WO2015014577A1 - Tyre, the crown region of which is provided with an inner layer that reduces rolling noise - Google Patents

Abstract

Tyre for motor vehicle comprising an inner crown layer placed circumferentially between the tread and the carcass reinforcement of the tyre, characterized in that this inner crown layer is a heat-expandable rubber composition comprising: - 50 to 100 phr of polyisoprene or polybutadiene; - optionally, 0 to 50 phr of another diene elastomer; - 20 to 80 phr of 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 tyre. This inner crown layer makes it possible to reduce the rolling noise of tyres without degrading the rolling resistance thereof.

Description

 TIRE WHERE THE TOP ZONE HAS AN INTERNAL LAYER REDUCING ROLL NOISE

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.

As regards the noise perceived 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. However, experience has shown that the use of a carboxylic acid as an expansion promoter has the drawback of significantly degrading (increasing) the hysteresis of vulcanized rubber compositions (i.e. state of foam) and thus ultimately penalize the rolling resistance of the tires, particularly when these foam rubber compositions are used as the inner crown layer in these tires.

3. BRIEF DESCRIPTION OF THE INVENTION

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 described above, that is to say to say to reduce rolling noise thanks to the use of a sufficiently expanded foam, this without significantly penalizing the rolling resistance of the tires.

Accordingly, the invention relates to a tire for a motor vehicle comprising: - a top (2) comprising a tread (3) provided with at least one portion (3 a) radially external intended to come into contact with the road;

 two inextensible beads (4), two flanks (5) connecting the beads (4) to the tread (3), a carcass reinforcement (6) passing through both flanks (5) and anchored in the beads (4) ;

 - A crown reinforcement or belt (7) disposed circumferentially between the portion (3a) radially outer of the tread (3) and the carcass reinforcement (6);

 a radially inner elastomeric layer (8) called "inner crown layer", of different formulation formulation of the radially outer portion (3 a) of the tread, this inner crown layer being itself circumferentially disposed between the part (3 a) radially outer of the tread (3) and the carcass reinforcement (6), this tire being characterized in that the inner layer of crown (8), adapted to reduce the rolling noise of the tire , is a heat-expandable rubber composition comprising:

50 to 100 phr of polyisoprene or polybutadiene;

 optionally, 0 to 50 phr of another diene elastomer;

 20 to 80 phr of a reinforcing filler;

between 2 and 25 phr of an expansion 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 from 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 tire, comprising at least the following steps: in a mixer, incorporating at least the reinforcing filler and the carboxylic acid into the diene elastomer or to the mixture of diene elastomers, thermomechanically kneading the whole, 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 incorporate the blowing agent to the mixture thus obtained and cooled, kneading thermomechanically all until a maximum temperature below 100 C;

 then incorporating 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;

 mix everything up to a maximum temperature below 120 ° C; extruding or calendering the rubber composition thus obtained in the form of a rubber profile;

 incorporating said rubber profile between the radially outer portion (3a) of the tread (3) and the carcass reinforcement (6) of the tire being manufactured.

The invention also relates to any tire in the vulcanized state obtained after firing a tire according to the invention.

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 the light of the following description and examples of embodiment, as well as of FIGS. 1 to 3 relating to these examples which diagrammatically show, in radial section, examples of radial tires according to FIG. invention. 4. DETAILED DESCRIPTION OF THE INVENTION

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

The abbreviation "pce" (usually "phr" in English) means parts by weight per hundred parts of elastomer or rubber (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 from more than a to 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 from a to b (i.e., including the strict limits a and b).

The tire of the invention therefore has the essential characteristic of being provided with an inner layer of crown adapted to reduce rolling noise and comprising, in the uncured state:

50 to 100 phr of a (at least one, ie one or more) polyisoprene or polybutadiene;

 optionally, 0 to 50 phr of a (at least one, that is to say one or more) other diene elastomer;

 20 to 80 phr of a (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;

 a sulfur-based crosslinking system and 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. All of these components 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, as well as, for example, copolymers of dienes and alpha-olefins of the EPDM type, fall into the category of essentially saturated diene elastomers, with a proportion of diene origin units which is low or very low, always less than 15% (% in moles). 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%.

These general definitions being recalled, the rubber composition forming the protective elastomeric underlayer has the essential first characteristic of comprising 50 to 100 phr of a diene elastomer of the polyisoprene or polybutadiene type. According to a preferred embodiment, this composition comprises 50 to 100 phr of natural rubber (NR) or of synthetic polyisoprene (IR), in particular an IR of the cis-1,4 type having more preferably a level (mol%) of cis-1,4 bonds greater than 90%), in particular greater than 98%, this NR or IR may for example be used in a blend (blend) with at most 50 phr of a polybutadiene.

According to another preferred embodiment, this composition comprises 50 to 100 phr of polybutadiene (BR), the latter being for example used in a blend (blend) with at most 50 phr of a natural rubber or synthetic polyisoprene as described previously. Among the polybutadienes are particularly suitable those having a cis-1,4 bond ratio (mol%) greater than 80%, in particular greater than 90%, those having a content (mol%) in -1,2 units between 4 % and 80%.

The other optional diene elastomer is preferably selected from the group consisting of butadiene copolymers, isoprene copolymers and mixtures of these elastomers.

Such copolymers are more preferably chosen from the group consisting of butadiene-styrene copolymers (SBR), isoprene-butadiene 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 especially those having a styrene content of between 5% and 50% by weight and a Tg of between -25 ° C and At -50 ° C., the isoprene-butadiene-styrene copolymers (SBIR) and in particular those having a styrene content of between 5% and 50% by weight, an isoprene content of between 15% and 60%); butadiene content between 5% and 50% by weight, 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 (mol%) in units -1.2 plus -3.4 of the isoprene part of between 5% and 70% and a content (mol%) in trans-1,4 units of the isoprene part of between 10%> and 50%>, and mixtures of such copolymers. Among the SBR copolymers, mention may be made more particularly of those having a styrene content of between 5% and 60% by weight and more particularly between 20% and 50%, a content (mol%) of -1,2 bonds. of the butadiene part of 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). .

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.

The level of total reinforcing filler (in particular silica or carbon black or a mixture of silica and carbon black) is in a range from 20 to 80 phr, more preferably from 25 to 75 phr. A content greater than 20 phr is favorable for good mechanical strength of the inner crown layer; above 80 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 30 to 70 phr, in particular from 30 to 60 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. For example, carbon blacks could 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, without any other means than an intermediate coupling agent, a rubber composition intended for the manufacture of tires, in other words able to replace, in its function of reinforcement, a conventional carbon black tire 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, 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 (called "HDS"), mention may be made, for example, of the "Ultrasil" 7000 and "Ultrasil" silicones 7005 from Evonik, the "Zeosil" 1165MP, 1135MP and 1115MP silicas from Rhodia, silica "Hi-Sil" EZ150G from the company PPG, the silicas "Zeopol" 8715, 8745 and 8755 of the Huber Company.

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, but not limited to, polysulfide silanes having the following 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, more particularly alkylene Ci-Cio, in particular C ₁ -C ₄ , especially propylene);

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

R1 R1 R2

 I I

 Si- -Si R2 Si-

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).

As examples of silane polysulfides, are more particularly the bis (mono, trisulfide or tetrasulfide) of bis (alkoxyl (Ci-C ₄₎ alkyl (Ci-C ₄₎ alkyl silyl (Ci-C ₄ )), such as polysulfides of bis (3-trimethoxysilylpropyl) or bis (3-triethoxysilylpropyl). Among these compounds, bis (3-triethoxysilylpropyl) tetrasulfide, abbreviated to TESPT, of formula [(C ₂ H ₅ O) 3 Si (CH 2) 3 S ₂ ] 2 or bis (triethoxysilylpropyl) disulfide, is especially used. abbreviated to TESPD, of formula [(C ₂ H ₅ O) 3Si (CH 2) 3 S] ₂ . Mention may also be made, by way of preferred examples, of polysulfides (in particular disulfides, trisulphides or tetrasulfides) of bis- (monoalkoxyl (Ci-C ₄ ) -dialkyl (Ci-C ₄ ) silylpropyl), more particularly bis-monoethoxydimethylsilylpropyl tetrasulfide. as described in 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, particular mention may be made of bifunctional POS (polyorganosiloxanes) or hydroxysilane polysulfides (R ² = OH in formula I above) as described by US Pat. 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 azodicarbonyl groups, such as described for example in patent applications WO 2006/125532, WO 2006/125533, WO 2006/125534. By way of 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. It is preferable to use chemical expansion agents, more preferably chemical expansion agents of the exothermic type, for example example of the compounds diazo, dinitroso, hydrazides, carbazides, semi-carbazides, tetrazoles, carbonates, citrates, as have been described in particular in the aforementioned application WO 2011/064128. 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 include functional groupings additional (other than COOH) such as hydroxyl groups (OH), ketone groups (C = O) or groups bearing ethylenic unsaturation.

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 preferred embodiment, combined or otherwise 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 (Cl 6), stearic acid (Cl 8), nonadecanoic acid (Cl 9), behenic acid (C 20) 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

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. For example, N-cyclohexylthiophthalimide sold under the name "Vulkalent G" may be mentioned. by Lanxess, N- (trichloromethylthio) benzenesulphonamide sold under the name "Vulkalent E / C" by Lanxess, or else phthalic anhydride sold under the name "Vulkalent B / C" by Lanxess. Preferably, N-cyclohexylthiophthalimide (abbreviated as "CTP") is used.

4.6. Various additives

The rubber composition of the inner crown layer may also comprise all or part of the usual additives usually used in tire rubber compositions, such as, for example, protective agents such as chemical antioxidants, anti-oxidants, plasticizers or extension oils, whether the latter are of aromatic or non-aromatic nature, in particular very slightly or non-aromatic oils, for example of the naphthenic or paraffinic type, with high or preferably low viscosity, non-aromatic oils such as that MES or TDAE oils, agents facilitating the implementation (processability) of the compositions in the green state, tackifying resins, reinforcing resins (such as resorcinol or bismaleimide), acceptors or methylene donors such as by example hexamethylenetetramine or hexamethoxymethylmelamine. The inner crown layers may also contain coupling enhancers when a coupling agent is used, inorganic filler recovery agents when an inorganic filler is used, or more generally, blending agents. in a known manner, by improving the dispersion of the filler in the rubber matrix and by lowering 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 forming the inner crown layer are manufactured in suitable mixers, for example using two successive preparation phases according to a general procedure well known to those skilled in the art: a first thermomechanical working or mixing phase (sometimes qualified of "non-productive" phase) at a high temperature, up to a maximum temperature of between 130 ° C and 200 ° C, preferably between 145 ° C and 185 ° C, followed by a second phase of mechanical work (sometimes described as a "productive" phase) at a lower temperature, typically less than 120 ° C., for example between 60 ° C. and 100 ° C., a finishing phase during which the crosslinking or vulcanization system is incorporated. 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 else calendered or extruded in the form of a thermo-expandable rubber profile that can be used directly as inner top layer, for example as a "base" part of a "cap-base" tread.

A method that can be used for the manufacture of tires in accordance with the invention preferably comprises the following steps: in a mixer, incorporate at least the reinforcing filler and the carboxylic acid into the diene elastomer or the mixture of diene elastomers, thermomechanically kneading the whole, 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 incorporating 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;

 mix everything up to a maximum temperature below 120 ° C; extruding or calendering the rubber composition thus obtained in the form of a rubber profile;

incorporating said rubber profile between the radially outer portion (3a) of the tread (3) and the carcass reinforcement (6) of the tire being manufactured. In the green state (that is to say uncured) and therefore unexpanded, the density or density denoted Di of the heat-expandable rubber composition is preferably between 1, 10 and 1, 40 g / cm ³ , more preferably in a range of 1, 15 to 1, 35 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.

5. EXAMPLES OF REALIZATION 5.1. Tires of the invention

The rubber composition described above is therefore used, in the tire of the invention, as an inner crown layer arranged circumferentially inside the crown of the tire, ie between firstly the most radially outer portion of its strip. of rolling, that is to say the portion intended to come into contact with the road during taxiing, and secondly the crown reinforcement or belt, or between this belt and the carcass reinforcement.

By "inner" top layer is meant any rubber part of the tire crown which does not extend to the outside of the tire, which is non-contact _Λ

 With air or inflation gas, in other words which is therefore located inside the internal structure of the crown of the tire.

It should therefore be understood that this inner crown layer is disposed: either in the tread itself, but in this case under the portion (that is to say radially inwardly with respect to this portion) of the tread which is intended to come into contact with the road during the rolling of the tire, throughout the lifetime of the latter;

 - either under the tread (that is to say radially internally relative to this tread), between the tread and the belt;

 between the belt and the carcass reinforcement of the tire.

The thickness of this inner crown layer is preferably between 0.3 and 3 mm, especially in a range from 0.5 to 2.0 mm, in the uncured state, and from 0.5 to 6 mm. , especially in a range of 1 to 5 mm, after baking the tire.

Figures 1 to 3 annexed represent in radial section, very schematically (especially without respecting a specific scale), three preferred examples of tires for motor vehicle with radial carcass reinforcement, according to the invention.

FIG. 1 illustrates a first possible embodiment of the invention, according to which the inner crown layer (8) is integrated with the tread (3) itself, but arranged under the portion (3 a) of the tread which is intended to come into contact with the road when driving, to form what is commonly called a sub-layer of a tread. It may also be recalled that, in such a case, the tread is also commonly known to those skilled in the art of tread with a "cap-base" structure, the word "cap" designating the carved portion of the tread intended to come into contact with the road and the term "base" designating the non-carved portion of the tread, of different formulation, which is in turn not intended to come into contact with the road.

In this FIG. 1, the tire (1) schematized comprises a crown (2) comprising a tread (3) (for simplicity, with a very simple sculpture), the radially outer portion (3a) of which is intended to come into contact two inextensible beads (4) in which is anchored a carcass reinforcement (6). The top (2), joined to said beads (4) by two sides (5), is known per se reinforced by a crown reinforcement or "belt" (7) at least partly metallic and radially external to the carcass reinforcement (6), constituted for example by at least two superposed cross plies reinforced by metal cables. The carcass reinforcement (6) is here anchored in each bead (4) by winding around two rods (4a, 4b), the upturn (6a, 6b) of this armature (6) being for example arranged outwardly tire (1) which is shown here mounted on its rim (9). The carcass reinforcement (6) consists of at least one ply reinforced by radial textile cords, that is to say that these cords are arranged substantially parallel to each other and extend from a bead to the other so as to form an angle of between 80 ° and 90 ° with the median circumferential plane (plane perpendicular to the axis of rotation of the tire which is located halfway between the two beads 4 and passes through the middle of the crown frame 7). Of course, this tire (1) further comprises, in known manner, a layer (10) of rubber or inner elastomer (commonly called "inner liner" or "inner liner") which defines the radially inner face of the tire and which is intended for protecting the carcass ply from the diffusion of air from the interior space to the tire.

This example of tire (1) according to the invention of Figure 1 is characterized in that the base portion (8) of its tread (3) is constituted by the inner crown layer which has been described in detail previously . FIG. 2 illustrates another possible embodiment of the invention, according to which the inner crown layer (8) is external to the tread (ie, distinct from the tread), this time arranged, always in the top ( 2), below the tread (ie, radially inwardly from the tread) and above the tread (ie, radially outwardly from the tread), in other words between the tread (3) and the belt (7).

FIG. 3 illustrates another possible embodiment of the invention, according to which the inner crown layer described above is disposed between the belt (7) and the carcass reinforcement (6) of the tire.

In all cases shown schematically by the figures discussed above, the inner layer of vertex, thanks to its sound barrier properties, is likely to contribute to reducing the noise emitted both inside and outside the vehicles when driving their tires.

5.2. Running tests

For the purposes of these tests, three rubber compositions (hereinafter C-1, C-2 and C-3) were first prepared whose formulation is given in the attached Table 1, the rate of the different products being expressed in phr (parts by weight per hundred parts of elastomer, here consisting of a blend of NR and BR).

For the manufacture of these compositions, the following procedure was carried out: the reinforcing filler (carbon black), the elastomer, were successively introduced into an internal mixer, the initial vessel temperature of which was approximately 60 ° C. diene (NR and BR), as well as the various other ingredients with the exception of the blowing agent (where appropriate, compositions C-2 and C-3) and the vulcanization system; the mixer was thus filled to about 70% (% by volume). Thermomechanical work (non-productive phase) was then carried out in one step of about 2 to 4 minutes, until a maximum temperature of "fall" of 165 ° C was reached. The mixture thus obtained was recovered, cooled, and sulfur and a sulfenamide type accelerator were incorporated on an external mixer (homo-finisher) at 30 ° C, mixing the whole (productive phase) for a few minutes. The control composition C-1 is totally devoid of the expansion system, it is therefore non-heat-expandable. Control composition C-2 is well provided with the expansion system (blowing agent and carboxylic acid), so it is heat-expandable; but its rate of accelerator (3 phr) is insufficient for the ratio (carboxylic acid / accelerator) to be less than 2.0 (2.7 in the present case).

Only the composition C-3 is in accordance with the invention; it contains an expansion agent (10 phr), a carboxylic acid (8 phr) and a vulcanization accelerator at a particularly high level (7 phr) such that the weight ratio (carboxylic acid / accelerator) is much lower than 2, 0 (1.1 in the present case).

Compositions C-2 and C-3 both had, after firing, once in the state of foam rubber, a markedly reduced density with a particularly high degree of expansion by about 80%, in accordance with the teachings. WO 2012/164001 cited in the introduction to this memo; this suggests a very good ability of the two compositions to reduce rolling noise.

The compositions of Table 1 were then calendered in the form of an underlayer or base (8) (thickness about 2.7 mm) of a cap-base type tread, then these underlays incorporated in the tire structure for a passenger vehicle (dimensions 225/55 RI 6) as shown diagrammatically in FIG. 1, the tread of which, for its radially external part (3a), consisted of a conventional rubber composition for "Green tire" with low rolling resistance, comprising an SBR BR cut as diene elastomer and silica as a reinforcing filler. The tires thus manufactured (test tires marked P-1 and P-2, tires of the invention rated P-3), of identical dimensions and construction, were all provided with the same tread of conventional formulation as described above. above and an underlayer (base 8) of which only the formulation differed (respectively consisting of compositions C1, C-2 and C-3).

To first characterize the noise reduction properties of the P-2 and P-3 tires compared to the P-1 control tires (without foam rubber), a rolling test was conducted in which the sound level emitted was evaluated. by the tires by measuring the sound pressure level, when driving the vehicle at a speed of 60 km / h, this thanks to several microphones arranged inside the vehicle ("road noise"). The vehicle used was a "Subaru" brand vehicle ("RI" model); the road surface used for this test corresponds to a semi-rough asphalt; when passing through the measuring area, the sound pressure recording is triggered. The results in Table 2 express the differences in sound level recorded between the P-2 and P-3 tires compared to the P-1 control tires, in a frequency range of 80 to 800 Hz. These differences are expressed in acoustic energy (dB (A)) which corresponds to the integration of the sound pressure as a function of frequency over the frequency ranges considered, a negative value indicating a reduction of the noise with respect to the reference (control tires P-1).

By reading Table 2, it can be seen that a noise reduction of 1 dB (A), which is quite significant for those skilled in the art, is obtained thanks to the presence of the inner layer of rubber top. foam, in the P-2 and P-3 tires.

Then rolling resistance measurements on a steering wheel (ISO 87-67 / 1992 method) were conducted on the three types of tires.

On reading the results of Table 3 (expressed in relative units, with a base 100 for the rolling resistance of the control tires P-1), it can be seen that the tires P-2 of the prior art show a marked degradation of the rolling resistance (15% increase) compared to the P-1 control tires (as a reminder, the inner crown underlayer is devoid of foam rubber), while, unexpectedly, no significant degradation is observed on the P-3 tires of the invention.

In conclusion, the incorporation into the structure of a tire of the inner crown layer according to the invention makes it possible, thanks to its very specific formulation, to reduce the rolling noise of the tires without degrading the rolling resistance. Table 1

 natural rubber (peptized);

 polybutadiene rubber with 0.3% 1-2; 2.7%> trans; 97%> cis 1-4 (Tg = -104 C);

 ASTM N774 grade (Cabot company);

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

 Sodium hydrogencarbonate ("Bifun Jiuso" from Asahi Glass); Citric acid (Kanto Kagaku company);

 N-cyclohexylthiophthalimide ("Vulkalent P" from the Lanxess Company);

 N-dicyclohexyl-2-benzothiazol sulfenamide ("Santocure CBS" from the company Flexsys).

Table 2

 (*) Noise measured inside the vehicle; noise difference

 between the P-2 and P-3 tires and the P-1 tires, inside the vehicle.

Table 3

 Tire: P-1 P-2 P-3

Rolling resistance (% o) 100 115 102

Claims

1. Tire (1) for a motor vehicle comprising

- a top (2) having a tread (3) provided with at least a portion (3 a) radially external intended to come into contact with the road;

 two inextensible beads (4), two flanks (5) connecting the beads (4) to the tread (3), a carcass reinforcement (6) passing through both flanks (5) and anchored in the beads (4) ;

 a crown reinforcement or belt (7) disposed circumferentially between the radially outer portion (3a) of the tread (3) and the carcass reinforcement (6);

 a radially inner elastomeric layer (8) called an "inner crown layer", of a formulation different from the formulation of the radially outer portion (3 a) of the tread, this inner crown layer being itself circumferentially disposed between the radially outer portion (3a) of the tread (3) and the carcass reinforcement (6), characterized in that this inner crown layer (8), adapted to reduce the rolling noise of the tire, is a heat-expandable rubber composition comprising:

50 to 100 phr of polyisoprene or polybutadiene;

 optionally, 0 to 50 phr of another diene elastomer;

 20 to 80 phr of 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 from 5 to 15 phr of a vulcanization accelerator;

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

2. Tire according to claim 1, the thermo-expandable composition comprising 50 to 100 phr of natural rubber or synthetic polyisoprene.

Tire according to claim 2, wherein the synthetic rubber or synthetic polyisoprene is used in a blend with at most 50 phr of a polybutadiene preferably having a cis-1,4 bond ratio of greater than 90%.

4. Tire according to claim 1, the thermo-expandable composition comprising 50 to 100 phr of a polybutadiene preferably having a cis-1,4 bond ratio greater than 90%.

Tire according to claim 4, wherein the polybutadiene is used in a blend with at most 50 phr of natural rubber or synthetic polyisoprene.

Tire according to any one of claims 1 to 5, wherein the reinforcing filler comprises an inorganic filler, carbon black or a mixture of inorganic filler and carbon black.

7. Tire according to any one of claims 1 to 6, wherein the level of reinforcing filler is in a range of 25 to 75 phr, preferably 30 to 70 phr.

A tire according to any one of claims 1 to 7, wherein the blowing agent is a carbonate or hydrogencarbonate, preferably a sodium, potassium or ammonium carbonate or hydrogencarbonate.

The tire of claim 8, wherein the blowing agent is selected from the group consisting of sodium carbonate, sodium hydrogencarbonate, potassium carbonate, potassium hydrogencarbonate, and mixtures of such carbonates.

10. Tire according to any one of claims 1 to 9, wherein the content of blowing agent is between 5 and 25 phr, preferably between 8 and 20 phr.

11. Tire according to any one of claims 1 to 10, wherein the carboxylic acid content is between 2 and 20 phr, preferably between 2 and 15 phr.

12. Tire according to any one of claims 1 to 11, wherein the total content of blowing agent and carboxylic acid is greater than 10 phr, preferably greater than 15 phr.

13. Tire according to any one of claims 1 to 12, wherein the melting temperature of the carboxylic acid is between 100 ° C and 200 ° C, preferably between 120 and 180 ° C.

14. Tire according to any one of claims 1 to 13, wherein the carboxylic acid has, along its hydrocarbon chain, from 2 to 22 carbon atoms.

The tire of claim 14, wherein the carboxylic acid is selected from the group consisting of palmitic acid, stearic acid, nonadecanoic acid, 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.

Tire according to claim 15, 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.

17. Tire according to any one of claims 1 to 16, wherein the rate of vulcanization accelerator is in a range of 5 to 10 phr.

Tire according to any one of claims 1 to 17, wherein the vulcanization accelerator is a sulfenamide accelerator.

A tire according to any one of claims 1 to 18, wherein the sulfenamide accelerator is selected from the group consisting of N-cyclohexyl-2-benzothiazyl sulfenamide (CBS), N, N-dicyclohexyl-2-benzothiazyl sulfenamide (DCBS), N-tert-butyl-2-benzothiazyl sulfenamide (TBBS), and mixtures of such compounds.

20. Tire according to any one of claims 1 to 19, wherein the weight ratio (carboxylic acid / accelerator) is less than 1.7, preferably less than 1.5.

21. Tire according to any one of claims 1 to 20, the thermoexpansible composition further comprising a vulcanization retarder, preferably at a rate between 0.5 and 10 phr.

22. A method for preparing a tire according to any one of claims 1 to 21, comprising at least the following steps:

 in a mixer, incorporating at least the reinforcing filler and the carboxylic acid into the diene elastomer or the mixture of diene elastomers, by 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; then incorporating 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;

 mix everything up to a maximum temperature below 120 ° C; extruding or calendering the rubber composition thus obtained in the form of a rubber profile;

 incorporating said rubber profile between the radially outer portion (3a) of the tread (3) and the carcass reinforcement (6) of the tire being manufactured.

23. Tire, in the vulcanized state, obtained after firing a tire according to any one of claims 1 to 21.