Piezoresistive lacquer and method for producing same

Abstract

The invention relates to a method for producing a piezoresistive lacquer comprising a polymer selected from the group consisting of polyurethane and silicone, a nanoparticle-based conductive filler selected from the group consisting of carbon black, graphite, carbon nanotubes, and a mixture thereof, and a solvent, wherein the conductive filler represents between 5% and 30% by weight relative to the polymer. The lacquer exhibits a heterogeneous dispersion of the nanoparticles of the filler in the polymer, and the amount of solvent is selected such that the lacquer has a viscosity of between 1 and 20000 mPa.s at 20°C and an electrical volume resistivity of between 5 and 200 ohm.cm, so as to obtain a mean lacquer hysteresis value of less than 5%, preferably less than 3%, and more preferably less than 1%.

Description

[0001] Description

[0002] Title: Piezoresistive lacquer and its manufacturing process

[0003] The field of the invention relates to a piezoresistive lacquer, as well as to its manufacturing method.

[0004] Piezoresistive lacquers are used as electronic components for various sensors. The sensors can be pressure, strain, tactile, or other sensors. They are used in particular in the railway, aeronautics, space, and automotive industries.

[0005] To manufacture a piezoresistive lacquer, a composite material is generally used. The composite mainly consists of a polymer in which a conductive filler is dispersed on a microscale. The conductive particles of the filler give the material conductivity and therefore electrical resistance. Through formulation and deposition techniques, an interconnected network between the conductive particles is formed, and any deformation and / or mechanical stress exerted modifies the contact between these particles, which modifies the electrical resistance of the material. It is then possible to detect mechanical interactions based on the measured change in electrical resistance.

[0006] In particular, piezoresistive lacquers find application in fields that seek to monitor the physical state of a given mechanical part. The mechanical part is coated with a thin layer of piezoresistive lacquer, which makes it possible to monitor the stresses that the part is subjected to in real time or in a delayed manner.

[0007] The lacquer thus allows the development of systems for capturing and / or monitoring (also called "monitoring" in the technical field of the invention) mechanical constraints such as pressure, compression, stretching, bending, deformation, and damage such as impact or cracking. Document WO2017114978 describes a lacquer based on styrene-butadiene-styrene polymers in which a conductive filler is dispersed.

[0008] One problem with existing lacquers is that they are subject to the phenomenon of hysteresis. Hysteresis is the failure to return to the initial value of electrical resistance after mechanical stress. More generally, it is the persistence of a phenomenon when the cause that produced it ceases.

[0009] The hysteresis of a lacquer is thus responsible for the appearance of a non-linearity in electrical measurements made of the electrical resistance, which makes it difficult to monitor the physical state of the supports on which the lacquer is applied. Indeed, a lacquer must present a certain deformation remanence even when the support has undergone a deformation (essentially elastic). If the remanence is insufficient, the deformation distorts the measurements and does not reflect the real state of the support. Thus, the results of measurements made on a lacquer exhibiting hysteresis provide an inaccurate signal feedback concerning the state of the support. Existing lacquers are not satisfactory in terms of hysteresis properties.

[0010] The present invention improves the situation.

[0011] In this respect, the invention relates to a method for manufacturing piezoresistive lacquer, comprising the following steps: a. providing a polymer selected from the group consisting of polyurethane and silicone; b. providing a predefined quantity of a nanoparticle conductive filler selected from the group consisting of carbon black, graphite, carbon nanotubes and a mixture thereof; c. mixing said polymer with said conductive filler until a paste is obtained; d. diluting said paste in an organic solvent to obtain a lacquer.

[0012] In the method of the invention: the predefined quantity of the conductive filler in step b. is chosen between 5 and 30% by mass relative to the polymer, the mixing in step c. is adjusted so that said paste obtained has a heterogeneous dispersion of said nanoparticles in the polymer, and the dilution in step d. in said organic solvent is carried out until a lacquer is obtained having a viscosity of between 1 and 20000 mPa. s (or cP) at 20°C, as well as a volume electrical resistivity of between 5 and 200 ohm. cm taking into account said chosen quantity of the conductive filler.

[0013] This allows to obtain a lacquer with an average hysteresis value of less than 5%.

[0014] The process of the invention thus makes it possible to manufacture a lacquer formulated to be applied in a thin layer (generally less than 50 μm) on supports. The lacquer is without or almost without hysteresis (less than or equal to 5%). An advantage of the lacquer of the invention is that after stress, the electrical resistance returns to its initial state without drift. The lacquer of the invention has a much wider elastic range than existing lacquers. This allows for better monitoring of the stresses undergone by the support, i.e. the material or part on which the lacquer is applied. Another advantage is that the manufacturing process has modular formulation steps and is relatively simple to implement.

[0015] The provision of polyurethane in step a. may comprise the following sub-steps under an inert atmosphere: a1. contacting a polyol with a diisocyanate, in the presence of an organic solvent, to obtain a polyurethane prepolymer; a2. contacting said polyurethane prepolymer with a chain extender to obtain a polyurethane.

[0016] The polyol may be selected from the group consisting of a linear saturated aliphatic polyol comprising one or more C2 to C5 units and a branched saturated aliphatic polyol comprising one or more C2 units, the polyol having a molar mass M of between 200 and 1000 g / mol; and the diisocyanate is of formula I:

[0017] O = C = N — R — N = C = O in which R is selected from a linear aliphatic hydrocarbon chain comprising a C2 to C4 unit, a methylene bis-cyclohexyl unit or a methylene bis-phenyl unit.

[0018] Preferably, the polyol is selected from the group consisting of polyethylene glycol, polypropanediol, polytetrahydrofuran, and polypropylene glycol.

[0019] The diisocyanate may be selected from the group consisting of hexamethylene diisocyanate, methylene-bis(phenyl isocyanate), 4,4'-methylene-bis(cyclohexyl isocyanate).

[0020] The chain extender may be selected from the group consisting of a linear C2 to C6 diol, a diol having a methylene bis-cyclohexyl unit and a diol having a methylene bis-phenyl unit.

[0021] Preferably, the chain extender is selected from the group consisting of ethylene glycol, butane-1,4-diol, hexane-1,6-diol and 4,4'-Isopropylidenedicyclohexanol.

[0022] Step a2. of bringing the prepolymer into contact with the extender may be carried out in the presence of a catalyst, preferably a tin-based catalyst. The catalyst is preferably chosen from dibutyltin dilaurate [DBTDL] and dibutyltin diacetate [DBTDA]. In a preferred embodiment, the polyurethane has a weighted average molar mass (Mn) of between 500 and 12,500 g / mol, preferably approximately 4,500 g / mol. Preferably, the polyurethane has a dispersity (D) of between 1.02 and 1.69, preferably approximately 1.20.

[0023] In one embodiment of the invention wherein the polymer is silicone, it may be a polydimethylsiloxane (PDMS) or a mixture of polydimethylsiloxanes.

[0024] The conductive filler nanoparticles preferably have a primary particle size between 10 nm and 100 nm as measured by transmission electron microscopy (TEM).

[0025] The nanoparticle conductive filler can be selected from the group consisting of: C-NERGY SUPER C65 Conductive (50nm), KETJENBLACK EC-300J Conductive (100nm), PRINTEX L6 powder (18nm), PRINTEX L6 powder (31nm), XPB 545 powder (12nm), PRINTEX XE2 B (3 Onm).

[0026] The mixing in step c. is a so-called "gentle" or "moderate" mixing. Preferably, the mixing is chosen from a mechanical mixing with a magnetic bar at a speed of between 250 and 750 rpm, preferably about 500 rpm, a mechanical mixing with a straight propeller blade, a mechanical mixing with a ring, a mechanical mixing with paddles, a mechanical mixing with anchors, and a low-intensity ultrasonic mixing.

[0027] The mixing in step c. is carried out in the presence of the solvent. This helps to promote mixing, for example by adjusting the viscosity by diluting with the solvent.

[0028] The mixing in step c. is preferably carried out at a temperature of approximately 60°C in the reaction medium.

[0029] In a preferred embodiment of the invention, the organic solvent is chosen from the group consisting of chloroform, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, methyl-tetrahydrofuran, ethyl acetate and butyl acetate when the polymer is polyurethane. When the polymer is polyurethane, the proportion of solvent added in step d. is preferably between 70% and 80% relative to said paste.

[0030] When the polymer is silicone, the proportion of solvent added in step d. is preferably between 40% and 70% relative to said paste.

[0031] In one embodiment of the invention, the lacquer obtained in step d. has an average hysteresis value of less than 3%, preferably less than 1%.

[0032] The invention also relates to a piezoresistive lacquer comprising a polymer chosen from the group consisting of polyurethane and silicone, a conductive filler with nanoparticles chosen from the group consisting of carbon black, graphite, carbon nanotubes and a mixture thereof and the solvent, characterized in that the conductive filler represents between 5% and 30% by mass relative to the polymer, and in that it has a heterogeneous dispersion of said nanoparticles of said filler in the polymer, the quantity of the solvent being chosen so that the lacquer has a viscosity of between 1 and 20,000 mPa.s (cP) at 20°C and a volume electrical resistivity of said lacquer of between 5 and 200 ohm. cm, so as to obtain an average value of the hysteresis of the lacquer of less than 5%, preferably less than 3%, and more preferably less than 1%.

[0033] Furthermore, the invention relates to a mechanical part covered at least partially with a piezoresistive lacquer described above and / or obtained by the method described above.

[0034] Other characteristics, details and advantages will appear on reading the detailed description below, and on analyzing the attached drawings in which:

[0035] Figure 1 shows a photograph of a polyurethane of the invention in solution;

[0036] Figure 2 shows a photograph of the polyurethane of Figure 1 after evaporation of the solvent; Figure 3 shows a photograph of a layer of a lacquer of the invention on PET (polyethylene terephthalate) film according to one embodiment (polyurethane);

[0037] Figure 4 shows an SEM (scanning electron microscopy) photograph of the lacquer in Figure 3;

[0038] Figure 5 shows a graph of an electrical signal as a function of a series of compressions of 0.05 MPa for a film comprising a lacquer of the invention (polyurethane);

[0039] Figure 6 shows a graph of an electrical signal at 40°C as a function of a series of compressions of 0.05 MPa for a film comprising a lacquer of the invention (polyurethane);

[0040] Figure 7 shows a graph of the electrical signal versus bending for a film comprising the lacquer of the invention;

[0041] Figure 8 shows a photograph of a layer of a lacquer of the invention on PET film according to one embodiment (with silicone);

[0042] Figure 9 shows a scanning electron microscopy photograph of the lacquer in Figure 8;

[0043] Figure 10 shows a graph of an electrical signal as a function of a compression of 0.05 MPa (400 to 800 seconds) for a film comprising a lacquer of the invention (silicone);

[0044] Figure 11 shows a graph of an electrical signal as a function of a bending of 2 to 5 mm for a silicone film for a film comprising a lacquer of the invention;

[0045] Figure 12 shows the evolution of the sensitivity of a lacquer of the invention as a function of the resistivity; and

[0046] Figure 13 shows characteristics of a lacquer of the invention (obtained by the process of the invention).

[0047] The figures, tables and description below contain, for the most part, elements of a certain nature. The figures and tables are an integral part of the description, and may therefore not only serve to better understand the present invention, but also contribute to its definition, where appropriate.

[0048] In the present description, reference is made to a piezoresitive composition, and more particularly to a piezoresitive lacquer. Depending on the application, the state of the art sometimes refers to a lacquer, a paint, a coating, a layer or even an ink respectively exhibiting piezoresistive behavior. Thus, the term lacquer used mainly in the present description is to be considered as almost equivalent to the terms paint, ink or coating.

[0049] Piezoresistiveness is a property of a material describing the change in electrical resistance resulting from a mechanical stress undergone by this material. This property is used in the manufacture of sensors generally called "piezoresistive sensors". There are currently three main categories of piezoresistive sensors: (1) inorganic sensors, (2) organic sensors and (3) mixed sensors comprising both organic and inorganic elements.

[0050] (1) Inorganic piezoresistive sensors.

[0051] These sensors are manufactured according to two main technological axes.

[0052] In the first axis, inorganic crystals based on lead, titanium or barium are synthesized [DOI: 10.7567 / JJAPS .26S2.174 | Chiaki Tanuma et al., 1987 Jpn . J. Appl . Phys. 26 174 ; DOI: 10.1038 / srep08595 | Deepam Maurya et al., Scientific Reports volume 5, Article number 8595 (2015) ; DOI: 10.1021 / acsami .7b08160 | Yingchun Liu et al., ACS Appl. Mater. Interfaces 2017, 9, 35, 29863-29871] . They have a relatively large piezoresistive response and are adjustable depending on the intended application. These crystals generally have relatively low hysteresis under mechanical stress. However, their use is accompanied by a number of problems. Indeed, the manufacturing processes require very rigorous control in order to obtain a given orientation of the crystals. Without precise orientation of the crystals, the piezoresistive response is unsatisfactory.Furthermore, the quantities produced then only represent small, non-flexible active surfaces (a few tens of square millimeters). This is due in particular to the complexity of controlling the growth of crystals while controlling their orientation. Furthermore, the acute toxicity of inorganic crystals makes the industrial development of this type of sensor difficult.

[0053] In the second axis, a silicon substrate is used as a base for the piezoresistive sensor [DOI: 10.1063 / 1.4921862 | Jin Woo Song et al., Rev Sci Instrum. 2015 Jun., 86(6):065003; US8261617] , Thanks to the architecture of these sensors, they generally have relatively high sensitivities. However, the manufacturing process is complex and requires successive protection / etching / deprotection steps. In addition, the silicon wafers used are not flexible, which significantly limits the surfaces that can be instrumented.

[0054] (2) Organic piezoresistive sensors.

[0055] In organic sensors, the active piezoresistive layer is a semiconducting polymer [DOI: 10.1016 / j .mee .2008.10.024 | Udo Lang et al., Microelectronic Engineering, Volume 86, Issue 3, March 2009, Pages 330-334 ; US5505093] . An advantage of this type of sensor is their relatively simple and versatile shaping. This allows them to be used in complex shapes without compromising their sensitivity. They are generally transparent and can be combined in optoelectronic applications. However, organic sensors are very sensitive to temperature, as well as humidity. This negatively affects their performance.

[0056] (3) Mixed piezoresistive sensors. Organic / inorganic piezoresistive sensors comprise a dispersion of conductive nanoparticles within an insulating polymer matrix. This combination presents the advantages of both types of materials: organic and inorganic. Indeed, inorganic materials generally have good electrical conductivity, while organic materials allow the development of flexible and thin matrices, as well as good handling and shaping practices (formulation and deposition). Together, inorganic and organic materials present improved resistance, low toxicity, while maintaining good shaping properties. These sensors can be classified into two categories according to their manufacturing process. There are (3i) sensors manufactured by volume shaping and (3ii) sensors manufactured in thin layers.

[0057] (3i) Volume forming consists of making a mixture between conductive nanoparticles and an insulating polymer matrix using a solvent. The mixture is cast into a mold to obtain a sensor of the desired configuration, which has a surface sensitive to deformations. Mold casting generally requires a large quantity of polymer matrix. The sensor is then deposited on a support whose state is to be monitored. Generally, mold casting provides relatively thick sensors ranging from a hundred micrometers to a few millimeters [DOI: 10.3390 / s22134765 | Zhiming Long et al., Sensors (Basel) , 2022 Jun 24 ; 22 ( 13 ) : 4765] . Today these sensors are widely used, in particular because of their ease of manufacture and low production costs. However, the performance of these sensors is limited by the polymer matrix.Indeed, the high mass proportion of the matrix results in low sensitivity, high response time and unsatisfactory temperature stability of the sensors. To compensate for these drawbacks, particularly the low sensitivity, alternative shapings have been explored. In particular, the insertion of cavities within the volume of the sensors, which improves their sensitivity to mechanical stress. The cavities are made by creating bubbles from a specific solvent or from a model agent such as sugar or salt [DOI: 10.1002 / smll .201901744 | Jinwon Oh et al., Small. 2019 Aug., 15 (33 ): el901744. ; DOI: 10.1016 / j . sna .2019.06.026 | Kyungseo Park et al., Sensors and Actuators A: Physical, Volume 295, 15 August 2019, Pages 541-550]. This cavity technique makes it possible to obtain sensors with a "foam" appearance.However, cavity sensors remain thick and, consequently, it is not possible to deposit them as is on a surface of a support whose state one would like to monitor.

[0058] (3ii) The manufacture of thin-film sensors (mono- or bi-layer) involves mixing conductive nanoparticles in an insulating polymer matrix using a solvent. The main difference with volume shaping (see 3i) lies in the deposition process. Indeed, deposition for the production of thin-film sensors allows thicknesses well below a hundred micrometers to be obtained. The deposition techniques used include: blade deposition, spin coating, dip coating or spray deposition. A continuous thin film is used as the active layer. Different types of insulating polymer matrix and different conductive nanoparticles can be used depending on the desired properties.The state of the art describes examples of thin-film piezoresistive films used in the design of piezoresistive sensors for various applications, including electronic, biomedical or automotive applications [cf. WO2017114978A1, US8661917B2, US8371174B2, US4765930, EP0236222A1]. However, problems remain, particularly due to the use of materials such as SBS, which is complex to work with. In particular, it is necessary to carry out a so-called "living" polymerization, which is relatively complex to implement. SBS is essentially obtained by molding or by thick deposition, which degrades thin-film deposition techniques. In addition, no sensor in the state of the art meets the requirement of almost no hysteresis. More generally, thin-film sensors are limited in their applications because they generally exhibit significant hysteresis.There is therefore a continuing need for a sensor that is free of hysteresis at room temperature or under thermal stress. Furthermore, known sensors with good sensitivity and low to moderate hysteresis are produced using complex and expensive manufacturing processes.

[0059] The Applicant has discovered, not without surprise, a manufacturing process which makes it possible to obtain a piezoresistive lacquer which solves the problems of the state of the art. Indeed, the lacquer of the present invention allows the suppression of hysteresis during measurements after mechanical stress on piezoresistive sensors. This absence of hysteresis of the lacquer occurs over the entire range between ambient temperature and the glass transition temperature of the polymer or substrate.

[0060] The method of the invention comprises several steps: a. providing a polymer selected from the group consisting of polyurethane and silicone, b. providing a predefined quantity of a nanoparticle conductive filler selected from the group consisting of carbon black, graphite, carbon nanotubes and a mixture thereof, c. mixing said polymer with said conductive filler until a paste is obtained, d. diluting said paste in an organic solvent to obtain a lacquer.

[0061] In the method of the invention, the predefined quantity of the conductive filler in step b. is chosen between 5 and 30% by mass relative to the polymer, the mixing in step c. is adjusted so that said paste obtained has a heterogeneous dispersion of said nanoparticles in the polymer, and the dilution in step d. in said organic solvent is carried out until a lacquer is obtained having a viscosity of between 1 and 20000 mPa.s at 20°C, as well as a volume electrical resistivity of between 5 and 200 ohm. cm taking into account said chosen quantity of the conductive filler.

[0062] This process makes it possible to obtain a piezoresistive lacquer with an average hysteresis value of less than 5%.

[0063] The specific choice of each component of the reaction medium, as well as the operating mode of each process step, contribute to the characteristic of very low hysteresis of the lacquer of the invention, or even the absence of hysteresis. Thus, the present description details below each step of the process of the invention.

[0064] Step a.

[0065] The choice of polymer is essential.

[0066] Reference is now made to the embodiment using polyurethane as the polymer.

[0067] In a preferred embodiment, the provision of polyurethane in step a. comprises the following sub-steps under an inert atmosphere: a1. contacting a polyol with a diisocyanate, in the presence of an organic solvent, to obtain a polyurethane prepolymer; a2. contacting said polyurethane prepolymer with a chain extender to obtain a polyurethane.

[0068] The polyol is chosen so as to have a saturated aliphatic chain comprising a maximum of five carbons (C2 to C5). When this chain is short, in particular two carbons (C2), methyl-type branches may be present. In addition, the polyol is chosen with a molar mass (Mn) of between 200 and 1000 g / mol. Beyond this molar mass, the microstructuring resulting in the absence of hysteresis is not satisfactory, and is generally absent. The diisocyanate (NCO) also has an impact on the property of absence of hysteresis of the lacquer of the invention. Thus, the NCO is chosen to be linear aliphatic with two to four carbons or composed of a methylene bis-cyclohexyl or methylene bis-phenyl type unit.

[0069] Thus, in one embodiment the polyol is chosen from the group consisting of a linear saturated aliphatic polyol comprising one or more C2 to C5 units and a branched saturated aliphatic polyol comprising one or more C2 units, the polyol having a molar mass M of between 200 and 1000 g / mol; and the diisocyanate is of formula I:

[0070] O = C = N — R — N = C = O in which R is selected from a linear aliphatic hydrocarbon chain comprising a C2 to C4 unit, a methylene bis-cyclohexyl unit or a methylene bis-phenyl unit.

[0071] Preferably, the polyol is selected from the group consisting of polyethylene glycol, polypropanediol, polytetrahydrofuran, and polypropylene glycol.

[0072] Polyethylene glycol can be chosen having a molar mass (Mn) of 200 g / mol, 400 g / mol or 1000 g / mol; polypropanediol of 400 g / mol, 600 g / mol or 1000 g / mol; polytetrahydrofuran of 250 g / mol, 650 g / mol or 1000 g / mol; and polypropylene glycol of 400 g / mol, 750 g / mol or 1000 g / mol. A molar mass Mn is the molar mass weighted according to the number of monomer units (n) contained in each of the chains.

[0073] The choice of polyol is important. In particular, polyols with a molar mass (Mn) greater than 1000 g / mol do not allow the invention to be carried out. Other polyols, such as polycaprolactone diol, do not allow the invention to be carried out.

[0074] Preferably, the diisocyanate is selected from the group consisting of hexamethylene diisocyanate, methylene-bis(phenyl isocyanate), 4,4'-methylene-bis(cyclohexyl isocyanate). Just like the choice of polyol, the choice of diisocyanate is also important. In particular, diisocynates such as tolylene-2,4-diisocyanate, isophorone diisocyanate, m-xylylene diisocyanate, 1,3-Bis(isocyanatomethyl)cyclohexane, 1,4-phenylene diisocyanate, trans-1,4-cyclohexylene diisocyanate do not allow the invention to be carried out.

[0075] In step al., which provides for bringing the polyol into contact with the diisocyanate, the viscosity of the reaction medium can be adjusted, for example between 1 and 3000 mPa.s.

[0076] The chain extender in step a2. is selected from the group consisting of a linear diol of two to six carbons (C2 to Ce), a diol with a methylene bis-cyclohexyl type unit and a diol with a methylene bis-phenyl type unit. Indeed, tests relating to the excessive variation of units within the polyurethane chain are not satisfactory and have not made it possible to obtain a lacquer without hysteresis.

[0077] Preferably, the chain extender is selected from the group consisting of ethylene glycol, butane-1,4-diol, hexane-1,6-diol and 4,4'-isopropylidenedicyclohexanol.

[0078] The choice of chain extender is important. Extenders such as cyclohexane dimethanol, hydroquinone bis(2-hydroxyethyl) ether, diethylene glycol, hexane-1,2-diol, benzene-1,2-dimethanol, or 4,4'-Methylene bis(cyclohexylamine) are not suitable for carrying out the invention.

[0079] In step a2., the prepolymer is brought into contact with the chain extender in the presence of a catalyst, preferably a tin-based catalyst. A tin-based catalyst allows rapid syntheses with a high conversion rate. The choice of the corresponding base (dilaurate or diacetate) does not impact the nature of the polyurethane.

[0080] In a preferred embodiment, the catalyst is chosen from dibutyltin dilaurate and dibutyltin diacetate.

[0081] Optionally, step a2. May include a sub-step (or a subsequent step) of evaporation of the solvent which was used in step a1. which reacts the polyol and the diisocyanate. This makes it easier to handle the polyurethane obtained in the rest of the process.

[0082] Reference is now made to the embodiment using silicone as the polymer.

[0083] The silicones used in the context of the present invention have

[0084] Preferably, the silicone is polydimethylsiloxane (PDMS) or a mixture of polydimethylsiloxanes.

[0085] More generally, the present invention uses siloxanes. To achieve the desired effect on hysteresis, the poly(dimethylsiloxanes) used in the invention form three-dimensional networks. To promote the formation of a three-dimensional network, the PDMS has at least one property among:

[0086] - a functionality of the siloxane monomers strictly greater than 2;

[0087] - a vinyl-type functional group present at the end of the chain of one of the monomers;

[0088] - a hydroxyl-type functional group present at the end of the chain of one of the monomers.

[0089] Other polysiloxanes are not satisfactory, in particular due to their characteristic(s): linear, functionality strictly equal to 2, absence of dimethyl group on the Si-O function, and / or absence of terminal vinyl or hydroxyl group).

[0090] In a preferred embodiment, the silicone called SYLGARD™ 184, available from DOW®, is used. This is a kit for silicone elastomers. The kit contains two chemicals: the base (Part A) and the curing agent (Part B), which are mixed in a mass ratio of 10:1. Both chemicals are transparent and viscous in nature. Part A comprises a mixture of polydimethylsiloxanes.En l'espèce la partie A comporte : poly (dimethylsiloxane ) , terminaison vinyle (vinyl terminated) [CAS: 68083-19-2] ; [ (dimethyl vinyl silyl)oxy] ; et du [ (trimethylsilyl) oxy] -modifié [CAS : 68988-89-6] ; Tetrakis (trimethylsilyloxy) silane [CAS: 3555-47-3] ; Ethylbenzène [CAS: 100-41-4] , La partie B comporte : Methylhydrosiloxane (3- 4%) + dimethylsiloxane (96-97%) copolymer [CAS: 68037-59-2] ; 2,4,6, 8-Tetramethyl-2 , 4,6, 8-tetravinylcyclotetrasiloxane [CAS : 2554-06-5] ; poly (dimethylsiloxane) , terminaison vinyle (vinyl terminated) [CAS: 68083-19-2] ; [ (dimethyl vinyl silyl)oxy] ; et [ (trimethylsilyl) oxy] -modified [CAS: 68988-89-6] ; Tetrakis (trimethylsilyloxy) silane [CAS: 3555-47-3] ; Ethylbenzène [CAS: 100-41-4] .

[0091] Other types of silicones can be used. In particular, poly(dimethylsiloxane), hydroxy terminated (hydroxy terminated) [CAS: 70131-67-8] (chain size viscosity 2.5k / 20k / 50k cSt) + PDMS [CAS: 63148-57-2] (chain size viscosity 15-40 cSt) + DBTDL [CAS: 77-58-7] in chloroform.

[0092] Other specific choices of the manufacturing process relate to both the embodiment in which the polymer is polyurethane and the embodiment in which the polymer is silicone.

[0093] In the following detailed description, reference is made to all embodiments of the invention, unless any limitation is specifically mentioned.

[0094] Step b.

[0095] The conductive filler chosen in step b. comprises nanoparticles. This filler is carbon black or graphite or carbon nanotubes. It is also possible to envisage a mixture of carbon black and graphite, carbon black and carbon nanotubes or graphite and carbon nanotubes. The choice of the conductive filler is important. A filler such as a filler with metal particles does not allow the invention to be carried out.

[0096] The nanoparticles used for the invention have a primary particle size between 10 nm and 100 nm measured by transmission electron microscopy (TEM). TEM images were obtained on Formvar grids with a transmission electron microscope (H7650, HITACHI (accelerating voltage 120 kV)), using the high contrast mode.

[0097] Primary particles are isolated particles. In fact, particles tend to clump together to form larger particles or irregular aggregates.

[0098] In one embodiment, the nanoparticle conductive filler is selected from the group consisting of: C-NERGY SUPER C65 Conductive (50nm), KETJENBLACK EC-300J Conductive (100nm), PRINTEX L6 powder (18nm), PRINTEX L6 powder (31nm), XPB 545 powder (12nm), PRINTEX XE2 B (30nm). The numbers in parentheses above correspond to the average diameter of the primary particles.

[0099] The quantity of nanoparticles plays a role in the sensitivity of the deposit to mechanical stresses.

[0100] To obtain a lacquer according to the invention, the quantity of the conductive filler in step b. is between 5% and 30% by mass relative to the polymer.

[0101] This quantity offers the possibility of adjusting the volume electrical resistivity of the lacquer of the invention between 5 and 200 ohm. cm. Controlling the amount of conductive filler is thus the essential parameter for obtaining the desired volume electrical resistivity. To further adjust the volume electrical resistivity, the amount of solvent added in process step d. can also be controlled, as described below.

[0102] Step c. Another key point of the invention is related to the mixing conditions in step c. of the process. The type of mixing has an influence on the quality of the lacquer and therefore the presence or absence of hysteresis. The mixing must be a gentle mixture. The mixing must not be too strong or too vigorous. The applicant discovered, not without surprise, that an overly homogeneous dispersion of the nanoparticles in the polymer is detrimental to the absence of hysteresis.

[0103] Thus, to improve the hysteresis-free properties, the nanoparticles of the conductive filler and the polymer are mixed to form a paste. This paste has a network of aggregates which makes it possible to obtain a lacquer without hysteresis. A network of aggregates corresponds to a heterogeneous dispersion of the nanoparticles in the polymer. The person skilled in the art knows how to adjust the mixture to obtain such a heterogeneous dispersion (sometimes described as non-homogeneous or not too homogeneous).

[0104] According to the invention, the mixing in step c. is adjusted so that said paste obtained has a heterogeneous dispersion of the nanoparticles in the polymer.

[0105] In one embodiment, the mixing in step c. is selected from a magnetic stir bar mechanical mixing at about 250-750 rpm, preferably 500 rpm, a straight propeller paddle mechanical mixing, a ring mechanical mixing, a paddle mechanical mixing, an anchor mechanical mixing, and a low intensity ultrasonic mixing. The mixing speed for the propeller paddle, ring, paddle, and anchor embodiments is also selected between 250 and 750 rpm depending on the viscosity of the solution / paste.

[0106] Mechanical mixing with a magnetic bar can be carried out with a 20L heating magnetic stirrer type RCT basic IRA or an economical rod stirrer type Heidolph 25 liters HEI-TORQUE CORE. Mechanical mixing with a blade can be carried out with a flat propeller stirring rod type R1375 - VWR 441-2942. Mechanical mixing with paddles can be carried out with a 4-blade propeller stirring rod type R1342 - VWR 441-2901. Mechanical mixing with an anchor can be carried out with an anchor propeller stirring rod type R1330 - VWR 441-2931. Low intensity ultrasonic mixing can be carried out with a Sonies vibra cell device type VCX500 (Power Output 500 W max - 20kHz - Amplitude 10% / pulse 30 -30 s).

[0107] The mixing method is important. Mixing methods using deflocculating blades - tooth saw, tri-cylinder, SpeedMixer (high intensity) or high intensity ultrasound - do not allow the invention to be implemented.

[0108] In a particular embodiment, solvent may be added in step c. to facilitate obtaining the paste. The solvent is generally the one also used in the subsequent step d. described below.

[0109] More particularly, the control of the dry extract (which corresponds to the polymer paste / conductive filler obtained in step c. is important to obtain a film with micro-structuring and therefore without hysteresis. In the case of polyurethanes, a dry extract content of between 20% and 30% is preferably used, while in the case of silicones a dry extract content of between 30% and 60% is preferably used. Outside these ranges, the films obtained by coating the lacquer on a support are not micro-structured (low value) or too friable (high value).

[0110] Step d.

[0111] The solvent is chosen so as to allow good dissolution of the reagents in step d. of dilution of the paste. The solvent is chosen according to its boiling point which must be greater than or equal to 60°C (with a maximum of approximately 205°C).

[0112] Preferably, when the polymer is polyurethane, the organic solvent is selected from the group consisting of chloroform, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, methyl-tetrahydrofuran, ethyl acetate and butyl acetate. The choice of solvent is important. Solvents, such as water and dimethyl sulfoxide, are not suitable for carrying out the invention. Preferably, the solvents are chosen according to the Hildebrand solubility parameter (not soluble if greater than >17 5d / MPa) 1 / 2). Water is a possible solvent and in combination with polyurethane polymers. However, the addition of dispersing agents should be considered due to the hydrophobic properties of certain conductive fillers such as carbon black.

[0113] Preferably, when the polymer is silicone, the organic solvent is selected from the group consisting of toluene, methyl tetrahydrofuran, ethyl acetate and butyl acetate. The choice of solvent is important. Solvents, such as water, chloroform, N-methyl-2-pyrrolidone or dimethyl sulfoxide do not allow the invention to be carried out.

[0114] For the invention, the organic solvent is therefore chosen from the group consisting of chloroform, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, methyl-tetrahydrofuran, ethyl acetate and butyl acetate when the polymer is polyurethane, or from the group consisting of toluene, methyl-tetrahydrofuran, ethyl acetate and butyl acetate when the polymer is silicone.

[0115] According to the invention, the viscosity of the piezoresistive lacquer must be between 1 and 20000 mPa.s at 20°C. The viscosity measurement can be carried out with a rotational viscometer of the ViscoQC 300R type available from the company Anton Paar.

[0116] A person skilled in the art knows how to adjust the viscosity. In particular, when the polymer is polyurethane, the proportion of solvent added in step d. is between 70% and 80% relative to said paste. When the polymer is silicone, the proportion of solvent added in step d. is between 40% and 70% relative to said paste.

[0117] Dissolving the paste obtained in step c. in solvent also makes it possible to adjust the desired volume electrical resistivity of the lacquer. According to the invention, the volume electrical resistivity of the lacquer is between 5 and 200 ohm. cm. A person skilled in the art knows how to adjust the volume electrical resistivity to the desired value, taking into account mainly the quantity of conductive filler made available in step b., then mixed with the polymer.

[0118] The volume electrical resistivity is defined as follows:

[0119] (Resistance x Section) / Length. It allows to predict the sensitivity of the lacquer (intensity of the resistance "jump"). The section (cross-sectional area) is defined by the following formula: Thickness x Width

[0120] Resistance measurement is performed with a 15XP-B multimeter available from Amprobe. Thickness measurement is performed with a Dektak XT stylus profilometer device available from Bruker. Width and length can be measured with a 150mm electronic digital caliper available from Brüder Mannesmann.

[0121] A piezoresistive lacquer is thus obtained having an average hysteresis value of less than 5%. Preferably, the respective selections and settings of each step of the method of the invention are chosen so as to obtain a lacquer having an average hysteresis value of less than 3%, preferably less than 1%.

[0122] An advantage of the lacquer of the invention is that after stress, the electrical resistance returns to its initial state without drift. In addition, its return to this initial state is almost instantaneous (or at least very rapid) after the end of a stress. Finally, the repeated sequence of stresses does not affect the electrical hysteresis of the lacquer of the invention, the latter retains a value of less than 5%. No drift of the electrical signal is noted during this type of cycling test.

[0123] More particularly, the method described above makes it possible to obtain a lacquer with characteristics making it possible to solve the problems of the lacquers of the state of the art. The piezoresistive lacquer of the invention comprises a polymer chosen from the group consisting of polyurethane and silicone, a conductive filler with nanoparticles chosen from the group consisting of carbon black, graphite, carbon nanotubes and a mixture thereof and the solvent, characterized in that the conductive filler represents between 5% and 30% by mass relative to the polymer, and in that it has a heterogeneous dispersion of said nanoparticles of said filler in the polymer, the quantity of the solvent being chosen so that the lacquer has a viscosity of between 1 and 20,000 mPa . s at 20°C and a volume electrical resistivity of said lacquer of between 5 and 200 ohm.cm, so as to obtain an average value of the hysteresis of the lacquer less than 5%, preferably less than 3%, and more preferably less than 1%.

[0124] In a preferred embodiment, the polyurethane is obtained by contacting a polyol with a diisocyanate so as to obtain a prepolymer which is contacted with a chain extender as described above.

[0125] Preferably, the polyurethane is {a,œ- dihydroxy oligo [ (ethylene glycol) -alt- (1, 4-butanediol ) ] -alt- (1, 6- diisocyanatohexane)} or { a, œ-dihydroxyoligo [ (propylene glycol) -alt- (1, 4-butanediol) ] -alt- (1, 6-diisocyanatohexane )} .

[0126] Silicone can be obtained using the following monomer pairs:

[0127] Poly (dimethylsiloxane), vinyl terminated + Tetrakis (trimethylsilyloxy) silane

[0128] Poly (dimethylsiloxane), hydroxy terminated + Poly (methylhydrosiloxane).

[0129] The lacquer of the invention is free of hysteresis. In other words, the lacquer exhibits a characteristic of absence of hysteresis. This characteristic is due particularly to the combination of sensitive control of synthesis parameters, formulation optimization and implementation during deposition.

[0130] When the lacquer is in the form of a thin layer applied to a substrate (i.e. in the form of a solid film), SEM microscopy shows a particular structure. The lacquer has cracks ranging from 1 to 100 pm in length and from 1 to 50 pm in width. In addition, aggregates are formed in the polymer structure creating irregularities (these include agglomerated carbon black particles). These structures resemble the interior appearance of white quartz geode-type stones.

[0131] The Applicant studied the molar masses of different polyurethanes of the invention, as well as their dispersity (D). Dispersity corresponds to a measure of the size distribution of macromolecules in a polymer sample. It is defined as follows: D = Mw / Mn. This made it possible to correlate the molar mass and / or the dispersity of the polyurethanes of the invention with the hysteresis value.

[0132] Thus, the polyurethanes of the invention have a weighted molar mass (Mn), also called number-average molecular mass, of between 500 and 12,500 g / mol. These polyurethanes can be obtained in particular by the reaction of diisocyanate with a polyol (diol) for 1 to 4 hours (i.e. step a1. of bringing a polyol into contact with a diisocyanate, in the presence of an organic solvent, to obtain a polyurethane prepolymer), according to the protocol described here. The subsequent contacting of the prepolymer with a chain extender (i.e. step a2. of bringing the polyurethane prepolymer into contact with a chain extender to obtain a polyurethane) is carried out for approximately 30 minutes to 1 hour. Preferably, an Mn equal to approximately 4,500 g / mol is targeted, which makes it possible to obtain a hysteresis value of less than or equal to 1%.

[0133] Furthermore, the polyurethanes of the invention have a dispersity (D) of between approximately 1.02 and approximately 1.69, preferably approximately 1.20.

[0134] The protocols described below in the embodiment examples show the different steps for obtaining a thin-layer piezoresistive film without hysteresis. The values ​​and / or reagents, in particular those in brackets [...], can be replaced by the values ​​or reagents previously cited in the description, without harming the performance or the absence of hysteresis of the lacquer obtained. The synthesis works particularly well up to a volume of half a liter (0.5 L).

[0135] PRODUCTION EXAMPLE 1: POLYURETHANE-BASED LACQUER

[0136] The following protocol describes the reaction for obtaining a polyurethane-based lacquer combining low hysteresis and high sensitivity to mechanical stress. The polyurethane is obtained by synthesizing a prepolymer. A polyol is brought into contact with a diisocyanate to form small polyurethane chains. If necessary, the viscosity is adjusted using an organic solvent, preferably between 1 and 3000 mPa.s at 60°C. Then, a chain extender is used to increase the chain length of the previously obtained polyurethane. At the end of the reaction, any solvent used can be evaporated.

[0137] The reaction takes place in a three-necked flask equipped with a condenser, a dropping funnel and a nitrogen / vacuum supply. Three nitrogen / vacuum cycles are carried out in order to obtain an inert atmosphere.

[0138] 0.018 mol of a polyol [here: polypropylene glycol] with a molar mass of 750 g / mol are introduced into an anhydrous organic solvent [here: chloroform]. The quantity of solvent is fixed in order to have a dry extract of reagent of 20% by mass. The dry extract represents the quantity of solid matter (present after evaporation of the solvent) over the total quantity (solid matter + solvent) in a paint / lacquer. 0.04 mol of 1,6-Diisocyanatohexane are introduced into the reaction mixture under moderate stirring (500 rpm).

[0139] It is preferable to use 20% dry extract because this provides good viscosity for the synthesis. The successive steps, particularly the one involving a chain extender, increase the viscosity of the solution significantly. Starting from 20%, no addition of solvent is necessary. For a quantity of solvent greater than 80% (dry extract less than 20%), the probability of the reactants meeting is reduced and therefore the synthesis time is increased. By carrying out syntheses without solvent, the production of the final product is drastically degraded. Without solvent, an insoluble and infusible gel is generally formed.

[0140] The reaction medium is brought to 60 °C, depending on the nature of the solvent, a slight reflux is possible. After 4 hours of reaction, 0.04 mol of 1,4-butanediol are introduced, the stirring speed is increased between 750 rpm and 1000 rpm. The heating of the reaction medium is stopped. 30 minutes after stopping the heating, 25 pL of catalyst [here: dibutyltin dilaurate] are introduced, if the viscosity increases significantly, solvent is then added to the reaction medium.

[0141] The reaction medium is left stirring until it returns to room temperature (approximately 30-45 min).

[0142] Figure 1 shows a photograph of the polyurethane in solution obtained at the end of the reaction.

[0143] The reaction solvent is evaporated to facilitate formulation, i.e. to adjust the dry extract.

[0144] Figure 2 shows a photograph of the polyurethane after the solvent has evaporated. This is the polyurethane in its solid state.

[0145] The resulting polyurethane is placed in a container along with 5% to 20% by mass of conductive filler (here: carbon black). Depending on the applications, the quantity of conductive filler can vary. Indeed, the latter affects the sensitivity as well as the mechanical resistance of the future thin layer of lacquer.

[0146] A mechanical mixture is then carried out on the two solids (here: polyurethane and carbon black). A fraction of solvent can be added to facilitate mixing. After the formation of a black paste (i.e.: absence of free carbon black powder), a volume of solvent is added to bring the dry extract to a value between 20% and 30% by mass depending on the deposition technique used subsequently.

[0147] This produces a lacquer according to the invention.

[0148] In this example, a 6 x 6 cm film 2 is obtained by blade deposition, it is possible by adjusting the previous parameters to use other deposition techniques. We can also consider deposits by brush coating, by aerosol (spray coating), by ink jet (ink-jet) or by dipping (dip coating).

[0149] For this example, a 30% mass dry extract (i.e. carbon black at 15% mass relative to polyurethane) in chloroform made it possible to obtain a layer of lacquer (or paint) with a thickness of 30 pm on a 100 pm PET film.

[0150] Figure 3 shows a photograph of the lacquer layer on the PET film.

[0151] Despite its homogeneity, the film macroscopically presents holes.

[0152] Figure 4 shows a scanning electron microscopy (SEM) photograph of the lacquer layer on PET film in Figure 3. Scanning electron microscopy analysis reveals the absence of material over only 20 pm. This is particularly the presence of crevices, extending to the PET substrate, distributed in a substantially homogeneous manner in the PU. The formation of PU islands separated by "void" can be distinguished. The presence of microstructuring is the cause of the absence of hysteresis.

[0153] Sensitivity and hysteresis measurements of the thin paint layer were carried out in compression using a ZwickRoell Z 5.0 equipped with a finger with a surface area of ​​one square centimeter.

[0154] In this example, 0.06 MPa of compression was applied four times to the active surface. An electrical signal is sent through the film during the compression test. The variation in the film response is plotted in blue and represents the voltage variations (from which the electrical resistance is deduced) of the film. Depending on the formulation, a pressure ranging from 0.01 MPa to 1 MPa can be applied to these thin layers of paint (20 to 80 μm thick).

[0155] Hysteresis is evaluated using the following formula: (signal at rest after compression - signal at rest before compression) / signal at rest before compression *100 (percentage).

[0156] The average value of hysteresis obtained is less than 1%.

[0157] Figure 5 shows a graph of the electrical signal as a function of a compression of 0.05 MPa for a polyurethane lacquer film formulated at room temperature (approximately 25°C). Figure 5 shows the return to an initial state almost instantaneously after the end of a stress. Figure 5 also shows that no drift of the electrical signal is noted during the cycling test.

[0158] Figure 6 shows a graph of the electrical signal versus 0.05 MPa compression for the film of Figure 5 at 40°C.

[0159] In this example, a 2 to 5 mm bending force was applied (ascending order) to the active surface. An electrical signal is sent through the film during the bending test. The variation in the film response is plotted in blue and represents the variations in the electrical resistivity of the film.

[0160] Hysteresis is evaluated using the following formula: (signal at rest after compression - signal at rest before compression) / signal at rest before compression *100 (percentage)

[0161] The average value of hysteresis obtained is less than 1%.

[0162] Figure 7 shows a graph of the electrical signal as a function of a flexion of 2 to 5 mm for a polyurethane lacquer film formulated at room temperature. The Applicant carried out gel permeation chromatography (GPC, also called size exclusion chromatography) analyses. This allowed access to the molar masses of different polyurethanes of the invention, as well as their dispersity (D). The analyses focus on the influence of the length of the polymer chain (reaction time between the diisocyanate and the polyol, such as a diol) on the hysteresis value of the final lacquer.

[0163] The chromatography device used is: HPCL UltiMati 3000 LC System (Thermo Fisher) equipped with a thermostated Tosoh G HHR GPC column with a DMF + lg / L LiBr solution as eluent. A differential refractometer detector (refractomax 520) is used. A standard calibration was carried out with polystyrene standards. The molar mass values ​​are therefore given in polystyrene equivalent.

[0164] In particular, the analysis targets a diisocyanate / polyol couple studied, which is the following in particular: Hexamethylene diisocyanate + Polypropylene glycol (750g / mol).

[0165] The results obtained are as follows:

[0166] Mn between 500 and 12,500 g / mol.

[0167] D between 1.02 and 1.69 (the weight average molar mass, also called weight average molecular mass, (Mw) can be calculated from the D).

[0168] The results obtained can be transposed to other types of polyurethanes obtained with different diisocyanates and / or polyols.

[0169] PRODUCTION EXAMPLE 2: SILICONE-BASED LACQUER

[0170] The silicone used here is a two-component product called Sylgard 184 from the Dow company. The product consists of a base (part A) and a curing agent (part B). Part A of Sylgard 184 is placed in a container along with 10% to 30% by mass of conductive filler [here: carbon black]. Depending on the application, the quantity of conductive filler can vary. The conductive filler has a direct effect on the sensitivity and mechanical strength of the future thin layer of lacquer.

[0171] Mechanical mixing is then carried out until a black paste is formed (i.e. until there is no free carbon black powder). A volume of solvent is added to bring the dry extract to a value between 30% and 60% by mass depending on the deposition technique used subsequently. Finally, part B is added to have an A:B ratio of 10:1.

[0172] In this example, the film is obtained using a blade deposition; it is possible to use other deposition techniques by adjusting the previous parameters.

[0173] A 35% mass dry extract (20% mass carbon black compared to part A) in solvent [here: ethyl acetate] makes it possible to obtain a layer of lacquer with a thickness of 50 pm on a 100 pm PET film.

[0174] Figure 8 shows a photograph of the lacquer layer on the PET film. Despite its homogeneity, the lacquer film macroscopically shows holes.

[0175] Figure 9 shows a scanning electron microscopy photograph of the lacquer on film in Figure 8. Electron microscopy analysis confirms the absence of material only over less than 5 pm.

[0176] Sensitivity measurements of the thin layer of lacquer are carried out in compression using a 500 g mass placed on a surface of one square centimeter (1 cm 2 ) .

[0177] In this example, 0.05 MPa of compression was applied to the active surface. Depending on the lacquer formulation, a pressure ranging from 0.01 MPa to 1 MPa can be applied to thin lacquer layers (20 μm to 80 μm thick).

[0178] Figure 10 shows a graph of the electrical signal versus 0.05 MPa compression (400 to 800 seconds) for the silicone lacquer film.

[0179] The average value of hysteresis obtained is less than 3%.

[0180] In this example, a 2 mm to 5 mm bending is applied (ascending order) to the active surface. An electrical signal is sent through the lacquer film during the bending test. The variation in the film response is plotted in blue and represents the variations in the electrical resistivity of the film.

[0181] Figure 11 shows a graph of the signal versus a 2-5 mm bending for a silicone lacquer film.

[0182] The hysteresis value is evaluated according to the following formula: (signal at rest after compression - signal at rest before compression) / signal at rest before compression *100 (percentage).

[0183] The average value of hysteresis obtained is less than 1%.

[0184] The table below shows the names, CAS numbers, reference and supplier of the reagents used in the working examples.

[0185] [Tables 1]

[0186] Table 1: List of reagents for PUs and PDMS

[0187] According to the invention, the dilution in step d. in the organic solvent is carried out until a lacquer is obtained having a viscosity of between 1 and 20,000 mPa.s at 20°C, as well as a volume electrical resistivity of between 5 and 200 ohm. cm taking into account the chosen quantity of the conductive filler.

[0188] The skilled person knows that to obtain a viscosity between 1 and 20000 mPa.s at 20°C. In particular, the skilled person has the ability to adjust the viscosity of a lacquer by adding a solvent. It is known that lacquers, paints and / or inks can be diluted to meet expectations. This is notably referenced in the technical data sheets accompanying conductive lacquers / paints / inks (see for example the data sheet "Conductive paint U-466" version 11 / 2020 from the company RESCOLL®). More generally, with his experience, the skilled person can determine the viscosity of a mixture using a mechanical mixer and observing the flow of the fluid. The viscosity can also be measured using a viscosity cup commonly available in premises preparing lacquers / paints / inks.

[0189] The skilled person also knows that for a volume electrical resistivity of between 5 and 200 ohm. cm taking into account said chosen quantity of the conductive filler. In particular, the skilled person knows that the value of the resistivity is directly proportional to the quantity of conductive filler. This value is obtained by a measurement known as a commonly used four-point measurement. By an empirical and iterative approach, it is possible to adjust the quantity of conductive filler. The literature describes the direct link between the quantity of carbon black and the electrical resistivity [Nagel, J., et al. (2022) - Enhanced PTC Effect in Polyamide / Carbon Black Composites - Materials, 15, 5400; Kupke, Michael & Wentzel, Hans-Peter & Karl, Schulte (1998) - Electrically conductive glass fiber reinforced epoxy resin - Material Research Innovations. 2. 164- 169. 10.1007 / 3100190050079] .

[0190] Figure 12 shows a graph of the evolution of the sensitivity (dU / U) of a lacquer of the invention as a function of the resistivity (ohm. cm).

[0191] The resistivity value has a direct influence on the sensitivity (dU / U) of the lacquer as shown in the graph. More generally, the defined values ​​of electrical resistivity correspond to the performance limits of an acquisition system, i.e. here the lacquer. Furthermore, according to the invention the mixing in step c. is adjusted so that the resulting paste has a heterogeneous dispersion of the nanoparticles in the polymer.

[0192] The skilled person knows the characteristics of a solution whether it is homogeneous or heterogeneous. He thus knows how to prepare a homogeneous or heterogeneous lacquer. In particular, the skilled person considers a heterogeneous solution as a solution having a large number of aggregates. In practice, the surface of the solution is not smooth in the case of a heterogeneous lacquer. As mentioned above, the skilled person normally seeks to eliminate heterogeneity in order to prepare a solution, and therefore a lacquer, that is as homogeneous as possible. The invention proposes the opposite.

[0193] Generally, to obtain the homogeneity of a solution, the person skilled in the art follows, for example, the procedure described in "Pigment dispersion II, testing" - Clifford K. Schoff in COATINGS TECH, Vol. 17, No. 7 (07 / 2020). The following steps can be performed:

[0194] - Visual observation: this visual analysis makes it possible to define the nature of the solution in 90% of cases. The presence of particles or aggregates visible to the eye indicates the nature of the solution;

[0195] - Observation with an instrument: it is possible by depositing the lacquer on a grindometer to define the presence of aggregates as well as the size of the particles;

[0196] - Observation by microscopy: using microscopic tools, it is possible to determine the presence of aggregates and define the non-homogeneity of the solution. Using optical and / or electron microscopy, heterogeneity can be qualified;

[0197] - Observation of the lacquer layer obtained: The visual analysis of the lacquer layer obtained (or the film) after deposition makes it possible to define whether the solution used was homogeneous or not. The presence of aggregates and / or discontinuity of the layer makes it possible to declare whether the lacquer used is homogeneous or not.

[0198] Although the steps described above are implemented in the art to obtain homogeneous solutions / lacquers, they can be implemented within the scope of the present invention to obtain heterogeneous solutions / lacquers.

[0199] The settings of the process of the invention make it possible to obtain a heterogeneous lacquer. In particular, the mixing technique for obtaining the lacquer of the invention must be gentle, without shear, in order to be able to obtain the appearance described as heterogeneous, that is to say in particular with the presence of aggregates. In practice and according to the invention, the opposite of the mixing methods described in the art is done, as for example in Tamburrano, A., Proietti, A., Fortunato, M., Pesce, N., & Sarto, MS (2022) - Exploring the Capabilities of a Piezoresistive Graphene-Loaded Waterborne Paint for Discrete Strain and Spatial Sensing - Sensors, 22 (11) , 4241 - https: / / doi.org / 10.3390 / s22114241.

[0200] In the invention, to obtain the desired structure, mechanical mixing with a magnetic bar is favored at a speed of between 250 and 750 rpm, preferably around 500 rpm. Indeed, mechanical mixing with a magnetic bar prevents the solution from being homogenized. Conventionally, mixing is carried out for a few minutes. Beyond 5 min at 750 rpm, the characteristic structure of the conductive particles may be damaged.

[0201] It is also possible to achieve mechanical mixing using paddles, anchors, straight propeller blades or rings. The absence of shear allows the aggregation of lacquer particles to be preserved. Beyond a stress of 10 min (at 500 rpm) or a stress of more than 750 rpm (2 min), the structure risks deteriorating.

[0202] In the case of mixing with an ultrasonic probe (20 kHz), a maximum power of approximately 500W with an intensity of 20% is recommended with a pulse of 30 seconds, and with 30 seconds of rest. Beyond a power of 100W (20% of 500W) or an active phase of more than 30 seconds without rest, the structure of the aggregates is possibly compromised.

[0203] Figure 13 shows characteristics of a lacquer of the invention obtained by mixing using a magnetic bar as described above.

[0204] The section marked (a) of figure 13 shows a photograph of the deposited lacquer, the section marked (b) shows a representation of the signal as a function of pressure with a mass of 500g on a 6 x 6 cm film 2, the section marked (c) shows an image obtained by optical microscopy (endoscope with the same scale) and the section marked (d) shows an image obtained by scanning electron microscopy (black scale = 100pm).

[0205] The adjustments of the parameters or variables defined in the method of the invention described above make it possible to obtain a lacquer having an average hysteresis value of less than 5%. In other words, a lacquer which has an average hysteresis value of less than 5% is the direct result of carrying out the steps of the method of the invention described. This lacquer particularly has a heterogeneous dispersion of filler nanoparticles in a polymer.

[0206] Generally speaking, and in view of the photographs relating to the invention (in particular Figure 4), the lacquers of the invention have at least approximately 80% aggregates (formed by a plurality of nanoparticles), preferably at least 90% aggregates, and more preferably 100% or almost 100% aggregates. The aggregates have at least an average diameter of approximately 10 μm. More particularly, the aggregates of the lacquers of the invention generally have an average diameter of between 10 μm and a diameter greater than or equal to 40 μm. Preferably, the average diameter is between 20 μm and an average diameter greater than or equal to 40 μm, and more preferably between an average diameter of 30 μm and an average diameter greater than or equal to 40 μm.In particular embodiments, the lacquers of the invention may have more than 80% or 90% of aggregates having average diameters of approximately 20 to 30 μm, and possibly occasionally contain aggregates having average diameters beyond 30 μm.

[0207] The conductive lacquer or paint of the invention is based on a polyurethane (PU) or silicone matrix comprising nanoparticles of carbon black and / or graphite and / or carbon nanotubes. The lacquer can be deposited directly or by means of a PET type transfer substrate on a surface of interest of a material, such as a substrate or a mechanical part. After encapsulation of the lacquer, and thanks to its piezoresistive behavior, monitoring of mechanical deformations or damage can be carried out. After stressing the substrate such as bending, an impact or even pressure via a weight or a finger, the electrical resistance will return to its initial state without drift, allowing better monitoring of the stresses undergone by the material.

[0208] The choices made during the chemical synthesis of the lacquer of the invention, the formulation and the application of the lacquer make it possible to obtain a macroscopically discontinuous homogeneous film. The relatively simple implementation makes it possible to obtain repeatability of the phenomenon for different polymers with variable rates of conductive fillers. Thus, a low hysteresis can be obtained for a thin layer. It follows that the lacquer can be deposited on complex three-dimensional surfaces. In addition, the property of absence of hysteresis of the lacquer is resistant to temperature. The absence of hysteresis remains observable under variable temperature conditions (room temperature up to the glass transition temperature of the polymer or the support).

[0209] The lacquer of the invention is part of a problem of capturing mechanical stresses repeated over time. In order to be able to study the phenomena that can cause mechanical stresses (compression, traction, bending, damage) on a part of interest, it is essential to have a return to an initial state when the stresses are not destructive. Thus, it is possible to validate whether the part has suffered irreversible damage or not. The lacquer of the invention responds to this problem.

[0210] The invention makes it possible to meet various needs, in particular:

[0211] - the absence of hysteresis while maintaining sensitivity to mechanical constraints to improve the reliability of the devices (reduce false positives), improve the fluidity of interactions and allow feedback to be triggered;

[0212] - temperature stability with no hysteresis;

[0213] - the consistency of the lacquer which allows it to be applied to a substrate and this while obtaining a low thickness of the layer of lacquer / paint / ink;

[0214] - ease of implementation.

[0215] Particularly advantageous points of the invention compared to known piezoresitive compositions: the absence of hysteresis; the temperature stability of the absence of hysteresis; the ease (or non-complexity) of stacking; the final thickness of the lacquer layer less than or equal to 50 μm; and the variety of possibilities for formulating the lacquer.

[0216] A distinction is sometimes made between hysteresis described as "mechanical" and hysteresis described as "electrical" (the one which is of main interest to the invention). Mechanical hysteresis and electrical hysteresis are not necessarily correlated. Thus, there are lacquers with an absence of mechanical hysteresis but a presence of electrical hysteresis. The polymer matrix returns to its initial state but the current paths, formed by the conductive particles, are no longer the same (electrical hysteresis present). This is the case of lacquers of the prior art. The present invention solves this problem. The lacquer of the invention has an absence of hysteresis, and this of "mechanical" and especially "electrical" hysteresis.

Claims

Claims

1. A method for manufacturing piezoresistive lacquer, comprising the following steps: a. providing a polymer selected from the group consisting of polyurethane and silicone; b. providing a predefined amount of a nanoparticle conductive filler selected from the group consisting of carbon black, graphite, carbon nanotubes and a mixture thereof; c. mixing said polymer with said conductive filler until a paste is obtained; d. diluting said paste in an organic solvent to obtain a lacquer; characterized in that the predefined amount of the conductive filler in step b. is selected from 5 to 30% by mass relative to the polymer, the mixing in step c. is adjusted so that said obtained paste has a heterogeneous dispersion of said nanoparticles in the polymer, and the dilution in step d.in said organic solvent is carried out until a lacquer is obtained having a viscosity of between 1 and 20000 mPa. s at 20°C, as well as a volume electrical resistivity of between 5 and 200 ohm. cm taking into account said chosen quantity of the conductive filler, so as to obtain a lacquer having an average hysteresis value of less than 5%.

2. A method according to claim 1, wherein the provision of polyurethane in step a. comprises the following substeps under an inert atmosphere: al. contacting a polyol with a diisocyanate, in the presence of an organic solvent, to obtain a polyurethane prepolymer; a2. contacting said polyurethane prepolymer with a chain extender to obtain a polyurethane.

3. The method of claim 2, wherein the polyol is selected from the group consisting of a linear saturated aliphatic polyol comprising one or more C2 to C5 units and a branched saturated aliphatic polyol comprising one or more C2 units, the polyol having a molar mass M of between 200 and 1000 g / mol; and the diisocyanate is of formula I: O = C = N — R — N = C = O in which R is selected from a linear aliphatic hydrocarbon chain comprising a C2 to C4 unit, a methylene bis-cyclohexyl unit or a methylene bis-phenyl unit.

4. The method of either of claims 2 and 3, wherein the polyol is selected from the group consisting of polyethylene glycol, polypropanediol, polytetrahydrofuran, and polypropylene glycol.

5. A method according to one of claims 2 to 4, wherein the diisocyanate is selected from the group consisting of hexamethylene diisocyanate, methylene-bis (phenyl isocyanate), 4,4'-methylene-bis (cyclohexyl isocyanate).

6. A method according to one of claims 2 to 5, wherein the chain extender is selected from the group consisting of a linear C2 to C6 diol, a diol having a methylene bis-cyclohexyl unit and a diol having a methylene bis-phenyl unit.

7. A method according to one of claims 2 to 6, wherein the chain extender is selected from the group consisting of ethylene glycol, butane-1,4-diol, hexane-1,6-diol and 4,4'-Isopropylidenedicyclohexanol.

8. Method according to one of claims 2 to 7, in which step a2. of bringing the prepolymer into contact with the extender is carried out in the presence of a catalyst, preferably a tin-based catalyst.

9. The method of claim 8, wherein the catalyst is selected from dibutyltin dilaurate and dibutyltin diacetate.

10. Method according to one of the preceding claims, in which the polyurethane has a weighted average molar mass (Mn) of between 500 and 12,500 g / mol, preferably approximately 4500 g / mol.

11. Method according to one of the preceding claims, in which the polyurethane has a dispersity (B) of between 1.02 and 1.69, preferably of approximately 1.

20.

12. The method of claim 1, wherein the silicone is a polydimethylsiloxane (PDMS) or a mixture of polydimethylsiloxanes.

13. Method according to one of the preceding claims, in which said nanoparticles of the conductive filler have a primary particle size of between 10 nm and 100 nm measured by transmission electron microscopy (TEM).

14. Method according to one of the preceding claims, wherein said nanoparticle conductive filler is selected from the group consisting of: C-NERGY® SUPER C65 Conductive (50nm), KETJENBLACK® EC-300J Conductive (100nm), PRINTEX® L6 powder (18nm), PRINTEX® L6 powder (31nm), XPB 545 powder (12nm), PRINTEX® XE2 B (30nm).

15. Method according to one of the preceding claims, in which the mixture in step c. is chosen from a mechanical mixture with a magnetic bar at a speed of between 250 and 750 rpm, preferably approximately 500 rpm, a mechanical mixture with a straight propeller blade, a mechanical mixture ring mixer, mechanical paddle mixer, mechanical anchor mixer, and low intensity ultrasonic mixer.

16. Method according to one of the preceding claims, in which the mixing in step c. is carried out in the presence of the solvent.

17. Process according to one of the preceding claims, in which the mixing in step c. is carried out at a temperature of approximately 60°C in the reaction medium.

18. A method according to any preceding claim, wherein the organic solvent is selected from the group consisting of chloroform, tetrahydrofuran, N-methyl-2-pyrrolidone, toluene, methyl-tetrahydrofuran, ethyl acetate and butyl acetate when the polymer is polyurethane.

19. Method according to one of claims 1 to 11 and 13 to 18, in which, when the polymer is polyurethane, the proportion of solvent added in step d. is between 70% and 80% relative to said paste.

20. Method according to one of claims 1, and 12 to 19, in which, when the polymer is silicone, the proportion of solvent added in step d. is between 40% and 70% relative to said paste.

21. Method according to one of the preceding claims, in which said lacquer obtained in step d. has an average hysteresis value of less than 3%, preferably less than 1%.

22. Piezoresistive lacquer comprising, a polymer selected from the group consisting of polyurethane and silicone, a conductive nanoparticle filler selected from the group consisting of carbon black, graphite, carbon nanotubes and a mixture thereof and the solvent, characterized in that the conductive filler represents between 5% and 30% by mass relative to the polymer, and in that it has a heterogeneous dispersion of said nanoparticles of said filler in the polymer, the quantity of solvent being chosen so that the lacquer has a viscosity of between 1 and 20,000 mPa.s at 20°C and a volume electrical resistivity of said lacquer of between 5 and 200 ohm. cm, so as to obtain an average value of the hysteresis of the lacquer of less than 5%, preferably less than 3%, and more preferably less than 1%.

23. Mechanical part covered at least partially with a piezoresistive lacquer according to claim 22.

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