PIEZOELECTRIC COMPOSITE MATERIALS WITH IMPROVED PIEZOELECTRIC PROPERTIES

Description

[0001] The present invention relates to a piezoelectric composite material, its method of obtaining and the use of such a material in devices such as piezoelectric actuators and / or detectors, energy generators.

[0002] 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 insulation materials, in piezoelectric actuators and / or detectors used for example in atomic force electron microscopy or used in tires to monitor the behavior of the latter.

[0003] Piezoelectricity is a physical phenomenon that corresponds to the appearance of an electrical polarization induced by an external mechanical deformation. It 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 inverse piezoelectric effect.

[0004] A variation of the macroscopic polarization when a stress is applied to the sample characterizes the piezoelectric effect. In a system of orthogonal axes, the polarization and the stress are linked in matrix notation by a tensor of rank 2 called piezoelectric tensor d ij with i and j corresponding respectively to the axis of polarization (1, 2, 3) and of application of the stress (1, 2, 3, 4, 5, 6) as shown in the figure 1 .

[0005] Materials with piezoelectric properties are classified into three broad classes: inorganic piezoelectrics, piezoelectric polymers, and piezoelectric composite materials.

[0006] These three major classes of piezoelectric materials do not have the same properties of dielectric permittivity, remanent polarization, coercive field, etc.

[0007] The properties of inorganic piezoelectrics, such as lead titanozirconates (PTZ), are very often linked to their crystalline structure, while those of piezoelectric polymers (organic materials) come from the presence of intrinsic permanent dipoles in the monomers constituting these polymers. The best-known piezoelectric polymer is PVDF.

[0008] 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 combination results in 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 (US2015 / 134061 A1, WO2016 / 157092 A1) 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 polyethylene (Rujijanagul et al.Journal of Materials Science Letters 2001, 20, 1943-1945), a polymer matrix comprising cyanoethylated polyvinyl alcohol (EP2654094 A1) or a polymer matrix comprising a diene elastomer (WO2019 / 243750 A1).

[0009] Document US2019 / 0284423 A1 describes a piezoelectric composite comprising a piezoelectric polymer and fillers in the form of particles dispersed in the polymer, in which the filler is in the form of micro-nanoparticles.

[0010] Document US2019 / 058108 A1 discloses a device comprising a piezoelectric material and two layers each forming the electrodes. The piezoelectric material is a dielectric polymer composition obtained by polymerization of at least one vinyl monomer in the presence of an ionic liquid and document EP2071584 A1 discloses a piezoelectric device comprising a piezoelectric actuating element surrounded by electrodes and an electrolyte polymer comprising an ionic liquid and a specific copolymer block.

[0011] Piezoelectric properties are used in many applications with increasing miniaturization, which requires composite materials with increasingly high piezoelectric properties.

[0012] The present invention therefore aims to address this need and to propose a new piezoelectric composite material having improved piezoelectric properties compared to the piezoelectric composite materials of the prior art.

[0013] The applicant found that the addition of a specific compound, an ionic liquid, to a piezoelectric composite material surprisingly improved its piezoelectric properties.

[0014] Thus the invention relates to a piezoelectric composite material based on a polymer matrix and piezoelectric inorganic fillers, characterized in that said material further comprises at least one ionic liquid of general formula Q +< A -< , in which Q +< represents a cation chosen from quaternary ammonium cations and quaternary phosphonium cations and A -< represents any anion which is suitable for forming a liquid salt at a temperature below 100°C.

[0015] More particularly, the piezoelectric composite material according to the invention can advantageously have at least one of the following preferred characteristics taken alone or in combination.

[0016] Preferably, the cation Q+ is a quaternary ammonium cation chosen from the group consisting of imidazolium cations, pyrazolium cations, pyridinium cations, pyrimidinium cations, tetra-alkyl(C1-C6)ammonium cations, guanidium cations and pyrrolidium cations.

[0017] Preferably, the cation Q+ is an imidazolium cation corresponding to the formula (I) in which R1 represents an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 10 carbon atoms, optionally substituted by one or more C6-C30 aryl, thiol or hydroxyl groups or interrupted by one or more oxygen or sulfur atoms or by one or more NR' groups, R2, R3, R4, R5 and R', identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms, or a C6-C30 aryl group, optionally substituted by one or more C1-C4 alkyl groups.

[0018] Preferably, the cation Q+ is chosen from the group consisting of 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-m-methylimidazolium, 1-octyl-3-methylimidazolium, 1-decyl-3-methylimidazolium, 1,3-dibutylimidazolium and 1-butyl-2,3-dimethylimidazolium.

[0019] Preferably, the anion A -< is chosen from the group consisting of acetate, trifluoroacetate, propionate, chloride, hydroxyl, sulfate, trifluoromethanesulfonate, pentafluoromethanesulfonate and phosphates; preferably A -< is acetate or propionate.

[0020] Preferably, the level of ionic liquid is greater than or equal to 0.1 parts by weight per hundred parts by weight of polymer, preferably is within a range from 0.1 to 15 parts by weight per hundred parts by weight of polymer, more preferably from 0.5 to 12 parts by weight per hundred parts by weight of polymer, more preferably still from 1 to 10 parts by weight per hundred parts by weight of polymer.

[0021] Preferably, the piezoelectric inorganic filler is chosen from piezoelectric ceramics, advantageously chosen from ferroelectric oxides, advantageously having a perovskite structure.

[0022] Preferably, the piezoelectric inorganic filler is chosen from barium titanate, lead titanate, lead zirconate titano, lead niobate, lithium niobate, potassium niobate and mixtures thereof, more preferably the piezoelectric inorganic filler is chosen from barium titanate, lead zirconate titano, potassium niobate and mixtures thereof.

[0023] Preferably, the piezoelectric inorganic filler is chosen from the group consisting of barium titanate, potassium niobate and their mixtures.

[0024] Preferably, the size of the particles of the piezoelectric inorganic filler is in a range between 50 nm and 800 µm.

[0025] Preferably, the rate of piezoelectric inorganic charges is within a range from 5% to 80% by volume relative to the total volume of said material, more preferably from 6% to 60%, more preferably still from 7% to 50%.

[0026] Preferably, the polymer matrix comprises at least one polymer chosen from the group consisting of thermoplastic polymers, thermoplastic elastomers, thermosetting polymers, diene elastomers and mixtures of these polymers.

[0027] Preferably, the polymer matrix comprises at least one thermoplastic elastomer and / or one diene elastomer.

[0028] Preferably, the polymer matrix mainly comprises a diene elastomer.

[0029] Preferably, the polymer matrix comprises from 75 pce to 100 pce, more preferably from 90 pce to 100 pce, even more preferably from 95 to 100 pce of diene elastomer.

[0030] Preferably, the polymer matrix comprises at least one diene elastomer 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.

[0031] Preferably, the diene elastomer of the polymer matrix 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.

[0032] Preferably, the piezoelectric composite material further comprises at least one crosslinking system, preferably this crosslinking system comprises at least one peroxide.

[0033] The invention also relates to a device comprising at least one layer based on at least one piezoelectric composite material defined above and at least two electrodes arranged on either side of said layer.

[0034] Preferably, the electrodes are made of metal.

[0035] Preferably, the metal of the electrode is chosen from the group consisting of silver, gold, nickel, palladium, aluminum, copper, titanium and their mixture, preferably gold.

[0036] Preferably according to one embodiment, the electrode is flexible and is a layer E is a conductive rubber composition based on at least 50 pce of diene elastomer, a graphitized or partially graphitized carbon black and a crosslinking system.

[0037] Another object of the present invention is a tire comprising at least one piezoelectric device defined above. Another object of the present invention is the use of a piezoelectric composite material defined above for the manufacture of structural parts and supported films deposited on all or part of the surface of such a support.

[0038] Another object of the present invention relates to a method for manufacturing structural parts and supported films in which at least one piezoelectric composite material defined above is deposited on all or part of a support.

[0039] The composite material of the invention has the advantage of having improved piezoelectric properties compared to the composite materials of the prior art at iso polarization conditions. This makes it possible, for example, to improve the viability of the composite material or to use smaller piezoelectric composite materials to deliver the same electrical intensity, thus reducing production costs or miniaturizing the devices in which the piezoelectric composite material is used. Due to these improved piezoelectric properties, it is also possible to modulate the polarization step of the piezoelectric composite material of the invention to obtain the same properties as a composite material of the prior art, for example by reducing the polarization time or by reducing the electrical strength of the electric field.

[0040] There figure 1is a representation of the system of orthogonal axes for polarization, P, and stress. The indices 1, 2, 3 correspond respectively to the directions normal to the YOZ, XOZ and XOY planes and the indices 4, 5, 6 to the directions tangential to these same planes.

[0041] The invention relates to a piezoelectric composite material based on a polymer matrix and piezoelectric inorganic fillers, characterized in that said material further comprises at least one ionic liquid of general formula Q +< A -< , in which Q +< represents a cation chosen from quaternary ammonium cations and quaternary phosphonium cations and A -< represents any anion capable of forming a liquid salt at a temperature below 100°C.

[0042] In this description, any interval of values ​​designated by the expression "from a to b" represents the range of values ​​from a to b (i.e., limits a and b inclusive). Any interval "between a and b" represents the range of values ​​from more than a to less than b (i.e., limits a and b excluded).

[0043] The term "piezoelectric composite material" means a material based on at least one piezoelectric material which gives the composite material its piezoelectric activity, and based on one or more non-piezoelectric phases. This combination results in a piezoelectric composite material whose performance is increased compared to each phase alone.

[0044] The expression "piezoelectric composite material based on" means a piezoelectric composite material comprising the mixture and / or the in situ reaction product of the various basic constituents used, some of these constituents being able to react and / or being intended to react with each other, at least partially, during the various phases of manufacture of said composite, or where appropriate during subsequent curing, modifying said composite as it is initially prepared. Thus, the piezoelectric composite materials as used for the invention may be different, for example, where appropriate in the non-crosslinked state and in the crosslinked state.

[0045] The expression "consists essentially of" followed by one or more characteristics means that, in addition to the components or steps explicitly listed, components or steps which do not significantly modify the properties and characteristics of the invention may be included in the process or material of the invention.

[0046] The compounds mentioned in the description and used in the preparation of polymers or rubber compositions may be of fossil or bio-sourced origin. In the latter case, they may be, partially or totally, derived from biomass or obtained from renewable raw materials derived from biomass. This includes in particular polymers, fillers....

[0047] The piezoelectric composite material of the invention comprises a polymer matrix, i.e. a polymer or a mixture of polymers.

[0048] By "polymeric matrix" is meant the polymer or set of polymers which constitutes the non-piezoelectric phase of the piezoelectric composite material.

[0049] Advantageously, the polymer matrix comprises at least one polymer chosen from the group consisting of thermosetting polymers, thermoplastic polymers, elastomers and mixtures of these polymers.

[0050] Advantageously, the matrix of the piezoelectric composite material of the invention comprises at least one thermoplastic elastomer and / or one diene elastomer.

[0051] The choice of the nature of the polymer(s) to constitute the matrix will depend on the use of the device in which the piezoelectric composite material of the invention will be inserted. For example, when this device will be used to replace conventional batteries with a limited duration, it will be preferable to use a rigid polymer matrix, therefore for example, based on at least one thermosetting polymer or a thermoplastic polymer. On the other hand, when it is desired to use a device in a tire, a flexible matrix based on diene or thermoplastic elastomers, in particular diene elastomers, will be preferred.

[0052] Preferably, the thermosetting polymers may be chosen from polyurethanes, phenol-formaldehyde resins, urea-formaldehyde resins, melamine resins, diallylphthalates, epoxy resins, polyimides, and polyester resins. Among the polyurethanes, mention may be made of anionic, cationic, non-ionic or amphoteric polyurethanes, acrylic polyurethanes, polyvinylpyrrolidones polyurethanes, polyester polyurethanes, polyether polyurethanes, polyureas, polyurea / polyurethanes, silicone polyurethanes, and mixtures thereof.

[0053] Preferably, the thermoplastic polymers that can be used in the context of the present invention may be any polymer, copolymer or any mixture of polymers and / or copolymers having the property of being thermoplastic. Among the thermoplastic polymers, mention may in particular be made of low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), aliphatic polyamides and polyesters. Among the aliphatic polyamides, mention may in particular be made of polyamides 4-6, 6, 6-6, 11 or 12. Among the polyesters, mention may be made, for example, of PET (polyethylene terephthalate), PEN (polyethylene naphthalate), PBT (polybutylene terephthalate), PBN (polybutylene naphthalate), PPT (polypropylene terephthalate), PPN (polypropylene naphthalate).

[0054] Among the thermoplastic polymers that can be used in the context of the present invention, thermoplastic elastomers can also be used, preferably. Thermoplastic elastomers (abbreviated as "TPE") have an intermediate structure between thermoplastic polymers and elastomers. They are block copolymers, consisting of rigid, thermoplastic blocks, connected by flexible, elastomeric blocks.

[0055] The thermoplastic elastomer used for implementing the invention may be a block copolymer in which the chemical nature of the thermoplastic blocks and the elastomer blocks may vary.

[0056] As is known, TPEs have two peaks of glass transition temperature (Tg, measured according to ASTM D3418-1999), the lower temperature being relative to the elastomeric part of the TPE, and the higher temperature being relative to the thermoplastic part of the TPE. Thus, the soft blocks of TPE are defined by a Tg lower than room temperature (25°C), while the rigid blocks have a Tg higher than 80°C.

[0057] To be both elastomeric and thermoplastic in nature, the TPE must have sufficiently incompatible blocks (i.e. different due to their respective mass, polarity or Tg) to retain their own properties as elastomeric or thermoplastic blocks.

[0058] 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, in a star shape or branched. Typically, each of these segments or blocks often contains at least more than 5, generally more than 10 basic units (for example, styrene units and butadiene units for a styrene / butadiene / styrene block copolymer).

[0059] 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 500 to 5000 g / mol, these TPEs will be called multi-block TPEs hereinafter, and are a sequence of elastomer blocks - thermoplastic blocks.

[0060] The elastomer blocks of the TPE for the purposes of the invention may be any elastomer known to those skilled in the art. 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 Tg of the elastomer block of the TPE is greater than -100°C.

[0061] For the definition of thermoplastic blocks, the glass transition temperature (Tg) characteristic of the rigid thermoplastic block will be used. This characteristic is well known to those skilled in the art. In particular, it allows the industrial processing (transformation) temperature to be chosen. In the case of an amorphous polymer (or polymer block), the processing temperature is chosen to be significantly higher than the Tg. In the specific case of a semi-crystalline polymer (or polymer block), a melting temperature may be observed which is then higher than the glass transition temperature. In this case, it is rather the melting temperature (Tf) which allows the processing temperature of the polymer (or polymer block) in question to be chosen. Thus, subsequently, when we speak of "Tg (or Tf, where appropriate)", it must be considered that this is the temperature used to choose the processing temperature.

[0062] For the purposes of the invention, the TPE elastomers comprise one or more thermoplastic block(s) preferably having a Tg (or Tf, where applicable) greater than or equal to 80°C and made from polymerized monomers. Preferably, this thermoplastic block has a Tg (or Tf, where applicable) within a range varying from 80°C to 250°C. Preferably, the Tg (or Tf, where applicable) of this thermoplastic block is preferably from 80°C to 200°C, more preferably from 80°C to 180°C.

[0063] The proportion of thermoplastic blocks relative to the TPE, as defined for the implementation of the invention, is determined on the one hand by the thermoplasticity properties that said copolymer must have. The thermoplastic blocks having a Tg (or Tf, where applicable) greater than or equal to 80°C are preferably present in sufficient proportions to preserve the thermoplastic character of the elastomer according to the invention. The minimum level of thermoplastic blocks having a Tg (or Tf, where 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 capacity of the TPE to deform may also contribute to determining the proportion of thermoplastic blocks having a Tg (or Tf, where applicable) greater than or equal to 80°C.

[0064] For example, any TPE which is a copolymer whose elastomer part is saturated and comprising styrene blocks and alkylene blocks may be used for the polymer matrix of the piezoelectric material of the invention. More preferably, this TPE elastomer is chosen from the following group, consisting of diblock copolymers, 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.

[0065] According to another example, any TPE which is a copolymer whose elastomer part is unsaturated, and which comprises styrene blocks and diene blocks, these diene blocks being in particular isoprene or butadiene blocks, can be used for the polymer matrix of the piezoelectric material of the invention. More preferably, this TPE elastomer is chosen from the following group, consisting of diblock copolymers, 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.

[0066] For example, the TPE used may also be a linear or star copolymer whose elastomer part comprises a saturated part and an unsaturated part, such as styrene / butadiene / butylene (SBB), styrene / butadiene / butylene / styrene (SBBS) or a mixture of these copolymers.

[0067] Among the multi-block TPEs, copolymers containing statistical copolymer blocks of ethylene and propylene / polypropylene, polybutadiene / polyurethane (TPU), polyether / polyester (COPE), polyether / polyamide (PEBA) can be used.

[0068] It is also possible that the TPEs given as examples above are mixed together within the TPE matrix according to the invention.

[0069] Examples of commercially available TPE elastomers include SEPS, SEEPS, or SEBS type elastomers marketed by Kraton under the name “Kraton G” (products “G1650, G1651, G1654, G1730”) or Kuraray under the name “Septon” (“Septon 2007”, “Septon 4033”, “Septon 8004”); or SIS type elastomers marketed by Kuraray under the name “Hybrar 5125”, or marketed by Kraton under the name “D1161”, or linear SBS type elastomers marketed by Polimeri Europa under the name “Europrene SOL T 166” or star-shaped SBS marketed by Kraton under the name “D1184”. We can also mention the elastomers marketed by the company Dexco Polymers under the name “Vector” (“Vector 4114”, “Vector 8508”).Among the multi-block TPEs, we can cite the “Vistamaxx” TPE marketed by the Exxon company; the COPE TPE marketed by the DSM company under the name “Arnitel”, or by the Dupont company under the name “Hytrel”, or by the Ticona company under the name “Riteflex”; the PEBA TPE marketed by the Arkéma company under the name “PEBAX”; the TPU TPE marketed by the Sartomer company under the name “TPU 7840”, or by the BASF company under the name “Elastogran”.

[0070] Preferably, the polymer matrix comprises at least one diene elastomer.

[0071] By "diene elastomer (or indistinctly rubber), whether natural or synthetic, is understood in a known manner an elastomer consisting at least in part (i.e., a homopolymer or a copolymer) of diene monomer units (monomers carrying two carbon-carbon double bonds, conjugated or not).

[0072] These diene elastomers can be classified into two categories: "essentially unsaturated" or "essentially saturated". "Essentially unsaturated" generally means a diene elastomer derived at least in part from conjugated diene monomers, having a content of units or patterns of diene origin (conjugated dienes) which is greater than 15% (mol %); thus, diene elastomers such as butyl rubbers or copolymers of dienes and alpha-olefins such as EPDM do not fall within the preceding definition and can be described in particular as "essentially saturated" diene elastomers (low or very low content of patterns of diene origin, always less than 15 mol %).

[0073] As diene elastomer capable of being used in the polymer matrices of the piezoelectric composite material in accordance with the invention, the following are suitable: any homopolymer of a diene monomer, conjugated or not, having from 4 to 18 carbon atoms; any copolymer of a diene, conjugated or not, having from 4 to 18 carbon atoms and at least one other monomer.

[0074] The other monomer in a diene copolymer can be ethylene, an olefin, or a diene, conjugated or not.

[0075] Suitable conjugated dienes are conjugated dienes having 4 to 12 carbon atoms, in particular 1,3-dienes, such as 1,3-butadiene and isoprene.

[0076] Suitable non-conjugated dienes are non-conjugated dienes with 6 to 12 carbon atoms, such as 1,4-hexadiene, ethylidene norbornene, dicyclopentadiene.

[0077] Suitable olefins are vinylaromatic compounds with 8 to 20 carbon atoms and aliphatic α-monoolefins with 3 to 12 carbon atoms.

[0078] Suitable vinyl aromatic compounds are, for example, styrene, ortho-, meta-, para-methylstyrene, the commercial mixture "vinyl-toluene", para-tert-butylstyrene.

[0079] Suitable aliphatic α-monoolefins are, in particular, acyclic aliphatic α-monoolefins having from 3 to 18 carbon atoms.

[0080] More specifically, 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 conjugated dienes with each other or with one or more vinylaromatic 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 their mixture such as for example elastomers obtained from ethylene, propylene with a non-conjugated diene monomer of the aforementioned type.

[0081] 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 blends of these diene elastomers. Among the isoprene copolymers, mention will be made in particular of isobutene-isoprene (butyl rubber - IIR), isoprene-styrene (SIR), isoprene-butadiene (BIR) or isoprene-butadiene-styrene (SBIR) copolymers. Among the butadiene copolymers, butadiene-styrene (SBR), isoprene-butadiene (BIR) or isoprene-butadiene-styrene (SBIR) copolymers are particularly suitable. Among the diene and alpha-olefin copolymers, ethylene-propylene-diene monomer (EPDM) copolymers are preferred.

[0082] Even more preferably, 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.

[0083] Even more preferably, the diene elastomer is a styrene-butadiene copolymer. Particularly suitable are butadiene-styrene copolymers 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 in the butadiene part of between 4% and 75%, a content (mol%) of trans-1,4 bonds of between 10% and 80%.

[0084] The diene elastomer can be modified, i.e. either coupled and / or star-shaped, or functionalized, or coupled and / or star-shaped and simultaneously functionalized.

[0085] Thus, the diene elastomer can be coupled and / or star-shaped, for example by means of a silicon or tin atom which links the elastomer chains together.

[0086] The diene elastomer may be simultaneously or alternatively functionalized and comprise at least one 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, secondary or tertiary amine, cyclic or not, a thiol, an epoxide.

[0087] 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, in which case we will say 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, in which case we will say that the diene elastomer is coupled or functionalized in the middle of the chain, as opposed to the "end of the chain" position and although the group is 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-shaped or branched.

[0088] The diene elastomer can contain several functional groups, pendant or not, distributed along the main chain of the elastomer, we will then say that the diene elastomer is functionalized along the chain.

[0089] The polymer matrix may contain a single elastomer, in particular diene, or a mixture of several elastomers, in particular diene and / or thermoplastic.

[0090] According to one embodiment of the invention, if several polymers are used in the polymer matrix of the piezoelectric composite material of the invention, the diene elastomer(s) constitute the majority fraction by weight; they then represent more than 50% by weight, more preferably at least 75% by weight relative to the total weight of the polymers in the rubber composition. Also preferably, the diene elastomer(s) represent at least 90%, or even 95% (in particular 100%) by weight relative to the total weight of the polymers in the rubber composition.

[0091] Preferably, the polymer matrix comprises from 75 to 100 parts by weight of diene elastomer relative to 100 parts by weight of polymers, more advantageously from 90 to 100 parts by weight of diene elastomer relative to 100 parts by weight of polymers, even more advantageously from 95 to 100 parts by weight of diene elastomer relative to 100 parts by weight of polymers. In a preferred variant of the invention, the diene elastomer, or the mixture of diene elastomers, is the only elastomer of the polymer matrix.

[0092] The piezoelectric composite material of the invention is also based on at least one piezoelectric inorganic filler. These piezoelectric inorganic fillers can advantageously be dispersed in the polymer matrix, advantageously in the form of particles not bound to the polymer matrix.

[0093] By "particles not bound to the polymer matrix" is meant particles without covalent bonds between the piezoelectric inorganic filler and the polymer(s) constituting the polymer matrix of the piezoelectric composite material.

[0094] By "particles dispersed in the polymer matrix" is meant that the piezoelectric inorganic filler used in the context of the present invention is distributed substantially uniformly in the polymer matrix of the piezoelectric composite material. Thus, the average distance separating the particles of these adjacent fillers is substantially constant throughout the entire volume of said polymer matrix.

[0095] These piezoelectric inorganic fillers can be piezoelectric single crystals or piezoelectric ceramics.

[0096] Piezoelectric single crystals are mainly natural piezoelectric materials such as quartz or tourmaline. These crystals, ferroelectric, can have a domain structure. We can distinguish single-domain single crystals and polydomain single crystals depending on whether one or more polarization directions coexist in the crystal.

[0097] Advantageously, the piezoelectric inorganic filler used in the context of the invention can be chosen from piezoelectric ceramics.

[0098] 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 crystal lattice 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 possessing an electric polarization in the spontaneous state, which can also be reversed by the application of a sufficiently intense external electric field.

[0099] Advantageously, the piezoelectric inorganic filler can be chosen from ferroelectric oxides.

[0100] Ferroelectric oxides can in particular have a perovskite structure. They advantageously correspond to a general formula ABO 3 such as barium titanate (BaTiO 3 ), lead titanate (PbTiO 3 ), potassium niobate (KNbO 3 ), lead niobate (PbNbO 3 ), or bismuth ferrite (BiFeO 3 ). In this family of piezoelectric materials, we can also cite lead zirconate titanoate (PZT) with a Pb(Zr x Ti 1-x )O 3 structure in which x is between 0 and 1. It can be in pure form or in the form of a semiconductor doped either with acceptor dopants (to give a so-called hard PZT), such as Fe, Co, Mn, Mg, Al, In, Cr, Sc, Na or K, or with donor dopants (to give a so-called soft PZT), such as La, Nd, Sb, Ta, Nb or W.

[0101] Advantageously, the piezoelectric inorganic fillers usable in the context of the invention have a perovskite structure.

[0102] By way of non-limiting examples, the piezoelectric inorganic filler usable in the context of the invention may be chosen from the group consisting of barium titanate, lead titanate, lead zirconate titano (PZT), lead niobate, lithium niobate, potassium niobate and mixtures thereof.

[0103] The best-known piezoelectric ceramics are barium titanate (BaTiO 3 ) and lead zirconate titanate (PZT), which have a very good electromechanical coefficient and are manufactured using a variety of processes. These processes (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.

[0104] Advantageously, the piezoelectric inorganic filler may be chosen from barium titanate, potassium niobate, lead zirconate titanate and mixtures thereof. Even more preferably, the piezoelectric inorganic filler is chosen from barium titanate, potassium niobate and mixtures thereof.

[0105] In particular, piezoelectric inorganic fillers have particle sizes between 50 nm and 800 µm. The particle size corresponds to the average particle diameter. The average diameter is measured using scanning electron microscopy (SEM). Images are taken of powder samples. Image analysis is performed using software and provides access to the average diameter of the particles present.

[0106] The volume of piezoelectric inorganic fillers used in the material of the invention will depend on the use of said material.

[0107] Preferably, the rate of piezoelectric inorganic filler in the composite material of the invention is greater than or equal to 5% by volume, in particular greater than or equal to 6% by volume, more preferably still greater than or equal to 7% by volume relative to the total volume of the piezoelectric composite material.

[0108] More preferably still, the piezoelectric inorganic filler content is within a range of 5% to 80% by volume relative to the total volume of the piezoelectric composite material, advantageously within a range of 6% to 60% by volume, more advantageously still from 7% to 50% by volume.

[0109] The piezoelectric composite material of the invention comprises at least one ionic liquid.

[0110] For the purposes of the present invention, the term "ionic liquid" means a non-aqueous ionized solvent which is liquid at a temperature below 100°C, i.e. a non-aqueous liquid substance for a temperature below 100°C (at atmospheric pressure) characterized by a dissociation of neutral species into various cations and anions.

[0111] Ionic liquids (called "Room Temperature Ionic Liquid" or "RTIL" in English) are therefore highly ionized molten non-aqueous salts which have a melting temperature of less than or equal to 150°C, preferably less than 100°C. The melting temperature is measured by differential scanning calorimetry, with a temperature rise rate of 10°C / minute, the melting temperature is then at a temperature corresponding to the top of the endothermic melting peak obtained during the measurement.

[0112] Ionic liquids should not be confused with salts, such as NaCl, NH 4 Cl, MgSO 4 etc... which are often solid at room temperature and dissolve in a solvent, especially in an aqueous solvent such as water. Ionic liquids have been described in more detail in the following documents: "Ionic Liquids, Physicochemical properties", Suojiang Zhang Xingmei Lu Qing Zhou Xiaohua Li Xiangping Zhang Shucai Li, Elsevier Science (2009); "Ionic liquids "in a nutshell'' (history, properties and development)", Pernak J., Rzemieniecki T., Materna K.,CHEMIK, 70 (9) 471-480 (2016); and in "An Introduction to Ionic Liquids", Freemantle, M., Royal Society of Chemistry; 1st edition (2010).

[0113] The ionic liquid can therefore be produced by combining a mineral or organic anion and a mineral or organic cation. Preferably, an organic molecule constitutes the cation of the ionic liquid and the anion of the ionic liquid can be mineral or organic.

[0114] The ionic liquid used in the context of the present invention is an ionic liquid of general formula Q +< A -< , in which Q +< represents a cation chosen from quaternary ammonium cations, quaternary phosphonium cations and A -< represents any anion capable of forming a liquid salt at a temperature less than or equal to 100°C, and advantageously less than or equal to 85°C, more preferably still less than or equal to 50°C.

[0115] The quaternary ammonium cations and the quaternary phosphonium cations Q+ preferably correspond to the general formulas N +< R a< R b< R c< R d< and P +< R a< R b< R c< R d< , or to the general formulas R a< R b< N +< =CR c< R d< and R a< R b< P +< =CR c< R d< where R a< , R b< , R c< and R d< , which may be identical or different, represent a hydrogen atom (with the exception of the NH4 +< cation for N +< R a< R b< R c< R d< , preferably a single substituent of R a< , R b< , R c< and R d< represents hydrogen), or hydrocarbyl radicals having from 1 to 30 carbon atoms. These hydrocarbyl radicals may be, for example, saturated or unsaturated C1-C30 alkyls, C3-C30 cycloalkyls, C5-C30 aromatic rings. These hydrocarbyl radicals may be substituted or not.

[0116] The substituents R a< , R b< , R c< and R d< can also form a nitrogen and / or phosphorus heterocycle ring containing 1, 2 or 3 nitrogen and / or phosphorus atoms, with general formulas in which the rings consist of 4 to 10 atoms, preferably 5 to 6 atoms, R a< and R b< are defined as previously.

[0117] Preferably, the ionic liquid(s) used in the context of the present invention has a quaternary ammonium cation Q+ chosen from the group consisting of imidazolium cations, pyrazolium cations, pyridinium cations, pyrimidinium cations, tetra-alkyl(C1-C6)ammonium cations, guanidium cations and pyrrolidium cations.

[0118] Imidazolium cations preferentially correspond to the following general formula (I): in which: R1 represents an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 10 carbon atoms, optionally substituted by one or more C6-C30 aryl, thiol or hydroxyl groups or interrupted by one or more oxygen or sulfur atoms or by one or more NR' groups, R2, R3, R4, R5 and R', identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms, or a C6-C30 aryl group, optionally substituted by one or more C1-C4 alkyl groups.

[0119] More preferably still, in formula (I), R1 represents an alkyl group comprising from 1 to 10 carbon atoms and R2, R3, R4, and R5, identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms.

[0120] More preferably still, in formula (I), R1 represents an alkyl group comprising from 1 to 10 carbon atoms; R2 and R3, identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 10 carbon atoms, preferably from 1 to 5 carbon atoms; R4 and R5 represent a hydrogen atom.

[0121] The imidazolium cations of general formula (I), more particularly preferred, are chosen from 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-m-methylimidazolium, 1-octyl-3-methylimidazolium, 1-decyl-3-methylimidazolium, 1,3-dibutylimidazolium and 1-butyl-2,3-dimethylimidazolium.

[0122] The ionic liquid(s) have an anion A -< capable of forming a liquid salt at a temperature less than or equal to 100°C, and advantageously less than or equal to 85°C, more preferably still less than or equal to 50°C. This anion may be a mineral anion or an organic anion.

[0123] More preferably, the anion A -< may be chosen from the group consisting of chloride anions (Cl -< ), bromide anions (Br -< ), tetrachloroaluminate anions (AlCl 4 -< ), tetrachloronickel anions (NiCl 4 -< ), perchlorate anions (ClO 4 -< ), nitrate anions (NO 3 -< ), nitrite anions (NO 2 -< ), sulfate anions (SO 4 -< ), methyl sulfate anions (CH 3 SO4 -< ), tetrafluoroborate anions (BF 4 -< ), phosphate anions (PO 4 3-< ), hexafluorophosphate anions (PF 6 -< ), dibutylphosphate anions and tris (pentafluoroethyl) trifluorophosphate anions, hexafluoroantimonate anions (SbF 6 -< ), triflate anions [NfO] (CF 3 SO 2 -< ), nonaflate anions [NfO] (CF 3 (CF 2 ) 3 SO 2 -< ), bis(trifyl)amide anions [Tf2N] (CF 3 SO 2 ) 2 N -< ), trifluoroacetate anions [TA] (CF 3 CO 2 -< ), heptafluorobutanoate [HB] (CF 3 (CF 2 ) 3 CO 2 -< ), acetate anions (CH 3 CO 2 -< ), trifluoroacetate anions (CF 3 CO 2 -< ),trifluoromethanesulfonate anions (CF 3 SO 3 -< ), dicyanamide anions, hydroxide anions (OH -< ), salicylate anions, L-(+)lactate anions, propionate anions (CH 3 CH 2 CO 2 -< ) and pentafluoromethanesulfonate anions.,

[0124] More preferably still, the anion A -< is chosen from the group consisting of acetate, trifluoroacetate, propionate, chloride, hydroxyl, sulfate, trifluoromethanesulfonate, pentafluoromethanesulfonate and phosphates; preferably the anion A -< is acetate or propionate.

[0125] More preferably still, the ionic liquid is of formula Q +< A -< in which Q +< is chosen from the group consisting of imidazolium cations, pyrazolium cations, pyridinium cations, pyrimidinium cations, tetra-alkyl(C1-C6)ammonium cations, guanidium cations and pyrrolidium cations and A -< is chosen from the group consisting of acetate, trifluoroacetate, propionate, chloride, hydroxyl, sulfate, trifluoromethanesulfonate, pentafluoromethanesulfonate and phosphates.

[0126] More preferably still, the ionic liquid is of formula Q +< A -< in which Q +< is chosen from imidazolium cations, preferably imidazolium cations of formula (I) and A -< is chosen from the group formed by acetate, trifluoroacetate and propionate.

[0127] In a non-exhaustive manner, the ionic liquid(s) used according to the invention may be chosen from the following compounds: 1-ethyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium bromide, 1-butyl-3-methylimidazolium chloride, 1-hexyl-3-methylimidazolium chloride, 1-methyl-3-octylimidazolium chloride, 1-decyl-3-methylimidazolium chloride, 1-decyl-3-methylimidazolium bromide, 1-dodecyl-3-methylimidazolium chloride, 1-methyl-3-tetradecylimidazolium chloride, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-pentyl-3-methylimidazolium tetrafluoroborate, 1-hexyl-3-methylimidazolium tetrafluoroborate, 1-heptyl-3-methylimidazolium tetrafluoroborate, 1-octyl-3-methylimidazolium tetrafluoroborate, 1-nonyl-3-methylimidazolium tetrafluoroborate, 1-decyl-3-methylimidazolium tetrafluoroborate,1-hexyl-3-ethylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, -butyl-3-methylimidazolium hexafluorophosphate, 1-pentyl-3-methylimidazolium hexafluorophosphate, 1-hexyl-3-methylimidazolium hexafluorophosphate, 1-heptyl-3-methylimidazolium hexafluorophosphate, 1-octyl-3-methylimidazolium hexafluorophosphate, 1-nonyl-3-methylimidazolium hexafluorophosphate, 1-decyl-3-methylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium methylsulfate, 1-methyl-3-butylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium nitrate, nitrite 1-ethyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium sulfate, 1-ethyl-3-methylimidazolium triflates, 1-ethyl-3-methylimidazolium nonaflates, 1-ethyl-3-methylimidazolium bis(trifyl)amide, trihexyl tetradecyl phosphonium chloride, tributyl tetradecyl phosphonium chloride,1-ethyl-3-methylimidazolium trifluoroacetate. 1-hexyl-2,3-dimethylimidazolium chloride, 1-ethyl-2,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium dicyanamide, 1-ethyl-3-methylimidazolium methylphosphonate and 1-butyl-3-methylimidazolium acetate.,

[0128] More preferably still, the ionic liquid is of formula Q +< A -< in which Q +< is selected from the group consisting of 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-m-methylimidazolium, 1-octyl-3-methylimidazolium, 1-decyl-3-methylimidazolium, 1,3-dibutylimidazolium and 1-butyl-2,3-dimethylimidazolium and A -< is selected from the group consisting of acetate, trifluoroacetate, propionate, chloride, hydroxyl, sulfate, trifluoromethanesulfonate, pentafluoromethanesulfonate and phosphates.

[0129] More preferably still, the ionic liquid is chosen from 1-ethyl-3-methylimidazolium trifluoroacetate, 1-butyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium trifluoroacetate.

[0130] A person skilled in the art knows how to adapt the level of ionic liquid in the piezoelectric composite material of the invention as a function of the electrical strength of the electric field used during the polarization phase of said composite material.

[0131] Preferably, the level of ionic liquid in the piezoelectric composite material is greater than or equal to 0.1 parts by weight per hundred parts by weight of polymer, preferably is within a range from 0.1 to 15 parts by weight per hundred parts by weight of polymer, more preferably from 0.5 to 12 parts by weight per hundred parts by weight of polymer, more preferably still from 1 to 10 parts by weight per hundred parts by weight of polymer.

[0132] Ionic liquids are commercially available from suppliers such as Sigma-Aldrich.

[0133] When the polymer matrix comprises at least one elastomer, in particular thermoplastic and / or diene, the crosslinking system may be any type of known system. It may in particular be based on sulfur, and / or peroxide and / or bismaleimides.

[0134] According to a preferred embodiment, the crosslinking system is sulfur-based, in which case it is referred to as a vulcanization system. The sulfur can be provided in any form, in particular in the form of molecular sulfur, or a sulfur-donating agent. At least one vulcanization accelerator is also preferably present, and, optionally, also preferably, various known vulcanization activators can be used, such as zinc oxide, stearic acid or any equivalent compound such as stearic acid salts and transition metal salts, guanidine derivatives (in particular diphenylguanidine), or even known vulcanization retarders.

[0135] When present, sulfur is used at a preferred rate of between 0.5 and 12 parts by weight per hundred parts by weight of polymer, in particular between 1 and 10 parts by weight per hundred parts by weight of polymer. The vulcanization accelerator is used at a preferred rate of between 0.5 and 10 parts by weight per hundred parts by weight of polymer, more preferably between 0.5 and 5.0 parts by weight per hundred parts by weight of polymer.

[0136] Any compound capable of acting as an accelerator for the vulcanization of diene elastomers in the presence of sulfur may be used as an accelerator, in particular accelerators of the thiazole type and their derivatives, accelerators of the sulfenamide, thiuram, dithiocarbamate, dithiophosphate, thiourea and xanthate types. Examples of such accelerators include, but are not limited to, the following compounds: 2-mercaptobenzothiazyl disulfide (abbreviated as "MBTS"), N-cyclohexyl-2-benzothiazyl sulfenamide ("CBS"), N,N-dicyclohexyl-2-benzothiazyl sulfenamide ("DCBS"), N-tert-butyl-2-benzothiazyl sulfenamide ("TBBS"), N-tert-butyl-2-benzothiazyl sulfenimide ("TBSI"), tetrabenzylthiuram disulfide ("TBZTD"), zinc dibenzyldithiocarbamate ("ZBEC") and mixtures of these compounds.

[0137] 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 comprise a vulcanization system, that is to say a sulfur-based crosslinking system.

[0138] The peroxide that can be used in the context of the invention can be any peroxide known to those skilled in the art.

[0139] Preferably, the peroxide is chosen from organic peroxides.

[0140] By "organic peroxide" is meant an organic compound, i.e. containing carbon, comprising an -OO- group (two oxygen atoms linked by a single covalent bond).

[0141] During the crosslinking process, the organic peroxide decomposes at its unstable OO bond into free radicals. These free radicals allow the creation of crosslinking bonds.

[0142] According to one embodiment, the organic peroxide is selected from the group consisting of dialkyl peroxides, monoperoxycarbonates, diacyl peroxides, peroxyketals, peroxyesters and mixtures thereof.

[0143] 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, α,α'-di-[(t-butyl-peroxy)isopropyl]benzene, α,α'-di-[(t-amyl-peroxy)isopropyl]benzene, di-t-amyl peroxide, 1,3,5-tri-[(t-butylperoxy)isopropyl]benzene, 1,3-dimethyl-3-(t-butylperoxy)butanol, 1,3-dimethyl-3-(t-amylperoxy)butanol and mixtures thereof.

[0144] Certain monoperoxycarbonates such as OO-tert-butyl-O-(2-ethylhexyl) monoperoxycarbonate, OO-tert-butyl-O-isopropyl monoperoxycarbonate, OO-tert-amyl-O-2-ethyl hexyl monoperoxycarbonate and mixtures thereof may also be used.

[0145] Among the diacyl peroxides, the preferred peroxide is benzoyl peroxide.

[0146] Among the peroxyketals, the preferred peroxides are selected from the group consisting of 1,1-di-(t-butylperoxy)-3,3,5-trimethylcyclohexane, n-butyl 4,4-di-(t-butylperoxy)valerate, 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 trimer cyclic peroxide), 3,3,5,7,7-pentamethyl-1,2,4-trioxepane, n-butyl 4,4-bis(t-amylperoxy)valerate, ethyl 3,3-di(t-amylperoxy)butyrate, 1,1-di(t-butylperoxy)cyclohexane, 1,1-di(t-amylperoxy)cyclohexane and mixtures thereof.

[0147] Preferably, the peroxyesters are selected from the group consisting of tert-butylperoxybenzoate, tert-butylperoxy-2-ethylhexanoate, tert-butylperoxy-3,5,5-trimethylhexanoate and mixtures thereof.

[0148] Particularly preferably, the organic peroxide is selected 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, OO-(t-butyl)-O-(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 thereof, still more preferably selected from the group consisting of dicumyl peroxide, n-butyl-4,4'-di(tert-butylperoxy)-valerate, OO-(t-butyl) O-(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 thereof.,

[0149] When present, the total level of peroxide in the composition is preferably greater than or equal to 0.3 parts by weight per hundred parts by weight of polymer, more preferably greater than or equal to 0.75 parts by weight per hundred parts by weight of polymer, preferably within a range from 0.5 to 5 parts by weight per hundred parts by weight of polymer, in particular from 0.5 to 3 parts by weight per hundred parts by weight of polymer.

[0150] The piezoelectric composite material of the invention can be manufactured by any known technique.

[0151] In general, the dispersion of the piezoelectric inorganic charges is carried out in the polymer matrix in the presence of the ionic liquid, the material is optionally crosslinked when the polymer matrix comprises at least one elastomer, in particular thermoplastic and / or diene, then the composite material is placed in an electric field suitable for allowing polarization of the piezoelectric inorganic charges and thus obtaining the piezoelectric composite material.

[0152] The dispersion of the piezoelectric inorganic fillers in the polymer matrix in the presence of the ionic liquid 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.

[0153] The step of polarizing the inorganic charges is carried out by means known and adapted for converting the piezoelectric inorganic charges into charges exhibiting piezoelectric properties on a macroscopic scale. The step of polarizing the device therefore corresponds to the application of an electric field to the terminals of the electroactive device to orient the dipoles of the piezoelectric charges in the same direction in order to obtain a macroscopic polarization of the composite material.

[0154] Polarization depends on the polarization temperature, the applied electric field, and the polarization time.

[0155] Advantageously, the polarization temperature is at least 5°C lower than the lowest Curie temperature, Tc, of the piezoelectric inorganic fillers, more advantageously at least 7°C lower, even more advantageously at least 10°C lower than the lowest Curie temperature, Tc, of the piezoelectric inorganic fillers. Indeed, close to the Curie temperature of the piezoelectric inorganic fillers, the agitation of the dipoles makes it more difficult to align them under an electric field.

[0156] The Curie temperature, Tc, of a piezoelectric material is the temperature at which the material becomes paraelectric. Thus, the hysteresis loop characteristic 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.

[0157] In particular, the electric field applied during the polarization step can be in a range from 0.1 to 10 kV / mm, advantageously from 0.5 to 5 kV / mm.

[0158] In particular, the applied electric field depends on the nature of the piezoelectric inorganic charge and the polarization time of the piezoelectric composite and the level of ionic liquid present in the material. A person skilled in the art knows how to adapt the electric field to the piezoelectric charge and the polarization time.

[0159] In particular, the polarization duration may be between 1 minute and 10 hours, preferably between 5 minutes and 2 hours.

[0160] Advantageously, the piezoelectric composite material according to the invention has 0-3 connectivity, comprising particles of piezoelectric fillers dispersed in the polymer matrix. The connectivity depends on the spatial organization of each phase constituting the composite material. A change in connectivity leads to major modifications in the physical properties of the 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 phase considered. Thus, a composite with 0-3 connectivity corresponds to a composite consisting of grains of piezoelectric powder dispersed in the polymer matrix. The main advantage of this type of composite is the ease of implementing the process, or the ease of producing complex shapes, such as curved surfaces.

[0161] The invention also relates to a device comprising at least one layer based on at least one piezoelectric composite material defined above and at least two conductive electrodes, in particular arranged on either side of said layer.

[0162] The electrodes are known to those skilled in the art. They are deposited on the faces of the piezoelectric composite material of the invention in order to collect the electrical charges emitted by said material. They may be opposite each other. Their dimensions are adapted to the intended application.

[0163] The electrodes may be made of metal or metal oxide in the form of a thin deposit of these metals or these metal oxides. For example, the electrodes may 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 may be carried out by a so-called additive process, for example by direct printing of a fluid or viscous composition comprising the metal or metal oxide composing the electrodes, at the desired locations, for example by inkjet printing, gravure printing, screen printing, flexography, spray coating, drop-casting or by chemical vapor deposition.The formation of the metal or metal oxide electrodes may correspond to a subtractive process, in which the material composing the electrodes is deposited on the entire piezoelectric composite material and in which the unused portions are then removed by photolithography or laser ablation for example. Depending on the metallic material considered, the deposition on the entire piezoelectric composite material of the invention may be carried out by liquid means, by cathode sputtering or by evaporation.

[0164] The electrodes may be flexible, such as a mixture of thermoplastic or diene elastomers made conductive, or such as a conductive polymer such as poly(3,4-ethylenedioxythiopene).

[0165] As flexible electrodes, it is possible, for example, to use a layer E comprising a conductive rubber composition based on at least 50 pce of diene elastomer, a crosslinking system and a graphitized or partially graphitized carbon black.

[0166] The diene elastomers suitable for the conductive rubber composition of the E-layer are those previously described for the piezoelectric composite material. The diene elastomer of the conductive rubber composition of the E-layer may be the same as or different from that of the piezoelectric composite material of the invention, preferably it is the same as the piezoelectric composite material of the invention.

[0167] The crosslinking system suitable for the conductive rubber composition of the E-layer is that previously described for the piezoelectric composite material of the invention. The crosslinking system of the conductive rubber composition of the E-layer may be identical to or different from that of the piezoelectric composite material of the invention, preferably it is identical to that of the piezoelectric composite material of the invention.

[0168] Graphitized or partially graphitized carbon black is any graphitized or partially graphitized carbon black having an oil absorption index OAN greater than or equal to 155 ml / 100 g, more preferably greater than or equal to 160 ml / 100 g.

[0169] Preferably, the graphitized or partially graphitized carbon black may have a particle size in a range of 50 to 500 µm. The amount of graphitized or partially graphitized black in the conductive rubber composition of layer E is in a range of 10% to 40% by volume, preferably 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 comprise a piezoelectric inorganic filler.

[0170] The conductive rubber composition of the E-layer is manufactured in a suitable 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 kneading phase (sometimes referred to as a “non-productive” phase) at high temperature where the constituents of the E-layer are mixed, with the exception of the crosslinking system, up to a maximum temperature of between 80°C and 190°C, preferably between 80°C and 150°C, followed by a second mechanical working phase (sometimes referred to as a “productive” phase) at a lower temperature, typically below 80°C, for example between 60°C and 80°C, a 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 E-layer.

[0171] The device of the invention can be manufactured by any means known to those skilled in the art. The E layers forming the electrodes are deposited on the faces of the piezoelectric composite material of the invention in order to collect the electrical charges emitted by said material. The device according to the invention is advantageously connected to an electronic component in order to capture the emitted electrical pulses and to use this information.

[0172] As an example of manufacturing the device with flexible E layers, the following protocol may be cited: the E layers and the piezoelectric composite material may be manufactured separately as explained above, then the piezoelectric composite material 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 optionally crosslinked to obtain the device according to the invention. The layers may be successively deposited in a suitable mold, called a curing mold, which may have any size. A pressure ranging from 1,000,000 to 20,000,000 Pa may be used, 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 may for example last from 5 min to 90 min.Crosslinking can be carried out by baking, 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 in particular on the baking temperature, the crosslinking system adopted and the crosslinking kinetics of the compositions considered. The pressurization and crosslinking steps can be simultaneous. For example, when the layers are deposited in a baking mold, this mold can be placed in a platen press where the assembly will be baked under pressure.

[0173] When the E layers are metallic layers, the piezoelectric composite material 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.

[0174] The method for preparing the device of the invention may advantageously comprise a polarization step described above.

[0175] The device is advantageously connected to an electronic organ in order to capture the electrical impulses emitted and to use this information.

[0176] The device according to the invention and mentioned above can be used in association with a sensor.

[0177] The invention also extends to the use of such a piezoelectric composite material defined above for the manufacture of structural parts and supported films deposited on all or part of the surface of such a support.

[0178] In particular, the piezoelectric composite material according to the invention and / or the device according to the invention are used to enable the detection of a mechanical stress on the surface of said composite material by direct piezoelectric effect.

[0179] 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.

[0180] The invention also relates to a tire comprising the above-mentioned device comprising the piezoelectric composite material according to the invention and electrodes. In particular, said device can be fixed for example on the inner sealed 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 the whitewash, the use of cold vulcanization or even the fusion of TPE. The fixing can be done by adhesive bonding.

[0181] Other advantages will become apparent upon reading the following description, which refers to the examples given without limitation. Examples:

[0182] The examples presented below are intended to compare the piezoelectric properties of the piezoelectric composite materials M1, M2 and M3, in accordance with the invention, with respect to a non-compliant piezoelectric composite material T.

[0183] Unless otherwise stated, the contents of the various constituents of the piezoelectric composite materials presented in Table 1 are expressed in parts by weight per 100 parts by weight of polymer. All composite materials have a content of piezoelectric inorganic fillers of 33% by volume relative to the total volume of the composite material. [Table 1] Material T M1 M2 M3 Polymeric matrix (1) 100,00 100,00 100,00 100,00 Piezoelectric inorganic filler (2) 320,10 322,80 326,30 334,60 Ionic liquid (3) (-) 1,00 2,00 5,00 Crosslinking system (4) 0,75 0,75 0,75 0,75 (1) Polymer matrix: Solution-polymerized styrene-butadiene copolymer (SBR) (S-SBR), non-functional, non-extended. Its microstructure is as follows: 24 mol% of 1,2-polybutadiene units relative to the butadiene part and 26.5 wt% 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 ASTM D3418 of 1999. The microstructure of S-SBR (relative distribution of 1,2-vinyl, 1,4-trans and 1,4-cis butadiene units) 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 uses a multivariate calibration [Vilmin, F.; Dussap, C.; Coste, N.Applied Spectroscopy 2006, 60, 619-29] using standard elastomers with a composition determined by 13< C NMR. The styrene content and microstructure are then calculated from the NIR spectrum of an elastomer film approximately 730 µm thick. The spectrum is acquired in transmission mode between 4000 and 6200 cm- 1< with a resolution of 2 cm -1< , using a Bruker Tensor 37 Fourier transform near infrared spectrometer equipped with a Peltier-cooled InGaAs detector. (2) Piezoelectric inorganic filler: BaTiO 3: average filler diameter: 500 nm, density 5.85 g / cm 3< marketed by Inframat Advanced Materials. (3) Ionic liquid: “ENIM Ac” (1-ethyl-3-methylimidazolium acetate) marketed by the company Sigma-Aldrich.Melting temperature = -20°C measured as described in the description (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. Preparation process of piezoelectric composite material

[0184] The piezoelectric composite materials are prepared in an internal “polylab” mixer of 85 cm 3< , filled to 70% and whose initial tank temperature is 80 ° C, the polymer matrix, the piezoelectric inorganic fillers and the ionic liquid for the composite materials M1, M2 and M3. Then, a thermomechanical work is carried out for 3 min at 80 rpm until reaching a maximum drop temperature of 150 ° C (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 crossed passes (productive phase). The materials thus obtained are then calendered into plates (thickness of 2 to 3 mm) and cooked using a press at 150°C for 20 minutes in a 330 cm 2 mold under 8 tonnes.

[0185] After this operation, it is quite possible to cut the laminates with a die cutter or any other cutting means to create a piezoelectric device with its two electrodes in the desired shape and size.

[0186] The device is then made. More precisely, parallelepipeds of 20 mm × 80 mm × 2 mm (width × length × thickness) (later also called test pieces) are then cut from the plates obtained previously. To facilitate polarization and allow measurements, the test pieces are metallized on the two larger sides. The metallization, here with gold, can be carried out manually with a lacquer, or by cathode sputtering or any other known method. In this case, the two gold electrodes are deposited using a metalizer ("DENTON DESK V" from the company DENTON VACUUM) with an amperage of 40 mA for 25 seconds.

[0187] The device is then placed in a silicone oil bath (“BLUESIL FLD 47V5000” marketed by Bluestar Silicones) for the polarization step. An electric field is applied with an electric generator “MCP Lab Electronics SPN6000A” for 10 min to the two terminals of the test piece (i.e. connected to the two metallized faces). Polarization is carried out at a temperature of 60°C. Two continuous electric field intensities are used: 1V / µm (condition A) and 4V / µm (condition B). Once polarized, the test pieces are short-circuited to evacuate as much residual charge as possible. Measurement of the piezoelectric coefficient d 33:

[0188] The piezoelectric coefficient d 33 makes it possible to determine the deformation power of a composite material, this deformation taking place parallel to the polarization axis.

[0189] The electromechanical response of the specimens is measured on a dynamic measurement bench. The sample is prestressed with a force of 10 N and then subjected to compression with a force of 5 N at a frequency of 1 Hz and a temperature of 23°C.

[0190] The signal generated by the piezoelectric composite material is recovered at the terminals of the sample by a specific jaw attachment, then amplified and measured on an oscilloscope.

[0191] From the peak-peak voltage read on the oscilloscope, the charge Q (pC picocoulomb) released at each mechanical stress is deduced. Thus the piezoelectric coefficient d 33 (pC / N (picocoulomb / newton)) can be calculated. The coefficient d 33 , known to those skilled in the art, represents the piezoelectric coefficient measured by applying a stress in the direction parallel to the polarization direction of the sample. In the case of a parallelepiped sample, the polarization direction corresponds to the smallest thickness (direction 3) and the application of the stress is done according to the same thickness (direction 3).

[0192] The following notation can be adopted: d 33 = ΔP 3 / Δσ 3 , with ΔP3 the variation of macroscopic polarization in direction 3 and Δσ3 the stress applied in direction 3.

[0193] The calculation of this coefficient is done using the following formula: d 33 = Q pC × épaisseur m / Force N × Longueur m in the case where the electrode covers the entire surface of the test piece. Results

[0194] Table 2 shows the results of piezoelectric coefficient measurement d 33 for the three piezoelectric composite materials of the invention compared to the control piezoelectric composite material, measured after polarization according to condition A or condition B. [Table 2] Piezoelectric coefficient d 33 T M1 M2 M3 Condition A: 1 V / µm 0,1 0,3 2,0 2,8 Condition B: 4V / µm 0,7 2,0 6,5 nm nm= not measured.

[0195] The results in Table 2 show that for a given polarization condition, for example condition A, the piezoelectric coefficient d 33 of the composite materials M1, M2, M3 according to the invention is very significantly improved compared to that of the composite material T outside the invention. This coefficient increases by at least a factor of 300% (comparison of the composite material M1 according to the invention compared to the composite material T outside the invention). The ionic liquid therefore makes it possible, surprisingly, to improve the piezoelectric properties of a composite material in which it is added, after polarization under the same conditions of temperature, time, and electric field intensity as a composite material not comprising ionic liquid.

[0196] The results presented in Table 2 also show that to obtain given piezoelectric properties for a composite material according to the invention (for example a piezoelectric coefficient d 33 = 0.2), it is possible either to modulate the electric strength of the electric field (composite material M1, condition B), or to modulate the amount of ionic liquid added (composite material M2, condition A). The piezoelectric properties of the composite materials of the invention can therefore be modulated easily and simply, which represents another advantage of the present invention.

Claims

1. Piezoelectric composite material based on a polymeric matrix and on piezoelectric inorganic fillers, characterized in that said material additionally comprises at least one ionic liquid of general formula Q+A-, in which Q+ represents a cation chosen from quaternary ammonium cations and quaternary phosphonium cations and A- represents any anion capable of forming together a liquid salt at a temperature of less than 100°C.

2. Piezoelectric composite material according to Claim 1, in which Q+ is a quaternary ammonium cation selected from the group consisting of imidazolium cations, pyrazolium cations, pyridinium cations, pyrimidinium cations, tetra(C1-C6)alkylammonium cations, guanidium cations and pyrrolidium cations.

3. Piezoelectric composite material according to either one of the preceding claims, in which Q+ is an imidazolium cation corresponding to the formula (I): in which R1 represents an alkyl group comprising from 1 to 15 carbon atoms, preferably from 1 to 10 carbon atoms, optionally substituted by one or more C6-C30 aryl, thiol or hydroxy groups or interrupted by one or more oxygen or sulfur atoms or by one or more NR' groups, R2, R3, R4, R5 and R', which are identical or different, each represent a hydrogen atom or an alkyl group comprising from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms, or a C6-C30 aryl group, optionally substituted by one or more C1-C4 alkyl groups.

4. Piezoelectric composite material according to Claim 4, in which Q+ is selected from the group consisting of 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-m-methylimidazolium, 1-octyl- 3-methylimidazolium, 1-decyl-3-methylimidazolium, 1,3-dibutylimidazolium and 1-butyl-2,3-dimethylimidazolium.

5. Piezoelectric composite material according to any one of the preceding claims, in which A- is selected from the group consisting of acetate, trifluoroacetate, propionate, chloride, hydroxide, sulfate, trifluoromethanesulfonate, pentafluoromethanesulfonate and phosphates; preferably, A- is acetate or propionate.

6. Piezoelectric composite material according to any one of the preceding claims, the content of ionic liquid is greater than or equal to 0.1 part by weight per hundred parts by weight of polymer, preferably is within a range extending from 0.1 to 15 parts by weight per hundred parts by weight of polymer, more preferentially from 0.5 to 12 parts by weight per hundred parts by weight of polymer, more preferentially still from 1 to 10 parts by weight per hundred parts by weight of polymer.

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

8. Piezoelectric composite material according to any one of the preceding claims, in which the piezoelectric inorganic filler is chosen from barium titanate, lead titanate, lead zirconate titanate, lead niobate, lithium niobate, potassium niobate and their mixtures; more preferentially, the piezoelectric inorganic filler is chosen from barium titanate, lead zirconate titanate, potassium niobate and their mixtures.

9. Piezoelectric composite material according to any one of the preceding claims, in which the content of piezoelectric inorganic fillers is within a range extending from 5% to 80% by volume, with respect to the total volume of said material, more preferentially from 6% to 60% and more preferentially still from 7% to 50%.

10. Piezoelectric composite material according to any one of the preceding claims, in which the polymeric matrix comprises at least one polymer selected from the group consisting of thermoplastic polymers, thermoplastic elastomers, thermosetting polymers, diene elastomers and the mixtures of these polymers.

11. Piezoelectric composite material according to any one of the preceding claims, in which the polymeric matrix comprises at least one thermoplastic elastomer and / or one diene elastomer.

12. Piezoelectric composite material according to any one of the preceding claims, in which the polymeric matrix comprises at least one diene elastomer selected from the group consisting of natural rubber, ethylene / propylene / diene monomer copolymers, synthetic polyisoprenes, polybutadienes, butadiene copolymers, isoprene copolymers and the mixtures of these diene elastomers.

13. Piezoelectric composite material according to either one of Claims 11 and 12, additionally comprising at least one crosslinking system; preferably, the crosslinking system comprises at least one peroxide.

14. Device comprising at least one layer based on at least one piezoelectric composite material and at least two electrodes positioned on either side of said layer, characterized in that said piezoelectric composite material is defined according to any one of Claims 1 to 13.

15. Tyre comprising at least one piezoelectric device defined according to Claim 14.