EP4094302A1

EP4094302A1 - Piezoelectric device having improved piezoelectric properties

Info

Publication number: EP4094302A1
Application number: EP20845597.2A
Authority: EP
Inventors: Guillaume PIBRE; François LAFORT
Original assignee: Compagnie Generale des Etablissements Michelin SCA
Current assignee: Compagnie Generale des Etablissements Michelin SCA
Priority date: 2019-12-23
Filing date: 2020-12-16
Publication date: 2022-11-30
Also published as: US20230122032A1; CN114846634A; FR3105589B1; WO2021130435A1; FR3105589A1; KR20220115808A

Abstract

The invention relates to a piezoelectric device comprising at least one piezoelectric layer P interposed between two conductive layers E, each layer E forming an electrode, characterised in that the layer P comprises at least one piezoelectric composition made from at least one elastomer matrix principally comprising at least one diene elastomer, a piezoelectric inorganic filler, a carbon black and a crosslinking system and in that the amount of piezoelectric inorganic filler is greater than or equal to 5 vol% relative to the total volume of the piezoelectric composition and the amount of carbon black is greater than or equal to 6 vol% relative to the total volume of the piezoelectric composition. The invention also relates to a tyre comprising at least one piezoelectric device defined above.

Description

PIEZOELECTRIC DEVICE WITH PIEZOELECTRIC PROPERTIES

IMPROVED

Technical area

The present invention relates to a piezoelectric device comprising at least one piezoelectric P layer of a piezoelectric composition interposed between two conductive electrodes.

Prior art

Many articles include piezoelectric composite materials for their operation. They are found, in fact, in acoustic transducers, in pressure and / or acceleration sensors, energy generators, sound insulating materials, in piezoelectric actuators and / or detectors used for example in construction. atomic force electron microscopy or else used in tires to monitor tire behavior.

Piezoelectricity is a physical phenomenon which corresponds to the appearance of an electrical polarization induced by an external mechanical deformation. This is an electromechanical coupling where the polarization is proportional to the mechanical stress applied up to a certain level. The piezoelectric effect is then said to be direct. This phenomenon is reversible: when the material is subjected to an external electric field, it deforms. This is the reverse piezoelectric effect.

A variation of the macroscopic polarization during the application of a stress on the sample characterizes the piezoelectric effect. In a system of orthogonal axes, the polarization and the stress are related in matrix notation by a tensor of rank 2 called the piezoelectric tensor d _ÿ with i and j corresponding respectively to the polarization axis (1, 2, 3) and d 'application of the constraint (1, 2, 3, 4, 5, 6) as shown in figure 1.

Materials that have piezoelectric properties are classified into three broad classes: inorganic piezoelectric, piezoelectric polymers, and piezoelectric composite materials.

These three major classes of piezoelectric materials in particular do not have the same properties of dielectric permittivity, remanent polarization, coercive field, etc. The properties of inorganic piezoelectrics, such as, for example, lead titanozirconates (PTZ), are very often linked to their crystalline structure, while those of piezoelectric polymers (organic materials) come from the presence of permanent dipoles intrinsic to the monomers constituting these polymers. The best known piezoelectric polymer is PVDF.

A piezoactive or piezoelectric composite material comprises at least one piezoelectric material, which gives the composite material its piezoelectric activity, and one or more non-piezoelectric phases. This association leads to a composite material whose performance is increased compared to each phase alone. The non-piezoelectric phase is generally an organic polymer matrix, in particular a rigid thermoplastic or thermosetting polymer matrix (US2015134061, WO2016 / 157092), which can be of the polyamide type (Capsal et al. Journal of non-crystalline solids 2010, 356, 629). -634), polyepoxy (Furukawa et al. Jpn. J. Appl. Phys. 1976, 15, 2119), polystyrene, polyurethane (Hanner et al. Ferroelectrics 1989, 100, 255-260), PVC (Liu et al. Materials Science and Engineering 2006, 127, 261-266) or also polyethylene (Rujijanagul et al. Journal of Materials Science Letters 2001, 20, 1943-1945), a polymer matrix comprising cyanoethylated polyvinyl alcohol (EP2654094) or a polymer matrix comprising a diene elastomer (PCT / FR2019 / 051514).

Piezoelectric properties are used in many applications with increasing miniaturization, which necessitates having devices having increasingly high piezoelectric properties.

The object of the present invention is therefore to remedy this need and to propose a new device comprising a piezoelectric composition whose piezoelectric properties are improved compared to the piezoelectric composite materials of the prior art.

Applicant has found that adding carbon black to a piezoelectric composition surprisingly improves its piezoelectric properties.

Summary of the invention

Thus, the subject of the invention is a piezoelectric device comprising at least one piezoelectric P layer interposed between two conductive E layers, each E layer forming an electrode, characterized in that the P layer comprises at least one piezoelectric composition based on at least an elastomeric matrix comprising mainly at minus a diene elastomer, a piezoelectric inorganic filler, a carbon black and a crosslinking system and in that the rate of piezoelectric inorganic filler being greater than or equal to 5% by volume relative to the total volume of the piezoelectric composition and the rate of carbon black being greater than or equal to 6% by volume relative to the total volume of the piezoelectric composition.

Preferably, the carbon black is a reinforcing carbon black.

Preferably, the carbon black has an oil absorption index OAN less than or equal to 154 ml / l 00g, more preferably within a range ranging from 35 to 150 ml / 100g, more preferably still ranging from 70 to 140 ml / 100g.

Preferably, the carbon black has a BET specific surface area of greater than 30 m ² / g, preferably within a range ranging from 70 to 150 m ² / g, more preferably still within a range ranging from 70 to 120 m ² / g.

Preferably, the carbon black has an oil absorption number OAN in a range ranging from 35 to 150 ml / 100 g and has a BET specific surface area in a range ranging from 70 to 150 m ² / g.

Preferably, the carbon black content is greater than or equal to 6.5% by volume relative to the total volume of the piezoelectric composition, more preferably greater than or equal to 7%.

Preferably, the carbon black content is less than or equal to 25% by volume relative to the total volume of the piezoelectric composition, more preferably less than or equal to 20% by volume, more preferably still less than or equal to 17% by volume.

Preferably, the level of carbon black is within a range ranging from 6 to 20% by volume relative to the total volume of the piezoelectric composition.

Preferably, the level of piezoelectric inorganic charge is within a range ranging from 5% to 80% by volume relative to the total volume of the piezoelectric composition, preferably ranging from 6% to 60% more preferably still ranging from 7% to 50 %.

Preferably, the piezoelectric inorganic filler is chosen from piezoelectric ceramics, advantageously from ferroelectric oxides, advantageously having a perovskite structure. Preferably, the piezoelectric inorganic filler is chosen from the group consisting of barium titanate, lead titanate, lead titano-zirconate, lead niobate, lithium niobate, potassium niobate and their mixtures, more preferably. the piezoelectric inorganic filler is chosen from the group consisting of barium titanate, lead titano-zirconate, potassium niobate and their mixtures.

Preferably, the piezoelectric inorganic filler is chosen from the group consisting of barium titanate, potassium niobate and their mixtures.

Preferably, the size of the particles of the piezoelectric inorganic filler is within a range ranging between 50 nm to 800 μm.

Preferably, the piezoelectric composition comprises from 75 phr to 100 phr, more preferably from 90 phr to 100 phr, more preferably still from 95 to 100 phr of diene elastomer.

Preferably, the diene elastomer of the piezoelectric composition is chosen from the group consisting of natural rubber, copolymers of diene and of alpha-olefins, synthetic polyisoprenes, polybutadienes, butadiene copolymers, isoprene copolymers and mixtures of these diene elastomers.

Preferably, the diene elastomer of the piezoelectric composition is chosen from the group consisting of natural rubber, ethylene-propylene-diene monomer copolymers, synthetic polyisoprenes, polybutadienes, butadiene-styrene copolymers, copolymers of butadiene-isoprene, butadiene-styrene-isoprene copolymers, isoprene-styrene copolymers and mixtures of these diene elastomers.

Preferably, the diene elastomer of the piezoelectric composition is chosen from the group consisting of polybutadienes, butadiene-styrene copolymers, butadiene-isoprene copolymers, butadiene-styrene-isoprene copolymers and mixtures of these diene elastomers, more preferably, the diene elastomer is the styrene-butadiene copolymer.

Preferably, the crosslinking system of the piezoelectric composition comprises a peroxide.

Preferably, the P layer is an electrically insulating piezoelectric layer. Preferably, each layer E is a conductive metal layer.

Preferably, the metal of layer E is chosen from the group consisting of silver, gold, nickel, palladium, aluminum, copper, titanium and their mixture, preferably is gold.

Preferably, each layer E is a conductive rubber composition based on at least 50 phr of diene elastomer, a graphitized or partially graphitized carbon black and a crosslinking system.

Preferably, the level of graphitized or partially graphitized carbon black in the conductive rubber composition of layer E is within a range ranging from 10 to 40% by volume relative to the total volume of the conductive rubber composition of layer E, preferably within a range ranging from 15 to 30% by volume.

Preferably, the system for crosslinking the conductive rubber composition of layer E comprises a peroxide.

Preferably, the diene elastomer of the piezoelectric composition of the E layer is co-crosslinked with the diene elastomer of the conductive rubber composition of each P layer.

Preferably, the graphitized or partially graphitized carbon black of the conductive rubber composition of layer E has an OAN oil absorption index of greater than or equal to 155 ml / 100g.

Another object of the present invention relates to a tire comprising at least one piezoelectric device defined above.

Brief description of the drawings

Figure 1 is a representation of the orthogonal axis system for polarization, P, and stress. The indices 1, 2, 3 correspond respectively to the directions normal to the planes YOZ, XOZ and XOY and the indices 4, 5, 6 to the directions tangential to these same planes.

detailed description

The invention relates to a piezoelectric device comprising at least one piezoelectric layer P interposed between two conductive E layers, each E layer forming an electrode, characterized in that the P layer comprises at least one piezoelectric composition based on at least one elastomeric matrix. mainly comprising at minus a diene elastomer, a piezoelectric inorganic filler, a carbon black and a crosslinking system and in that the rate of piezoelectric inorganic filler being greater than or equal to 5% by volume relative to the total volume of the piezoelectric composition and the rate of carbon black being greater than or equal to 6% by volume relative to the total volume of the piezoelectric composition.

In the present description, any range of values designated by the expression "from a to b" represents the range of values going from a to b (that is to say bounds a and b included). Any interval "between a and b" represents the range of values going from more than a to less than b (that is, bounds a and b excluded).

When reference is made to a “majority” compound, it is understood, within the meaning of the present invention, that this compound is in the majority among the compounds of the same type in the composition, that is to say that it is the one which represents the greatest amount by mass among compounds of the same type. Thus, for example, a major elastomer is Telastomer representing the greatest mass relative to the total mass of the elastomers in the composition. Likewise, a so-called majority filler is that representing the greatest mass among the fillers of the composition. By way of example, in a system comprising a single elastomer, the latter is in the majority within the meaning of the present invention; and in a system comprising two elastomers, the majority telastomer represents more than half of the mass of the elastomers. Preferably, the term “majority” is understood to mean present at more than 50%, preferably more than 60%, 70%, 80%, 90%, and more preferably the “majority” compound represents 100%.

The expression "consists essentially of" followed by one or more characteristics, means that may be included in the process or the material of the invention, in addition to the components or steps explicitly listed, components or steps which do not significantly modify the properties and characteristics of the invention.

By the expression "part by weight per hundred parts by weight of elastomer" (or phr), it should be understood within the meaning of the present invention, the part, by weight per hundred parts by weight of elastomers, whether they are. thermoplastic or not. In other words, thermoplastic elastomers are elastomers.

The compounds mentioned in the description and entering into the preparation of polymers or rubber compositions can be of fossil origin or biobased. In the latter case, they can be, partially or totally, derived from biomass or obtained from renewable raw materials from biomass. This concerns in particular polymers, fillers, etc.

Layer P

The P layer of the device according to the invention comprises at least, in particular consists, preferably, of a piezoelectric composition based on at least one elastomeric matrix mainly comprising at least one diene elastomer, a piezoelectric inorganic filler, a carbon black and a crosslinking system and in that the rate of piezoelectric inorganic charge is greater than or equal to 5% by volume relative to the total volume of the piezoelectric composition and the rate of carbon black is greater than or equal to 6% by volume relative to the total volume of the piezoelectric composition.

By "piezoelectric composition" is meant a composition based on at least one piezoelectric material which gives the composition its piezoelectric activity and based on one or more non-piezoelectric phases. This association leads to a composition the performance of which is increased compared to each phase alone.

By the expression “piezoelectric composition based on” is meant a piezoelectric composition comprising the mixture and / or the reaction product in situ of the various basic constituents used, some of these constituents being able to react and / or being intended to react. between them, at least partially, during the various phases of manufacture of said composition, or during the subsequent cooking, modifying said composition as it is prepared at the start. Thus, the piezoelectric compositions as used for the invention can be different in the uncrosslinked state and in the crosslinked state.

Elastomeric matrix

By "elastomeric matrix" is meant G elastomer or all of the elastomers which constitute the non-piezoelectric phase of the piezoelectric composition.

By "diene elastomer (or rubber)", whether natural or synthetic, should be understood in a known manner an elastomer consisting at least in part (ie, a homopolymer or a copolymer) of diene monomer units (monomers carrying two carbon-carbon double bonds, conjugated or not).

These diene elastomers can be classified into two categories: “essentially unsaturated” or “essentially saturated”. In general, the term “essentially unsaturated” is understood to mean a diene elastomer derived at least in part from conjugated diene monomers, having a level of units or units of diene origin (conjugated dienes) which is greater than 15% (% by moles); it is thus that diene elastomers such as butyl rubbers or copolymers of dienes and alpha-olefins of the EPDM type do not fall within the preceding definition and can in particular be qualified as “essentially saturated” diene elastomers (content of units of weak or very weak diene origin, always less than 15 mol%).

As diene elastomer capable of being used in the elastomer matrix, the piezoelectric composition in accordance with the invention is suitable:

- any homopolymer of a diene monomer, conjugated or not, having 4 to 18 carbon atoms;

- any copolymer of a diene, conjugated or not, having 4 to 18 carbon atoms and at least one other monomer.

The other monomer of the copolymer of a diene may be ethylene, an olefin or a diene, conjugated or not.

Suitable conjugated dienes are conjugated dienes having from 4 to 12 carbon atoms, in particular 1,3-dienes, such as in particular 1,3-butadiene and isoprene.

Suitable unconjugated dienes are unconjugated dienes having 6 to 12 carbon atoms, such as 1, 4-hexadiene, ethylidene norbomene, dicyclopentadiene.

Suitable olefins are vinyl aromatic compounds having from 8 to 20 carbon atoms and aliphatic α-monoolefins having from 3 to 12 carbon atoms.

Suitable vinyl aromatic compounds are, for example, styrene, ortho-, meta-, para-methylstyrene, the commercial mixture "vinyl-toluene", para-tert-butylstyrene.

Suitable aliphatic α-monoolefins in particular are acyclic aliphatic α-monoolefins having from 3 to 18 carbon atoms.

More particularly, the diene elastomer is:

- any homopolymer of a conjugated diene monomer, in particular any homopolymer obtained by polymerization of a conjugated diene monomer having from 4 to 12 carbon atoms;

- any copolymer obtained by copolymerization of one or more dienes conjugated with one another or with one or more vinyl aromatic compounds having from 8 to 20 carbon atoms; - any copolymer obtained by copolymerization of one or more dienes, conjugated or not, with ethylene, an α-monoolefin or a mixture thereof, such as for example elastomers obtained from ethylene or propylene with an unconjugated diene monomer of type cited above.

Preferably, the diene elastomer is chosen from the group consisting of natural rubber (NR), copolymers of dienes and alpha-olefins, synthetic polyisoprenes (IR), polybutadienes (BR), butadiene copolymers, isoprene copolymers and mixtures of these diene elastomers. Among the isoprene copolymers, there may be mentioned in particular the copolymers of isobutene-isoprene (butyl rubber - IIR), of isoprene-styrene (SIR), of isoprene-butadiene (BIR) or of isoprene-butadiene-styrene. (SBIR). Among the copolymers of butadiene, mention will in particular be made of copolymers of butadiene-styrene (SBR), isoprene-butadiene (BIR) or isoprene-butadiene-styrene (SBIR). Of the copolymers of dienes and alpha-olefins, ethylene-propylene-diene monomer (EPDM) copolymers are preferred.

More preferably still, the diene elastomer is chosen from the group consisting of natural rubber, ethylene-propylene-diene monomer copolymers, synthetic polyisoprenes, polybutadienes, styrene-butadiene copolymers, isoprene-styrene copolymers. , isoprene-butadiene-styrene copolymers, isoprene-butadiene copolymers and mixtures of these diene elastomers.

More preferably still, the diene elastomer is a styrene-butadiene copolymer. In particular, butadiene-styrene copolymers are suitable and in particular those having a Tg (glass transition temperature (Tg, measured according to ASTM D3418-1999) of between 0 ° C and - 90 ° C and more particularly between - 10 ° C. and - 70 ° C, a styrene content of between 1% 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 diene elastomer can be modified, ie either coupled and / or star, or functionalized, or coupled and / or star and simultaneously functionalized.

Thus, the diene elastomer can be coupled and / or star-branched, for example by means of a silicon or tin atom which binds the elastomer chains together. The diene elastomer can be simultaneously or alternately functionalized and include at least a functional group. By functional group is meant a group comprising at least one heteroatom chosen from Si, N, S, O, P. Particularly suitable as functional groups are those comprising at least one function such as: silanol, an alkoxysilane, a primary amine , secondary or tertiary, cyclic or not, a thiol, an epoxide.

In the context of functionalized elastomers, that is to say comprising at least one functional group:

- The functional group can be located at the end of the elastomer chain, it will then be said that the diene elastomer is functionalized at the end or end of the chain; - The functional group can be located in the main linear elastomer chain, we will then say that the diene elastomer is coupled or functionalized in the middle of the chain, as opposed to the position "at the end of the chain" and although the group is not not located precisely in the middle of the main elastomer chain;

- The functional group can be central and link n elastomer chains (n> 2), the elastomer being star or branched;

The diene elastomer can have several functional groups, pendant or not, distributed along the main chain of the elastomer, we will say that the diene elastomer is functionalized along the chain.

The elastomeric matrix may contain a single diene elastomer or a mixture of several diene elastomers.

Preferably, the elastomer matrix comprises from 75 to 100 phr of diene elastomer (phr = part by weight relative to 100 parts by weight of elastomer), more advantageously from 90 to 100 phr, even more advantageously from 95 to 100 phr. In a preferred variant of the invention, the diene elastomer, or the mixture of diene elastomers, is the only elastomer in the elastomeric matrix (thus corresponding to 100 phr).

In a variant of the invention, the piezoelectric composition of the P layer can comprise another elastomer, in a content strictly less than 50 phr.

This other elastomer can, in particular, be a thermoplastic elastomer (abbreviated “TPE”). TPEs have an intermediate structure between thermoplastic polymers and elastomers. They are block copolymers, made up of rigid, thermoplastic blocks, connected by flexible, elastomeric blocks.

The thermoplastic elastomer used for the implementation of the invention is a block copolymer of which the chemical nature of the thermoplastic blocks and of the elastomer blocks can vary.

In a known manner, TPEs exhibit two glass transition temperature peaks (Tg, measured according to ASTM D3418-1999), the lower temperature being relative to the elastomeric part of the TPE, and the highest temperature being relative to the part. thermoplastic TPE. Thus, the flexible blocks of TPEs are defined by a Tg lower than the ambient temperature (25 ° C), while the rigid blocks have a Tg higher than 80 ° C.

To be both elastomeric and thermoplastic in nature, the TPE must be provided with sufficiently incompatible blocks (that is to say different because of their mass, their polarity or their respective Tg) to retain their specific properties of elastomer or thermoplastic block.

TPEs can be copolymers with a small number of blocks (less than 5, typically 2 or 3), in which case these blocks preferably have high masses, greater than 15,000 g / mol. These TPEs can be, for example, diblock copolymers, comprising a thermoplastic block and an elastomer block. They are often also triblock elastomers with two rigid segments connected by a flexible segment. The rigid and flexible segments can be arranged linearly, star or branched. Typically, each of these segments or blocks often contains at least more than 5, generally more than 10 base units (eg styrene units and butadiene units for a styrene / butadiene / styrene block copolymer).

The TPEs can also comprise a large number of smaller blocks (more than 30, typically 50 to 500), in which case these blocks preferably have low masses, for example from 500 to 5000 g / mol, these TPEs will be called Multiblock TPE thereafter, and are a concatenation of elastomeric blocks - thermoplastic blocks.

The elastomeric blocks of TPE for the purposes of the invention can be all the elastomers described above for the diene elastomer. They preferably have a Tg of less than 25 ° C, preferably less than 10 ° C, more preferably less than 0 ° C and very preferably less than -10 ° C. Also preferably, the elastomeric block Tg of the TPE is greater than -100 ° C. For the definition of thermoplastic blocks, the glass transition temperature characteristic (T g) of the rigid thermoplastic block will be used. This characteristic is well known to those skilled in the art. In particular, it makes it possible to choose the temperature for industrial use (transformation). In the case of an amorphous polymer (or of a block of polymer), the processing temperature is chosen to be substantially greater than the Tg. In the specific case of a semi polymer (or of a block of polymer). -crystalline, a melting temperature can then be observed which is then higher than the glass transition temperature. In this case, it is rather the melting temperature (Tm) which makes it possible to choose the processing temperature of the polymer (or polymer block) considered. Thus, subsequently, when we speak of “Tg (or Tf, if applicable)”, it will be necessary to consider that this is the temperature used to choose the implementation temperature.

For the purposes of the invention, the TPE elastomers comprise one or more thermoplastic block (s) preferably having a Tg (or Tm, where appropriate) greater than or equal to 80 ° C and constituted from of polymerized monomers. Preferably, this thermoplastic block has a Tg (or Tm, where appropriate) within a range varying from 80 ° C to 250 ° C. Preferably, the Tg (or Tm, where appropriate) of this thermoplastic block is preferably from 80 ° C to 200 ° C, more preferably from 80 ° C to 180 ° C.

The proportion of thermoplastic blocks relative to TPE, as defined for the implementation of the invention, is determined on the one hand by the thermoplastic properties that said copolymer must exhibit. Thermoplastic blocks having a Tg (or Tm, if applicable) greater than or equal to 80 ° C are preferably present in sufficient proportions to preserve the thermoplastic character of the elastomer. The minimum rate of thermoplastic blocks having a Tg (or Tm, if applicable) greater than or equal to 80 ° C. in the TPE may vary depending on the conditions of use of the copolymer. On the other hand, the ability of TPE to deform can also help determine the proportion of thermoplastic blocks having a Tg (or Tm, if applicable) greater than or equal to 80 ° C.

By way of example, it is possible to use, for the elastomer matrix of the piezoelectric composition, any TPE which is a copolymer in which the elastomer part is saturated, and comprising styrene blocks and alkylene blocks. The alkylene blocks are preferably ethylene, propylene or butylene. More preferably, this TPE elastomer is chosen from the following group, consisting of diblock, linear or star triblock copolymers: styrene / ethylene / butylene (SEB), styrene / ethylene / propylene (SEP), styrene / ethylene / ethylene / propylene (SEEP ), styrene / ethylene / butylene / styrene (SEBS), styrene / ethylene / propylene / styrene (SEPS), styrene / ethylene / ethylene / propylene / styrene (SEEPS), styrene / isobutylene (SIB), styrene / isobutylene / styrene (SIBS) and mixtures of these copolymers.

According to another example, any TPE can be used which is a copolymer whose elastomeric part is unsaturated, and which comprises styrene blocks and diene blocks, these diene blocks being in particular isoprene or butadiene blocks. More preferably, this TPE elastomer is chosen from the following group, consisting of diblock, linear or star triblock copolymers: styrene / butadiene (SB), styrene / isoprene (SI), styrene / butadiene / isoprene (SBI), styrene / butadiene / styrene (SBS), styrene / isoprene / styrene (SIS), styrene / butadiene / isoprene / styrene (SBIS) and mixtures of these copolymers.

For example also, the TPE can be a linear or star copolymer whose elastomeric part comprises a saturated part and an unsaturated part such as for example styrene / butadiene / butylene (SBB), styrene / butadiene / butylene / styrene (SBBS) or a mixture of these copolymers.

Among the multiblock TPEs, mention may be made of copolymers comprising blocks of random copolymer of ethylene and propylene / polypropylene, polybutadiene / polyurethane (TPU), polyether / polyester (COPE), polyether / polyamide (PEBA).

As examples of commercially available TPE elastomers, mention may be made of elastomers of the SEPS, SEEPS or SEBS type marketed by the company Kraton under the name “Kraton G” (products “G1650, G1651, G1654, G1730”) or the Kuraray company under the name “Septon” (“Septon 2007”, “Septon 4033”, “Septon 8004”); or the elastomers of the SIS type marketed by Kuraray, under the name “Hybrar 5125”, or marketed by Kraton under the name “DI 161” or else the elastomers of the linear SBS type marketed by Polimeri Europa under the name “Europrene SOL T 166” ”Or spangled SBS marketed by Kraton under the name“ DI 184 ”. Mention may also be made of the elastomers sold by the company Dexco Polymers under the name “Vector” (“Vector 4114”, “Vector 8508”). Among the multiblock TPEs, mention may be made of the “Vistamaxx” TPE marketed by the company Exxon; TPE COPE marketed by the company DSM under the name “Amitel”, or by the company Dupont under the name “Hytrel”, or by the company Ticona under the name “Riteflex”; the TPE PEBA marketed by the company Arkéma under the name “PEBAX”; TPE TPU marketed by the company Sartomer under the name “TPU 7840”, or by the company BASF under the name “Elastogran”.

Piezoelectric inorganic charges

The piezoelectric composition of the P layer comprises at least one piezoelectric inorganic filler at a rate greater than or equal to 5% by volume relative to the total volume of said composition.

The piezoelectric inorganic filler can advantageously be dispersed in the elastomeric matrix.

Advantageously, the inorganic piezoelectric filler can be in the form of particles not bound to the elastomeric matrix.

The term “particles not bound to the elastomeric matrix” is understood to mean particles without covalent bonds between the inorganic piezoelectric filler and to the constituent elastomer (s) of the elastomeric matrix of the piezoelectric composition.

The term “particles dispersed in the elastomeric matrix” is understood to mean that the inorganic piezoelectric filler used in the context of the present invention is distributed substantially uniformly in the elastomeric matrix of the piezoelectric composition. Thus, the average distance separating the particles from these adjacent piezoelectric inorganic charges is substantially constant throughout the entire volume of said elastomeric matrix.

These piezoelectric inorganic charges can be piezoelectric single crystals or piezoelectric ceramics.

The piezoelectric single crystals are in particular natural piezoelectric materials such as quartz or tourmaline. These ferroelectric crystals can have a domain structure. One can distinguish monodomain single crystals and polydomain single crystals according to one or more directions of polarization coexist in the crystal.

Advantageously, the piezoelectric inorganic filler used in the context of the invention can be chosen from piezoelectric ceramics.

Piezoelectric ceramics are materials with strong electromechanical coupling and high density. These ceramics derive their piezoelectric property from their crystalline structure, through the absence of symmetry of the crystal lattice which dissociates the centers of gravity of the positive and negative charges, each lattice then constituting an electric dipole. The mesh crystalline thus has a permanent dipole which gives these ceramics high dielectric permittivity values. Synthetic piezoelectric ceramics are in particular composed of ferroelectric oxides, which have the property of having an electrical polarization in the spontaneous state, which can also be reversed by the application of a sufficiently intense external electric field.

Advantageously, the piezoelectric inorganic filler can be chosen from ferroelectric oxides.

The ferroelectric oxides can in particular be endowed with a perovskite structure. They advantageously correspond to a general formula ABO3 such as barium titanate (BaTiCh), lead titanate (PbTiCL), potassium niobate (KNbCh), lead niobate (PbNbCL), or bismuth ferrite (BiFeCL) . In this family of piezoelectric materials, mention may also be made of lead titano-zirconate (PZT) with a Pb (Zr _x Tii_ _x ) 0 _{3 structure} in which x is between 0 and 1. It can be in pure form or in semiconductor form doped either with acceptor dopants (to give a so-called hard or "hard" PZT), such as Fe, Co, Mn, Mg, Al, In, Cr, Sc, Na or K, or with dopants donors (to give a soft or soft PZT), such as La, Nd, Sb, Ta, Nb or W.

Advantageously, the piezoelectric inorganic fillers which can be used in the context of the invention have a perovskite structure.

By way of nonlimiting examples, the piezoelectric inorganic filler which can be used within the framework of the invention can be chosen from the group consisting of barium titanate, lead titanate, lead titano-zirconate (PZT), niobate lead, lithium niobate, potassium niobate and mixtures thereof.

The best known piezoelectric ceramics are barium titanate (BaTiCL) and lead zirconate titanate (PZT), which have a very good electromechanical coefficient and have varied manufacturing processes. These (sol-gel process, hydrothermal synthesis, calcination, etc.) allow the dielectric, mechanical and piezoelectric properties to be modified depending on the intended application. Barium titanate, like potassium niobate, are lead-free piezoelectric materials. They have the advantage of being less toxic.

Advantageously, the piezoelectric inorganic filler can be chosen from barium titanate, potassium niobate, lead titano-zirconate and their mixtures. More preferably still, the piezoelectric inorganic filler is chosen from barium titanate, potassium niobate and their mixtures.

In particular, the piezoelectric inorganic charges have particle sizes between 50 nm and 800 µm. The size of the particles corresponds to the average diameter of the particles. The measurement of the average diameter is carried out by scanning electron microscopic analysis (SEM). Photographs are taken on powder samples. Image analysis is performed using software and provides access to the average diameter of the particles present.

The volume of piezoelectric inorganic charge used in the P layer of the device of the invention will depend on the use of said device.

Preferably, the rate of piezoelectric inorganic charge is within a range ranging from 5% to 80% by volume relative to the total volume of the piezoelectric composition, advantageously within a range ranging from 6% to 60% by volume, more advantageously still from 7% to 50% by volume.

Advantageously, the P layer has 0-3 connectivity, comprising particles of piezoelectric inorganic charges dispersed in the elastomeric matrix. Connectivity depends on the spatial organization of each phase that makes up the piezoelectric composition. A change in connectivity causes major changes in the physical properties of composites. In the case of two-phase systems, the nature of the connectivity is represented by two numbers (the first for the ceramic, the second for the matrix). They indicate the number of directions connected by the considered phase. Thus, a composite with 0-3 connectivity corresponds to a composite consisting of grains of piezoelectric powder dispersed in the elastomer matrix. The main advantage of this type of composition is the ease of implementation of the method, or the ease of producing complex shapes, such as curved surfaces.

Carbon black

The piezoelectric composition of the layer P comprises at least one carbon black at a rate greater than or equal to 6% by volume relative to the total volume of said composition.

Carbon black is elemental carbon in the form of colloidal particles produced by the partial combustion or thermal decomposition of liquid hydrocarbons or gaseous under controlled conditions. It is used as a filler or colorant in many industrial applications.

Preferably, the carbon black used in the context of the invention is a reinforcing carbon black. For the purposes of the present invention, the term “reinforcing carbon black” means a carbon black having the capacity to reinforce a rubber composition which can be used in particular for the manufacture of tires. The carbon black which can be used in the present invention is therefore a reinforcing filler.

Surprisingly, when a carbon black is added to a piezoelectric composition based on an elastomeric matrix comprising piezoelectric inorganic fillers, an improvement in the piezoelectric properties of said composition is observed with respect to a composition not containing carbon black. carbon. This improvement in properties makes it possible, for example, to increase the viability of said composition or to use smaller quantities of piezoelectric compositions to deliver the same electrical intensity, therefore to reduce production costs or to miniaturize the devices in which the composition piezoelectric is used.

Suitable carbon blacks are all carbon blacks, in particular the blacks conventionally used in tires. Among the latter, we can cite more particularly the reinforcing carbon blacks of the 100, 200, 300 series, or the blacks of the 500, 600 or 700 series (grades ASTM D-1765-2017), such as for example the blacks NI 15, N134 , N234, N326, N330, N339, N347, N375, N550, N683, N772). These carbon blacks can be used in the isolated state, as available commercially, or in any other form, for example as a support for some of the rubber additives used. The carbon blacks could, for example, already be incorporated into the diene elastomer, in particular isoprene, in the form of a masterbatch (see for example applications WO97 / 36724-A2 or WO99 / 16600-A1).

More preferably, the carbon black which can be used in the context of the present invention has an OAN oil absorption index of less than or equal to 154 ml / 100g, more preferably within a range ranging from 35 to 150 ml / 100g, more more preferably ranging from 70 to 140 ml / l 00g.

More preferably, the carbon black which can be used in the context of the present invention has a BET specific surface area greater than 30 m ² / g, preferably within a range ranging from 70 to 150 m ² / g, more preferably still within a range ranging from 70 to 120 m ² / g.

More preferably still, the carbon black which can be used in the context of the present invention has an oil absorption index OAN comprised in a range ranging from 35 to 150 ml / 100 g and a BET specific surface area ranging from 70 at 150 m ² / g.

The OAN oil absorption index is measured according to the D2414-2018 standard and the BET specific surface area according to the D6556-2017 standard.

The level of carbon black in the piezoelectric composition of the layer P is greater than or equal to 6% by volume relative to the total volume of the said composition. Below this threshold, improvements in the piezoelectric properties of the piezoelectric composition are not observed. Preferably, the carbon black content is greater than or equal to 6.5% by volume, more preferably greater than or equal to 7% by volume.

Preferably, the P layer is an electrically insulating layer. Those skilled in the art will therefore be able to adapt the maximum amount of carbon black used depending on the use of the device of the invention. For this, a person skilled in the art will know how to choose a maximum charge rate so that the electrical conductivity of the P layer is below the threshold of electric percolation of the charges of the P layer for a value of the electric field used during step polarization. Thus preferably, the carbon black content may be less than or equal to 25% by volume relative to the total volume of the piezoelectric composition, more preferably less than or equal to 20% by volume, more preferably still less than or equal to 17% by volume. volume. More preferably still, the level of carbon black is within a range ranging from 6% to 20% by volume relative to the total volume of the piezoelectric composition.

Cross-linking system

The system for crosslinking the piezoelectric composition can be any type of known system. It can in particular be based on sulfur, and / or peroxide and / or bismaleimides.

According to a preferred embodiment, the crosslinking system is sulfur-based, this is called a vulcanization system. The sulfur can be provided in any form, in particular in the form of molecular sulfur, or of a sulfur donor agent. At least one vulcanization accelerator is also preferably present, and, in a manner optional, also preferential, one can use various known vulcanization activators such as zinc oxide, stearic acid or any equivalent compound such as stearic acid salts and transition metal salts, guanide derivatives (in particular diphenylguanidine), or also known vulcanization retarders.

When present, sulfur is used at a preferential rate of between 0.5 and 12 phr, in particular between 1 and 10 phr. The vulcanization accelerator is used at a preferential rate of between 0.5 and 10 phr, more preferably between 0.5 and 5.0 phr.

Can be used as accelerator any compound capable of acting as vulcanization accelerator of diene elastomers in the presence of sulfur, in particular accelerators of the thiazole type as well as their derivatives, accelerators of the sulfenamide, thiuram, dithiocarbamate, dithiophosphates, thioureas and xanthates types. As examples of such accelerators, there may be mentioned in particular the following compounds: 2-mercaptobenzothiazyl disulfide (abbreviated "MBTS"), N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"), N, N-dicyclohexyl- 2-Benzothiazyl sulfenamide (“DCBS”), N-ter-butyl-2-benzothiazyl sulfenamide (“TBBS”), N-ter-butyl-2-benzothiazyl sulfenimide (“TBSI”), tetrabenzylthiuram disulfide (“TBZTD”) , zinc dibenzyldithiocarbamate ("ZBEC") and mixtures of these compounds.

According to another preferred embodiment, the crosslinking system preferably contains a peroxide. Advantageously, the peroxide is the only crosslinking agent. Thus, advantageously, according to this embodiment, the composition does not include a vulcanization system, that is to say a sulfur-based crosslinking system.

The peroxide which can be used in the context of the invention can be any peroxide known to those skilled in the art.

Preferably, the peroxide is chosen from organic peroxides.

By "organic peroxide" is meant an organic compound, that is to say, containing carbon, having an -O-O- group (two oxygen atoms linked by a single covalent bond).

During the crosslinking process, the organic peroxide decomposes at its unstable 0-0 bond to free radicals. These free radicals allow the creation of crosslinking bonds. According to one embodiment, the organic peroxide is chosen from the group consisting of dialkyl peroxides, monoperoxycarbonates, diacyl peroxides, peroxyketals, peroxyesters, and mixtures thereof.

Preferably, the dialkyl peroxides are selected from the group consisting of dicumyl peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, 2,5-dimethyl-

2.5-di (t-butylperoxy) hexane, 2,5-dimethyl-2,5-di (t-amylperoxy) -hexane, 2,5-dimethyl-

2.5-di (t-butylperoxy) hexyne-3, 2,5-dimethyl-2,5-di (t-amylperoxy) hexyne-3, a, a'-di - [(t-butyl-peroxy) isopropyl] benzene, a, a'-di - [(t-amyl-peroxy) isopropyl] benzene, di-t-amyl peroxide, l, 3,5-tri - [(t-butylperoxy) isopropyl] benzene, 1,3-dimethyl-3- (t-butylperoxy) butanol, 1,3-dimethyl-3- (t -amylperoxy) butanol and mixtures thereof.

Certain monoperoxycarbonates such as 00-tert-butyl-0- (2-ethylhexyl) monoperoxycarbonate, OO-tert-butyl-O-isopropyl monoperoxycarbonate, OO-tert-amyl-O-2-ethyl hexyl monoperoxycarbonate and their mixtures, can also be used.

Among the diacyl peroxides, the preferred peroxide is benzoyl peroxide.

Among the peroxyketals, the preferred peroxides are chosen from the group consisting of 1,1-di- (t-butylperoxy) -3,3,5-trimethylcyclohexane, 4,4-di- (t-butylperoxy) valerate of n - butyl, ethyl 3,3-di- (t-butylperoxy) butyrate, 2,2-di- (t-amylperoxy) -propane, 3,6,9-triethyl-3,6,9- trimethyl-1,4,7-triperoxynonane (or methyl ethyl ketone cyclic trimer peroxide), 3,3,5,7,7-pentamethyl-1,2,4-trioxepane, 4,4-bis (t- amylperoxy) n-butyl valerate, ethyl 3,3-di (t-amylperoxy) butyrate, 1,1-di (t-butylperoxy) cyclohexane, 1,1- di (t-amylperoxy) cyclohexane and their mixtures.

Preferably, the peroxyesters are chosen from the group consisting of tert-butylperoxybenzoate, tert-butyleperoxy-2-ethylhexanoate, tert-butyleperoxy-3,5,5-trimethylhexanoate and their mixtures.

Particularly preferably, the organic peroxide is chosen from the group consisting of dicumyl peroxide, aryl or diaryl peroxides, diacetyl peroxide, benzoyl peroxide, dibenzoyl peroxide, ditertbutyl peroxide, tertbutylcumyl peroxide, 2,5-bis (tertbutylperoxy) -2,5-dimethylhexane, n-butyl-4,4'- di (tert-butylperoxy) valerate, 00- (t-butyl) -0- ( 2-ethylhexyl) monoperoxycarbonate, tert-butyl peroxyisopropylcarbonate, tert-butyl peroxybenzoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, 1,3 (4) -bis (tert-butylperoxyisopropyl) benzene and mixtures of the latter, still preferably chosen from the group consisting of peroxide of dicumyl, n-butyl-4,4'-di (tert-butylperoxy) -valerate, OO- (t-butyl) 0- (2-ethylhexyl) monoperoxycarbonate, tertio-butyl peroxyisopropylcarbonate, tertio-butyl peroxybenzoate, tert-butyl peroxy-3,5,5-trimethylhexanoate, 1,3 (4) -bis (tert-butylperoxyisopropyl) benzene and mixtures thereof.

When it is present, the total level of peroxide in the composition is preferably greater than or equal to 0.3 phr, more preferably greater than or equal to 0.75 phr, preferably within a range ranging from 0.5 to 5 phr, in particular 0.5 to 3 phr.

Other additives

The piezoelectric composition of the P layer of the device according to the invention may optionally also comprise one or more additives, such as for example pigments, protective agents such as anti-ozone waxes, chemical anti-ozonants, anti-oxidants, agents. plasticizers such as plasticizing oils or hydrocarbon resins, reinforcing resins, acceptors (eg phenolic novolak resin) or methylene donors (eg HMT or H3M).

Advantageously, the piezoelectric composition of the P layer does not include graphitized or partially graphitized carbon black. By "graphite or partially graphite black" is meant within the meaning of the present invention, a carbon black comprising graphite and whose oil absorption index is greater than or equal to 155 ml / 100g. In the case where graphitized or partially graphitized carbon black is also present in this composition of the layer P, the level of this graphitized or partially graphitized carbon black is preferably chosen by those skilled in the art so that the piezoelectric composition remains insulating. electrically. Thus preferably, the level of graphitized or partially graphitized carbon black is less than or equal to 5% by volume relative to the total volume of the piezoelectric composition.

Manufacturing of the P layer

The P layer of the device of the invention can be manufactured by any known technique.

In general, the dispersion of the inorganic piezoelectric charges in the elastomeric matrix is carried out in the presence of the carbon black which can be used in the context of the present invention, the composition obtained from the previous step is optionally crosslinked.

The dispersion of the piezoelectric inorganic charges in the elastomeric matrix in the presence of carbon black which can be used in the context of the present invention is carried out by any means known per se to those skilled in the art, in particular for example by twin-screw extrusion or by mixing in an internal mixer.

By way of example, the layer P is manufactured in an appropriate mixer, 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 referred to as "non-productive" phase) at high temperature of the diene elastomer, of the inorganic piezoelectric filler and of the carbon black, as well as of the other ingredients if possibly present except for the crosslinking system, up to a maximum temperature between 80 ° C and 190 ° C, preferably between 80 ° C and 150 ° C, followed by a second phase of mechanical work (sometimes qualified as “productive” phase) at a lower temperature, typically less than 80 ° C , for example between 60 ° C and 80 ° C, finishing phase during which the crosslinking system is incorporated into the mixture obtained in the non-productive phase, then at the end of the productive phase by extruding or calendering the com position obtained to form the P layer.

Layer E forming an electrode

The electrodes are known to those skilled in the art. They are deposited on the faces of the P layer in order to collect the electric charges emitted by the piezoelectric composition of this P layer. They can be facing each other. Their dimensions are suitable for the intended application.

The electrodes can be made of metal or metal oxide in the form of a thin deposit of these metals or of these metal oxides. For example, the electrodes can be made of indium tin oxide, a metallic material, such as silver, gold, nickel, palladium, aluminum, copper, titanium or an alloy or a mixture of at least two of these materials. The formation of the metal or metal oxide electrodes can be carried out by a so-called additive process, for example by direct printing of a fluid or viscous composition comprising the metal or the metal oxide composing the electrodes, at the desired locations, by for example by inkjet printing, heliography, screen printing, flexography, spray coating (in English "spray coating"), deposit of drops (in English "drop-casting") or by chemical vapor deposition. The formation of the metal or metal oxide electrodes can correspond to a subtractive process, in which the material composing the electrodes is deposited on the entire P layer and in which the unused portions are then removed by photolithography or laser ablation by example. According to the material metal considered, the deposition on the entire P layer can be carried out by liquid, by cathode sputtering or by evaporation.

The electrodes can be flexible, such as a mixture of thermoplastic or diene elastomers made conductive, or such as a conductive polymer such as, for example, poly (3,4-ethylenedioxythiopene).

As flexible electrodes, one can for example use a layer E comprising a conductive rubber composition based on at least 50 phr of diene elastomer, a crosslinking system and a graphitized or partially carbon black. graphite.

The diene elastomers which are suitable for the conductive rubber composition of the layer E are those previously described for the piezoelectric composition of the layer P. The diene elastomer of the conductive rubber composition of the layer E may be the same or different from that of the piezoelectric composition of the P layer, preferably it is identical to that of the piezoelectric composition of the P layer.

The crosslinking system which is suitable for the conductive rubber composition of the layer E is that previously described for the piezoelectric composition of the layer P. The crosslinking system of the conductive rubber composition of the layer E may be the same or different from that. of the piezoelectric composition of the P layer, preferably it is identical to that of the piezoelectric composition of the P layer.

Graphitized or partially graphitized carbon black is any graphitized or partially graphitized carbon black having an OAN oil absorption index greater than or equal to 155 ml / 100 g, more preferably greater than or equal to 160 ml / 100 g.

Preferably, the graphitized or partially graphitized carbon black may have a particle size ranging from 50 to 500 mhi. The amount of graphitized or partially graphitized black in the conductive rubber composition of layer E is within a range ranging from 10% to 40% by volume, preferably from 15% to 30% by volume relative to the total volume of the conductive rubber composition. Preferably, the conductive rubber composition of layer E does not include any piezoelectric inorganic filler.

The conductive rubber composition of layer E is manufactured in an appropriate mixer, 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 referred to as "non-productive" phase) at high temperature where the constituents of layer E are mixed with the exception of the crosslinking system, up to a maximum temperature between 80 ° C and 190 ° C, preferably between 80 ° C and 150 ° C, followed by a second phase of mechanical work (sometimes referred to as “productive” phase) at a lower temperature, typically less than 80 ° C, for example between 60 ° C and 80 ° C, finishing phase during which the crosslinking system is incorporated. At the end of the productive phase, the conductive rubber composition is extruded or calendered to form a flexible layer E.

Manufacture of the device of the invention.

The device of the invention can be manufactured by any means known to those skilled in the art. The layers E forming the electrodes are deposited on the faces of the layer P comprising the piezoelectric composition in order to collect the electric charges emitted by said composition. The device according to the invention is advantageously connected to an electronic device in order to capture the electrical impulses emitted and to use this information.

As an example of manufacturing the device with flexible E layers, the following protocol can be cited: the E and P layers can be manufactured separately as explained above then the P layer comprising the piezoelectric composition is placed between two E layers , identical or different, preferably identical, conductive to obtain an assembly, then pressure is applied to the assembly and the assembly is crosslinked to obtain the device according to the invention. The layers can be successively deposited in a suitable mold, called a baking mold, which can have any dimension. It is possible to use a pressure ranging from 1,000,000 to 20,000,000 Pa, advantageously ranging from 1,500,000 to 10,000,000 Pa. This pressure is applied to the assembly. The duration of the compression is adapted according to the chosen pressure; it can for example last from 5 min to 90 min. The crosslinking can be carried out by curing, that is to say by heating the assembly to a temperature generally between 130 ° C and 200 ° C, for a sufficient time which can vary for example between 5 and 90 min depending on in particular the baking temperature, the crosslinking system adopted and the crosslinking kinetics of the compositions considered. The pressurizing and crosslinking steps can be simultaneous. For example, when the layers are placed in a baking mold, this mold can be placed in a plate press where the assembly will be baked under pressure. In a variant of the device, the elastomer of the composition of the layer P can be co-crosslinked with the diene elastomer of each composition of the flexible diene layer E. Thus the different layers can be advantageously linked together in a covalent manner, making it possible to obtain a device which is cohesive. When the E layers are metal layers, the P layer can be manufactured as explained above, then the E layers are applied according to one of the manufacturing processes for these layers as explained above.

The method of preparing the device of the invention can advantageously comprise a polarization step. The device polarization step is the application of an electric field across the electroactive device to orient the dipoles of the inorganic piezoelectric charges in the same direction to achieve macroscopic polarization of the device.

The step of polarizing the inorganic piezoelectric charges is carried out by known means suitable for converting the inorganic piezoelectric charges into charges exhibiting piezoelectric properties on a macroscopic scale. The device polarization step therefore corresponds to the application of an electric field across the electroactive device to orient the dipoles of the inorganic piezoelectric charges in the same direction in order to obtain a macroscopic polarization of the piezoelectric composition.

The polarization depends on the polarization temperature, the applied electric field and the polarization time.

Advantageously, the polarization temperature may be at least 5 ° C lower than the Curie temperature, Te, the lower of the piezoelectric inorganic charges, more preferably at least 7 ° C lower, even more advantageously at least 10 ° C less than the Curie temperature, Te, the lower of the piezoelectric inorganic charges. Indeed, close to the Curie temperature of piezoelectric inorganic charges, the stirring of the dipoles makes their alignment more difficult under an electric field.

The Curie temperature, Te, of a piezoelectric material is the temperature at which the material becomes paraelectric. Thus, the characteristic hysteresis cycle of the piezoelectric material, which is obtained by plotting the polarization as a function of the electric field applied to the material, disappears when the Curie temperature is reached. The Curie temperature is a characteristic of the piezoelectric material.

In particular, the electric field applied during the polarization step can be in a range ranging from 0.1 to 10 kV / mm, advantageously from 0.5 to 5 kV / mm.

In particular, the applied electric field depends on the nature of the piezoelectric inorganic charge and on the duration of polarization of the piezoelectric composition and on the rate of carbon black present in the piezoelectric composition. Those skilled in the art know how to adapt the electric field to the device of the invention and to the duration of polarization.

In particular, the polarization time can be between 1 minute and 10 hours, preferably between 5 minutes and 2 hours.

Use of the device according to the invention

The device according to the invention and mentioned above can be used in combination with a sensor.

The device of the invention allows the detection of a mechanical stress on the surface of said piezoelectric composition by direct piezoelectric effect. The detection of mechanical stress can be carried out in a very wide field of technical applications such as aeronautics, automobiles, health, tires, transport, etc.

A subject of the invention is also a tire comprising the device mentioned above comprising said piezoelectric composition and electrodes. In particular, said device can be fixed, for example, on the inner waterproof layer of said tire, that is to say on the layer which is in contact with the inflation air of the tire. The fixing can be carried out by conventional means known to those skilled in the art such as scraping whitewash, the use of cold vulcanization or else the melting of TPE. Fixing can be done by adhesive bonding.

Other advantages will become apparent on reading the following description, which refers to the examples given without limitation.

Examples:

The aim of the test presented below is to compare the piezoelectric properties of the piezoelectric composition C1, in accordance with the invention, and forming the layer P of the device of the invention with respect to a device having the same electrodes but having a layer P of a non-conforming piezoelectric composition T.

Unless otherwise stated, the levels of the various constituents of the piezoelectric composite materials presented in Table 1 are expressed in phr (part by weight per 100 parts by weight of elastomer).

[Table 1]

(1) Elastomeric matrix: Styrene-Butadiene Copolymer (SBR) polymerized in solution (S-SBR), non-functional, non-extended. Its microstructure is as follows: 24 mol% of 1,2 polybutadiene units relative to the butadienic part and 26.5% by weight of styrene units relative to the total weight of the copolymer. It has a Tg = -48 ° C. The glass transition temperature, Tg, is measured in a known manner by DSC (“Differential Scanning Calorimetry) according to the ASTM D3418 standard of 1999. The microstructure of S-SBR (relative distribution of butadiene units 1, 2-vinyl, 1, 4 -trans and 1, 4-cis) and the quantitative determination of the mass content of styrene in S-SBR are determined by near infrared spectroscopy (NIR). The principle of the method is based on the Beer-Lambert law generalized to a multi-component system. The method being indirect, it makes use of a multivariate calibration [Vilmin, F .; Dussap, C .; Coste, N. Applied Spectroscopy 2006, 60, 619-29] produced using standard elastomers with a composition determined by ¹³ C NMR. The styrene content and the microstructure are then calculated from the NIR spectrum of a film. of elastomer approximately 730 mhi thick. The spectrum is acquired in transmission mode between 4000 and 6200 cm- ¹ with a resolution of 2 cm ¹ , using a near infrared Fourier transform spectrometer Bruker Tensor 37 equipped with an InGaAs detector cooled by Peltier effect.

(2) Piezoelectric inorganic charge: BaTiCfi: average diameter of the charges: 500 nm, density 5.85 g / cm ³ sold by Inframat Advanced Materials.

(3) Carbon black: Carbon black reinforcing N347 (ASTM DI 765-2017) marketed by Cabot, having an O AN index = 124 ml / 100g measured according to standard D2414-2018 and a BET specific surface area = 85 m ² / g measured according to standard D6556-2017.

(4) Crosslinking system: “Luperox 231” (1,1-di- (t-butylperoxy) -3,3,5-trimethylcyclohexane supported at 40% by weight on calcium carbonate) marketed by the company Arkéma. Process for preparing piezoelectric compositions

The piezoelectric compositions are prepared in an internal “polylab” mixer of 85 cm ³ , filled to 70% and whose initial tank temperature is 80 ° C, the elastomeric matrix, the piezoelectric inorganic fillers and the carbon black for the composition. piezoelectric Cl. Then, thermomechanical work is carried out for 3 min at 80 revolutions / min until a maximum drop temperature of 150 ° C. is reached (non-productive phase). The mixture thus obtained is recovered, cooled and then the crosslinking system is added on an external mixer (homo-finisher) at a temperature of 25 ° C., mixing the whole in 12 cross passes (productive phase). The materials thus obtained are then calendered in the form of plates (thickness of 2 to 3 mm) and baked using a press at 150 ° C. for 20 minutes in a 330 cm ² mold under 8 tonnes.

At the end of this operation, it is quite possible to cut the laminates with a cookie cutter or any other cutting means to make a piezoelectric device with its two electrodes in the desired shape and size.

The device is then made. More precisely, parallelepipeds of 20 mm × 80 mm × 2 mm (width × length × thickness) (then also called specimens) are then cut from the plates obtained previously. To facilitate polarization and allow measurements, the specimens are metallized on both sides of larger dimensions. Metallization, here with gold, can be done manually with a lacquer, or by sputtering or any other known method. In this case, the two gold electrodes are deposited using a metallizer ("DENTON DESK V" from the company DENTON VACUUM) with an amperage of 40 mA for 25 seconds.

The device is then placed in a bath of silicone oil ("BLUESIL FLD 47V5000" sold by the company Bluestar Silicones) for the polarization step. An electric field is applied with an "MCP Lab Electronics SPN6000A" electric generator for 10 min to the two terminals of the test tube (i.e. connected to the two metallized faces). Polarization takes place at a temperature of 60 ° C. Two DC electric field strengths are used: 1V / pm (condition A) and 4V / pm (condition B). Once polarized, the test pieces are short-circuited to evacuate a maximum of residual charges.

Measurement of the piezoelectric coefficient d33: The piezoelectric coefficient (I33 makes it possible to determine the deformation power of a piezoelectric composition, this deformation taking place parallel to the axis of polarization.

The measurement of the electromechanical response of the specimens is carried out on a dynamic measurement bench. The sample is prestressed with a force of 10 N and then it is subjected to compression with a force of 5 N at a frequency of 1 Hz and at a temperature of 23 ° C.

The signal generated by the piezoelectric composition is recovered at the terminals of the sample by a specific jaw deck, then amplified and measured on an oscilloscope.

From the peak-peak voltage read on the oscilloscope, we deduce the charge Q (pC picocoulomb) released at each mechanical stress. Thus, the piezoelectric coefficient d33 (pC / N (picocoulomb / newton)) can be calculated. The coefficient d33, known to those skilled in the art, represents the piezoelectric coefficient measured by applying a stress in the direction parallel to the direction of polarization of the sample. In the case of a parallelepipedal sample, the direction of polarization corresponds to the smallest thickness (direction 3) and the application of the stress is done according to the same thickness (direction 3).

The following notation can be adopted: d ₃₃ = DR3 / Ds3, with DR3 the variation of macroscopic polarization in direction 3 and Ds3 the stress applied in direction 3. The calculation of this coefficient is done by the following formula: d33 = [ Q (pC) x thickness (m)] / [Force (N) x Length (m)] if the electrode covers the entire surface of the specimen.

Results

Table 2 shows the measurement results of piezoelectric coefficient d33 for the piezoelectric composition of the invention compared to that of the control, measured after polarization according to condition A or condition B.

[Table 2] nm = short circuit during polarization.

The results of Table 2 show that for a given polarization condition, for example condition A, the piezoelectric coefficient d33 of the piezoelectric composition C1 according to the invention is very significantly improved compared to that of the piezoelectric composition T outside the invention. This coefficient increases by at least a factor of 300% (comparison Cl versus T condition A). Carbon black therefore makes it possible, surprisingly, to improve the piezoelectric properties of a piezoelectric composition in which it is added, after polarization under the same conditions of temperature, time and intensity of the electric field as a piezoelectric composition not comprising carbon black.

Claims

1. Piezoelectric device comprising at least one piezoelectric P layer interposed between two conductive E layers, each E layer forming an electrode, characterized in that the P layer comprises at least one piezoelectric composition based on at least one elastomeric matrix comprising mainly at minus a diene elastomer, a piezoelectric inorganic filler, a carbon black and a crosslinking system and in that the rate of piezoelectric inorganic filler is greater than or equal to 5% by volume relative to the total volume of the piezoelectric composition and the rate of carbon black is greater than or equal to 6% by volume relative to the total volume of the piezoelectric composition.

2. A piezoelectric device according to claim 1, wherein the carbon black is a reinforcing carbon black.

3. Piezoelectric device according to any one of the preceding claims, in which the carbon black has an oil absorption index OAN less than or equal to 154 ml / 100g, more preferably within a range ranging from 35 to 150 ml. / 100g, more preferably still ranging from 70 to 140 ml / 100g.

4. Piezoelectric device according to any one of the preceding claims, in which the carbon black has a BET specific surface area greater than 30 m ² / g, preferably in a range ranging from 70 to 150 m ² / g, more preferably. still within a range ranging from 70 to 120 m ² / g.

5. Piezoelectric device according to any one of the preceding claims, in which the rate of piezoelectric inorganic charge is within a range ranging from 5% to 80% by volume relative to the total volume of the piezoelectric composition, preferably ranging from 6%. at 60% more preferably still ranging from 7% to 50%.

6. Piezoelectric device according to any one of the preceding claims, in which the inorganic piezoelectric filler is chosen from piezoelectric ceramics, advantageously from ferroelectric oxides, advantageously having a perovskite structure.

7. Piezoelectric device according to any one of the preceding claims, wherein the inorganic piezoelectric filler is selected from the group consisting of barium titanate, lead titanate, lead titano-zirconate, lead niobate, niobate. lithium, potassium niobate and their mixtures, more preferably the inorganic piezoelectric filler is chosen from the group consisting of barium titanate, lead titano-zirconate, potassium niobate and their mixtures.

8. Piezoelectric device according to any one of the preceding claims, in which the diene elastomer of the piezoelectric composition is chosen from the group consisting of natural rubber, ethylene-propylene-diene monomer copolymers, synthetic polyisoprenes, polybutadienes, butadiene copolymers, isoprene copolymers and mixtures of these diene elastomers.

9. A piezoelectric device according to any preceding claim, wherein the crosslinking system comprises a peroxide.

10. A piezoelectric device according to any preceding claim, wherein each layer E is a conductive metal layer.

11. Piezoelectric device according to one of claims 1 to 9, wherein each layer

E is a conductive rubber composition based on at least 50 phr of diene elastomer, a graphitized or partially graphitized carbon black and a crosslinking system.

12. A piezoelectric device according to claim 11, wherein the crosslinking system of the conductive rubber composition of layer E comprises a peroxide.

13. The piezoelectric device of claim 11 or 12, wherein the diene elastomer of the piezoelectric composition of the P layer is co-crosslinked with the diene elastomer of the conductive rubber composition of each E layer.

14. Piezoelectric device according to one of claims 11 to 13, wherein the graphitized or partially graphitized carbon black has an oil absorption index OAN greater than or equal to 155 ml / 100g.

15. A tire comprising at least one piezoelectric device according to any one of claims 1 to 14.