only releasing carbon dioxide and water during its decomposition; it is therefore particularly favorable to the environment. Hydrogen carbonate or sodium bicarbonate (NaHCO 3) is particularly used.

Preferably, the content of this blowing agent is between 5 and 25 phr, more preferably between 8 and 20 phr. 4.4. Carboxylic acid

An essential characteristic of the invention is to add to the blowing agent previously described, as a hot-melt compound, a carboxylic acid whose melting point is between 60 ° C. and 220 ° C., preferably between 100 ° C. and 100 ° C. ° C and 200 ° C, more preferably between 120 ° C and 180 ° C.

Melting temperature is a well-known basic physical constant (available for example in "Handbook of Chemistry and Physics"); it can be controlled by any known method, for example by the Thiele method, the Kofler bench method or by DSC analysis.

By dispersing homogeneously in the composition, during its melting in the specific temperature range indicated above, this carboxylic acid has the function of chemically activating (ie, by chemical reaction) the blowing agent which, during its thermal decomposition, will release much more gas bubbles (C0 ₂ and H ₂ 0) than if it was used alone. The level of this carboxylic acid is between 2 and 25 phr, preferably between 2 and 20 phr, especially between 2 and 15 phr.

The carboxylic acids may be monoacids, diacids or triacids, they may be aliphatic or aromatic; they may also comprise additional functional groups (other than COOH) such as hydroxyl groups (OH), ketone groups (C = O) or groups bearing ethylenic unsaturation.

P10-3103 PCT According to a preferred embodiment, the pKa (Ka acid constant) of the carboxylic acid is greater than 1, more preferably between 2.5 and 12, in particular between 3 and 10. According to another embodiment preferential, combined or not with the preceding, the carboxylic acid comprises, along its hydrocarbon chain, from 2 to 22 carbon atoms, preferably from 4 to 20 carbon atoms.

The aliphatic monoacids preferably comprise, along their hydrocarbon chain, at least 16 carbon atoms; mention may be made, for example, of palmitic acid (C16), stearic acid (Cl8), nonadecanoic acid (Cl9), behenic acid (C20) and their various mixtures. The aliphatic diacids preferably comprise, along their hydrocarbon chain, from 2 to 10 carbon atoms; mention may be made, for example, of oxalic acid (C2), malonic acid (C3), succinic acid (C4), glutaric acid (C5), adipic acid (C6), pimellic acid (C7), suberic acid (C8), azelaic acid (C9), sebacic acid (C10) and their various mixtures. As aromatic monoacid, there may be mentioned for example benzoic acid. The acids having functional groups may be monoacids, diacids or triacids, of the aliphatic type as aromatic; examples that may be mentioned include tartaric acid, malic acid, maleic acid, glycolic acid, α-ketoglutaric acid, salicylic acid, phthalic acid or citric acid; .

Preferably, the carboxylic acid is chosen from the group consisting of palmitic acid, stearic acid, nonadecanoic acid, behenic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimellic acid, suberic acid, azelaic acid, sebacic acid, benzoic acid, tartaric acid, malic acid, maleic acid, glycolic acid, α-ketoglutaric acid, salicylic acid, phthalic acid, citric acid and mixtures of these acids.

More particularly, the carboxylic acid is selected from the group consisting of malic acid, α-ketoglutaric acid, citric acid, stearic acid and mixtures thereof. More preferably still, citric acid, stearic acid or a mixture of these two acids is used.

Preferably, the total amount of blowing agent (in particular of carbonate or hydrogencarbonate) and of carboxylic acid is greater than 10 phr, preferably of between 10 and 40 phr. This total amount is more preferably greater than 15 phr, in particular between 15 and 40 phr.

4.5. Crosslinking system

P10-3103 PCT The crosslinking system is a vulcanization system based on sulfur (that is to say sulfur or a sulfur-donor agent) and 5 to 15 phr of a vulcanization accelerator, the accelerator rate being chosen such that the weight ratio (carboxylic acid / accelerator) is less than 2.0, preferably less than 1.7, more preferably still less than 1.5.

The sulfur content is preferably between 0.5 and 5 phr, that of the vulcanization accelerator is preferably within a range of 5 to 10 phr.

To this vulcanization system can be added, incorporated during the first non-productive phase and / or during the productive phase, various known vulcanization activators such as zinc oxide, stearic acid, guanidine derivatives (in particular particular diphenylguanidine), etc.

Any compound (or mixture of compounds) capable of acting as an accelerator for vulcanizing diene elastomers in the presence of sulfur, especially thiazole accelerators and their derivatives, thiuram accelerators, dithiocarbamates, may be used as accelerator. of zinc. 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, N dicyclohexyl-2-benzothiazylsulfenamide ("DCBS"), N-tert-butyl-2-benzothiazylsulfenamide ("TBBS"), N-tert-butyl-2-benzothiazylsulfenamide ("TBSI"), zinc dibenzyldithiocarbamate (" ZBEC ") and mixtures of these compounds.

Preferably, an accelerator of the family of sulfenamides, in particular chosen from the group consisting of CBS, DCBS, TBBS and mixtures of these compounds, is used.

Since the carboxylic acid has the possible effect of reducing the induction time (that is to say the time required for the beginning of the vulcanization reaction) during the baking of the composition, it is advantageous to use a retarder of beginning of vulcanization to counteract this phenomenon, and thus to provide the rubber composition the time necessary for full expansion before vulcanization.

The level of this vulcanization retarder is preferably between 0.5 and 10 phr, more preferably between 1 and 5 phr, in particular between 1 and 3 phr.

Vulcanization retarders are well known to those skilled in the art. Mention may be made, for example, of N-cyclohexylthiophthalimide sold under the name "Vulkalent G" by the company Lanxess, N- (trichloromethylthio) benzenesulfonamide sold under the name "Vulkalent E / C" by Lanxess, or else marketed phthalic anhydride.

P10-3103 PCT under the name "Vulkalent B / C" by Lanxess. Preferably, N-cyclohexylthiophthalimide (abbreviated "CTP") is used.

4.6. Various additives

The heat-expandable rubber composition of the invention may also comprise all or part of the usual additives usually used in tire foam rubber compositions, such as, for example, protective agents such as antiozone waxes, chemical antiozonants, oxidants, plasticizers.

According to a preferred embodiment, the thermo-expandable rubber composition also comprises a liquid plasticizer (at 20 ° C) whose function is to soften the matrix by diluting the diene elastomer and the reinforcing filler; its Tg is by definition less than -20 ° C, preferably less than -40 ° C.

According to another preferred embodiment, this liquid plasticizer is used at a relatively reduced rate, such that the weight ratio reinforcing filler on liquid plasticizer is greater than 2.0, more preferably greater than 2.5, in particular greater than 3 , 0. Any extender oil, whether aromatic or non-aromatic, any liquid plasticizer known for its plasticizing properties vis-à-vis diene elastomers, is usable. At ambient temperature (20 ° 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.

Particularly suitable liquid plasticizers selected from the group consisting of naphthenic oils (low or high viscosity, including hydrogenated or not), paraffinic oils, MES oils (Medium Extracted Solvates), oils DAE (Distillate Aromatic Extracts), Treated Distillate Aromatic Extracts (TDAE) oils, Residual Aromatic Extracts (RAE) oils, Treated Residual Aromatic Extracts (TREE) oils, Safety Residual Aromatic Extracts (SRAE) oils, mineral oils, vegetable oils, ethers plasticizers, ester plasticizers (for example plasticizers of the phosphates or sulfonates type) 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.

Examples of phosphate plasticizers are those containing from 12 to 30 carbon atoms, for example trioctylphosphate. By way of examples of ester plasticizers, mention may be made especially of the compounds chosen from the group consisting of

P10-3103 PCT trimellitates, pyromellitates, phthalates, 1,2-cyclohexane dicarboxylates, adipates, azelates, sebacates, glycerol triesters and mixtures thereof. Among triesters above, there may be mentioned include glycerol triesters, preferably consisting predominantly (for more than 50%, more preferably more than 80% by weight) of an unsaturated fatty acid Ci ₈ is that is to say selected 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.

According to another preferred embodiment, the rubber composition of the invention may also comprise, as solid plasticizer (at 23 ° C.), a hydrocarbon resin having a Tg greater than + 20 ° C., preferably greater than + 30 ° C, as described for example in the applications WO 2005/087859, WO 2006/061064 or WO 2007/017060.

Hydrocarbon resins are polymers that are well known to those skilled in the art, essentially based on carbon and hydrogen, and therefore inherently miscible in diene (s) elastomer compositions when they are further qualified as "plasticisers". ". They may be aliphatic, aromatic or aliphatic / aromatic type that is to say based on aliphatic and / or aromatic monomers. They may be natural or synthetic, whether or not based on petroleum (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, the plasticizing hydrocarbon resin has at least one, more preferably all, of the following characteristics: a Tg greater than 20 ° C (more preferably between 40 and 100 ° C);

 a number-average molecular weight (Mn) of between 400 and

2000 g / mol (more preferably between 500 and 1500 g / mol);

 a polymolecularity index (Ip) of less than 3, more preferably less than 2 (booster: Ip = Mw / Mn with Mw weight average molecular weight).

The Tg of this resin is measured in a known manner by DSC (Differential Scanning Calorimetry), according to the ASTM D3418 standard. The macrostructure (Mw, Mn and Ip) of the hydrocarbon resin is determined by steric exclusion chromatography (SEC): tetrahydroiurane solvent; temperature 35 ° C; concentration 1 g / 1; flow rate 1 ml / min; filtered solution on

P10-3103 PCT 0.45 μηι 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").

According to a particularly preferred embodiment, the plasticizing hydrocarbon resin is chosen from the group consisting of cyclopentadiene homopolymer or copolymer resins (abbreviated to CPD), dicyclopentadiene homopolymer or copolymer resins (abbreviated to DCPD), terpene homopolymer or copolymer resins, homopolymer or C5 cut copolymer resins, homopolymer or C9 cut copolymer resins, alpha-methyl-styrene homopolymer or copolymer resins and mixtures thereof. resins. Among the above copolymer resins are more preferably used those selected from the group consisting of (D) CPD / vinylaromatic copolymer resins, (D) CPD / terpene copolymer resins, copolymer resins (D) CPD / C5 cut, (D) CPD / C9 cut copolymer resins, terpene / vinylaromatic copolymer resins, terpene / phenol copolymer resins, C5 / vinylaromatic cut copolymer resins, C9 / vinylaromatic cut copolymer resins, and mixtures of these resins. The term "terpene" here combines in a known manner the alpha-pinene, beta-pinene and limonene monomers; preferably, a limonene monomer is used which is in a known manner in the form of three possible isomers: L-limonene (laevorotatory enantiomer), D-limonene (dextrorotatory enantiomer), or the dipentene, racemic of the dextrorotatory and levorotatory enantiomers. . Suitable vinylaromatic monomers are, for example, styrene, alpha-methylstyrene, ortho-, meta-, para-methylstyrene, vinyltoluene, para-tert-butylstyrene, methoxystyrenes, chlorostyrenes, hydroxystyrenes, vinylmesitylene, divinylbenzene, vinylnaphthalene, any vinylaromatic monomer from a C ₉ cut (or more generally from a C ₈ to C ₁₀ cut). Preferably, the vinyl-aromatic compound is styrene or a vinylaromatic monomer derived from a C ₉ cut (or more generally from a C ₈ to C ₁₀ cut). Preferably, the vinylaromatic compound is the minor monomer, expressed as a mole fraction, in the copolymer under consideration.

The content of hydrocarbon resin is preferably between 3 and 60 phr, more preferably between 3 and 40 phr, especially between 5 and 30 phr.

In the case where it is desired to increase the rigidity of the composition once expanded, without reducing the content of liquid plasticizer above, it will be advantageous to incorporate reinforcing resins (for example acceptors and donors of methylene) as described. for example in WO 02/10269 or US 7 199 175.

P10-3103 PCT 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, filler agents. implementability (processability) likely in 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.

4.7. Production of compositions

The rubber compositions of the invention are manufactured in suitable 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 "phase"). non-productive ") at a high temperature, up to a maximum temperature between 130 ° C and 200 ° C, preferably between 145 ° C and 185 ° C, during which is incorporated in particular the expansion activator ( carboxylic acid), followed by a second phase of mechanical work (sometimes called "productive" phase) at low temperature, typically below 120 ° C, for example between 60 ° C and 100 ° C, finishing phase during which is incorporated the blowing agent and the crosslinking or vulcanization system.

A method that can be used for the manufacture of such compositions comprises, for example, and preferably the following steps: in a mixer, incorporating in the diene elastomer or in the mixture of diene elastomers, at least the filler and the carboxylic acid by thermomechanically mixing the all, in one or more times, until reaching a maximum temperature of between 130 ° C and 200 ° C;

 cool the assembly to a temperature below 100 ° C;

 then incorporating the blowing agent into the mixture thus obtained and cooled, kneading thermomechanically all until a maximum temperature below 100 C;

 - Then incorporate the sulfur-based crosslinking system and 5 to 15 phr of vulcanization accelerator, the weight ratio (carboxylic acid / accelerator) being less than 2.0;

 mix everything up to a maximum temperature below 120 ° C; extruding or calendering the resulting rubber composition.

P10-3103 PCT 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, falling and cooling of the mixture thus obtained, it is then preferably incorporated in this order, the blowing agent, then the vulcanization retarder (if such a compound is used), finally the rest of the vulcanization system. (sulfur and accelerator) at low temperature, usually in an external mixer such as a roll mill; the whole is then mixed (productive phase) for a few minutes, for example between 5 and 15 min.

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 calendered or extruded in the form of a thermo-expandable rubber profile.

In the green state (that is to say unvulcanized) and therefore not expanded, the density or density denoted Di of the heat-expandable rubber composition is preferably between 1, 100 and 1, 400 g / cm ³ , more preferably in a range from 1, 150 to 1, 350 g / cm ³ .

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 fired (ie vulcanized) state, the density denoted D ₂ of the rubber composition once expanded (ie, in the foam rubber state) is preferably between 0.500 and 1 000 g / cm ³ , more preferably in a range from 0.600 to 0.850 g / cm ³ .

Its volume expansion rate noted T _E (expressed in%) is preferably between 30% and 150%, more preferably in a range of 50 to 120%, this expansion ratio T _E being calculated in a known manner from densities Di and D ₂ above, as follows:

T _E = [(D ₁ / D ₂ ) - 1] x 100.

Preferably, its Shore A hardness (measured according to ASTM D 2240-86) is in a range from 45 to 60.

P10-3103 PCT Thanks to the heat-expandable rubber composition thus formulated, it is possible to reduce the rolling noise of the tires without substantially degrading their rolling resistance, in particular when such a composition is used to form all or part of a tire. tire tread.

P10-3103 PCT

Claims

1. A thermo-expandable rubber composition, usable especially for the manufacture of tires, comprising at least: a diene elastomer;

 a reinforcing filler;

 between 2 and 25 phr of an expanding agent;

 between 2 and 25 phr of a carboxylic acid whose melting temperature is between 60 ° C and 220 ° C;

 a sulfur-based crosslinking system and 5 to 15 phr of a vulcanization accelerator;

 the weight ratio (carboxylic acid / accelerator) being less than 2.0.

The composition 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. Composition according to claims 1 or 2, comprising 50 to 100 phr of a copolymer based on styrene and butadiene, and optionally 0 to 50 phr of another diene elastomer.

The composition of claim 3, wherein the styrene-butadiene copolymer is selected from the group consisting of styrene-butadiene copolymers, styrene-butadiene-isoprene copolymers and mixtures of such copolymers.

The composition of claim 4, wherein the styrene-butadiene-based copolymer is a styrene-butadiene copolymer (SBR).

6. Composition according to any one of claims 3 to 5, wherein the copolymer based on styrene and butadiene has a glass transition temperature greater than -40 ° C, preferably in a range of -30 ° C to + 30 ° C.

The composition according to any one of claims 1 to 6, wherein the other optional diene elastomer is natural rubber, a synthetic polyisoprene or a polybutadiene preferably having a cis-1,4 bond ratio greater than 90%.

P10-3103 PCT

The composition of any one of claims 1 to 7, wherein the reinforcing filler comprises an inorganic filler, carbon black, or a mixture of inorganic filler and carbon black.

9. Composition according to any one of claims 1 to 8, wherein the reinforcing filler content is greater than 40 phr, preferably between 50 and 150 phr.

10. Composition according to any one of claims 1 to 9, wherein the blowing agent is a carbonate or hydrogencarbonate, preferably a carbonate or hydrogen carbonate sodium, potassium or ammonium.

The composition of claim 10, wherein the blowing agent is selected from the group consisting of sodium carbonate, sodium hydrogencarbonate, potassium carbonate, potassium hydrogencarbonate, and mixtures of such carbonates.

12. Composition according to any one of claims 1 to 11, wherein the content of blowing agent is between 5 and 25 phr, preferably between 8 and 20 phr.

13. Composition according to any one of claims 1 to 12, wherein the carboxylic acid level is between 2 and 20 phr, preferably between 2 and 15 phr.

14. Composition according to any one of claims 1 to 13, wherein the total content of blowing agent and carboxylic acid is greater than 10 phr, preferably greater than 15 phr.

15. Composition according to any one of claims 1 to 14, wherein the melting temperature of the carboxylic acid is between 100 ° C and 200 ° C, preferably between 120 and 180 ° C.

16. Composition according to any one of claims 1 to 15, wherein the carboxylic acid comprises, along its hydrocarbon chain, from 2 to 22 carbon atoms.

The composition of claim 16, wherein the carboxylic acid is selected from the group consisting of palmitic acid, stearic acid, nonadecanoic acid, behenic acid, oxalic acid, malonic acid, and the like. , succinic acid, glutaric acid, adipic acid, pimellic acid, suberic acid, azelaic acid, sebacic acid, benzoic acid, tartaric acid, malic acid , maleic acid, glycolic acid, α-ketoglutaric acid, salicylic acid, phthalic acid, citric acid and mixtures of these acids.

P10-3103 PCT

The composition of claim 17, wherein the carboxylic acid is selected from the group consisting of malic acid, α-ketoglutaric acid, citric acid, stearic acid and mixtures of these acids.

19. Composition according to any one of claims 1 to 18, wherein the rate of vulcanization accelerator is in a range of 5 to 10 phr.

20. Composition according to any one of claims 1 to 19, wherein the vulcanization accelerator is a sulfenamide accelerator.

21. Composition according to any one of claims 1 to 20, wherein the weight ratio (carboxylic acid / accelerator) is less than 1.7, preferably less than 1.5.

22. Composition according to any one of claims 1 to 21, further comprising a vulcanization retarder, preferably at a rate between 0.5 and 10 phr.

23. A tire, in the unvulcanized state, comprising a heat-expandable rubber composition according to any of claims 1 to 22.

24. A tire according to claim 23, the heat-expandable rubber composition forming all or part of the tread of the tire.

A process for preparing a heat-expandable rubber composition according to any one of claims 1 to 22, comprising at least the following steps: in a mixer, incorporating at least one diene elastomer or diene elastomer mixture, at least the reinforcing filler and the carboxylic acid, 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 ° C;

 then incorporating the blowing agent into the mixture thus obtained and cooled, kneading thermo-mechanically all until a maximum temperature of less than 100 C;

 then incorporating the sulfur-based crosslinking system and 5 to 15 phr of the vulcanization accelerator, the weight ratio carboxylic acid / accelerator being less than 2.0;

 mix everything up to a maximum temperature below 120 ° C; extruding or calendering the resulting rubber composition.

P10-3103 PCT