Thermally conductive vitreous composite material

Abstract

Thermally Conductive Vitrimer Composite Material The present invention relates to a thermally conductive vitrimer composite material comprising: (a) a crosslinked epoxy vitrimer resin matrix and (b) thermally conductive fillers, the content of thermally conductive filler (b) being between 70% and 90% by weight relative to the total weight of the thermally conductive vitrimer composite material, and said thermally conductive vitrimer composite material being obtainable by crosslinking an epoxy resin comprising said fillers with a crosslinking agent of the following general formula I: H2N-ASSA-H2N (I) in which A represents a phenyl group or a linear or branched C1-C6 alkyl group, the phenyl or alkyl groups optionally being substituted by one or more substituents selected from a C1-C20 alkyl group, a C5-C14 aryl group, an -OR2 group, an -(CO)R3 group, an -O(CO)R4 group, a group -(SO)R5, a group –NH-CO-R6, a group -COOR7,An -NR8R9 group, an -NO2 group, and a halogen atom (R2 to R9) independently represent a hydrogen atom, a C1-C20 alkyl group, or a C5-C14 aryl group. It also relates to a process for preparing this composite material, its use, a kit for its preparation, and a process for insulating the stator of an electric motor using this composite material. Finally, it relates to a non-crosslinked mixture of an epoxy resin comprising thermally conductive fillers and 4,4'-dithiodianiline, the thermally conductive filler content being between 70% and 90% by weight relative to the total weight of the mixture.

Description

Title of the invention: Thermally conductive vitrimer composite material Technical field

[0001] The present invention relates to the general field of thermally conductive composite materials, and even more particularly it relates to a thermally conductive vitrimer composite material. Prior art

[0002] Hydraulic and pneumatic energy are gradually being replaced by electrical energy on board aircraft. In order to improve the performance and efficiency of electrical machines, thermal management at the level of their notches is essential.

[0003] When a simple varnishing of the coils of a stator is used for electrical insulation, the vibrations in operation cause wear and possible short circuits at the coils. In addition, various chemical substances and vapors can also damage the coils. Thus, encapsulating epoxy resins provide an answer to the problems mentioned above. Epoxy resins offer excellent electrical insulation, good mechanical characteristics, chemical resistance and thermal endurance. As encapsulation materials for electrical equipment that can release abundant heat, these resins are often of the composite type in order to have a thermally conductive character (addition of thermally conductive fillers such as aluminum oxide or boron nitride for example).

[0004] However, exposure of aeronautical electrical machines to varying environmental conditions such as temperature, humidity, chemicals and radiation causes the formation and propagation of microcracks within the composite epoxy resins.

[0005] Over the past two decades, many efforts have been made to produce recyclable and self-healing epoxy resins by introducing exchangeable bonds into polymer networks. For example, vitrimers are crosslinked networks containing dynamic covalent bonds. At operating temperature, vitrimers behave like traditional thermosets, exhibiting good thermal and mechanical properties. Upon heating to a certain temperature (or with the aid of a catalyst), exchange reactions occur. Under the influence of an external force, the exchange reaction induces a rearrangement of the topology and rapid stress relaxation. The viscosity of this type of resin decreases pro progressively as a function of temperature, generally following an Arrhenius law, just like vitreous silica (which led to the name “vitrimer”).

[0006] The phenomenon of self-repair of vitrimers has been widely reported in the literature and some examples report a self-repair process without the input of external forces. While there are many studies on composite epoxy vitrimers, few studies concern thermally conductive composite materials with vitrimer properties. Indeed, the problem encountered is the charge rate of these composites which generally remains low to preserve the vitrimer property of said composite. Not only can the high percentage of charge induce unsuitable crosslinking behavior, but it can also restrict the molecular mobility necessary for the proper progress of the macroscopic flow.

[0007] In particular, the preparation of an epoxy-based vitrimer composite material containing at least 70% by weight of fillers has never been reported. However, the presence of a high proportion of filler is important to ensure good thermal diffusion of the resin, particularly in the context of the insulation of electric motors.

[0008] Now the inventors have surprisingly discovered that it is possible to prepare an epoxy-based vitrimer composite material containing at least 70% by weight of fillers using a disulfide derivative such as 4,4'-dithiodianiline as crosslinking agent.

[0009] Indeed, the thermal activation of the disulfide bonds present within the crosslinked matrix will induce bond exchanges in order to provide the desired properties, in particular the self-repair of microcracks and the possibility of recycling the resin at the end of the life of the electrical machine. This activation will also prevent the formation of microcracks which may appear during the encapsulation process, that is to say during the implementation of the resin, and particularly during cooking, due to the different expansion coefficients of the materials. Statement of the invention

[0010] The present invention therefore relates to a thermally conductive vitrimer composite material comprising, advantageously consisting essentially of, in particular consisting of: - (a) a crosslinked epoxy vitrimer resin matrix and - (b) thermally conductive fillers, the content of thermally conductive filler (b) being between 70% and 90% by weight relative to the total weight of the thermally conductive vitrimer composite material and said thermally conductive vitrimer composite material being capable of being obtained by crosslinking an epoxy resin comprising said fillers with a re- crosslinker of the following general formula I:

[0011] [Chem.l] H2N-ASS-TO-H2N (I)

[0012] in which A represents a phenyl group or a linear or branched C1-C6 alkyl group, the phenyl or alkyl groups possibly being substituted by one or more substituents chosen from a C1-C20 alkyl group, a C5-C14 aryl group, an -OR2 group, a -(CO)R3 group, an -O(CO)R4 group, an -(SO)R5 group, an -NH-CO-R6 group, a -COOR7 group, an -NR8R9 group, an -NO2 group and a halogen atom and R2 to R9 independently represent a hydrogen atom, a C1-C2 alkyl group or a C5-C14 aryl group.

[0013] In the present application, the expressions "between ... and..." and "from ... to..." must be understood to include the limits unless explicitly stated otherwise.

[0014] In the context of the present invention, the term "thermally conductive vitrimer composite material" means a composite material whose matrix (i.e. the continuous phase) is a vitrimer polymer and whose fillers (i.e. the discontinuous phase) are thermally conductive fillers, which makes it possible to give the composite material the vitrimer properties of the matrix and the thermally conductive properties of the fillers.

[0015] In the context of the present invention, the term "vitrimer" means any covalently bonded crosslinked polymer which behaves like a traditional thermosetting polymer in terms of its thermal and mechanical properties at the operating temperature, but which contains dynamic covalent disulfide (SS) bonds which will allow it to flow like glass in terms of its malleability and recycling properties from a certain temperature thanks to bond exchange reactions. The associative mechanism breaks the original bond only when a new covalent bond is formed elsewhere, which makes it possible to maintain a constant crosslinking density, the material therefore follows the Arrhenius law during heating.

[0016] In the context of the present invention, the term "epoxy vitrimer resin" means any epoxy resin having a vitrimer behavior as defined above, i.e. having dynamic covalent disulfide bonds (SS). Advantageously, the epoxy resin, in particular of the matrix (a) of the composite material according to the invention, is a glycidyl epoxy resin, advantageously a glycidyl epoxy ether resin such as bisphenol A diglycidyl ether or epoxy novolac resin, more advantageously an epoxy novolac resin. Epoxy novolac resin or Poly[(phenyl glycidyl ether)-co-formaldehyde] is a glycidyl ether of phenolic novolac resin. It is obtained by reacting phenol with for maldehyde in the presence of acid catalyst to produce phenolic novolac resin, followed by reaction with epichlorohydrin in the presence of sodium hydroxide as catalyst. It has the following general formula (1):

[0017] [Chem.2]

[0018] In the context of the present invention, the term "Cx-Cy alkyl group" means any alkyl group of x to y carbon atoms, linear or branched, in particular, the methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, t-butyl, n-pentyl, n-hexyl groups. Advantageously, it is a methyl group.

[0019] In the context of the present invention, the term "Cx-Cy aryl group" means one or more aromatic rings having x to y carbon atoms, which may be joined or fused. In particular, the aryl groups may be monocyclic or bicyclic groups, preferably phenyl, naphthyl, tetrahydronaphthyl or indanyl. Advantageously, it is a phenyl or naphthyl group, more particularly a phenyl group.

[0020] In the context of the present invention, the term “halogen atom” means any halogen atom, advantageously chosen from Cl, Br, I or F, in particular Cl.

[0021] Advantageously, the vitrimer epoxy resin according to the invention is obtained by crosslinking an epoxy resin with a crosslinking agent of general formula I.

[0022] The crosslinking agent of general formula I used in the context of the present invention therefore has a disulfide bridge. It may advantageously be an aromatic disulfide crosslinking agent of the following general formula II:

[0023] [Chem.3] ' \ / (II)

[0024] in which Ri and R'i independently represent a hydrogen atom, a C1-C20 alkyl group, a C5-C14 aryl group, a -OR2 group, a -(CO)R 3 group, a -O(CO)R4 group, a -(SO)R5 group, a -NH-CO-R6 group, a -COOR 7 group, a -NR8R9 group, a -NO2 group and a halogen atom; R2 to R9 independently represent a hydrogen atom, a C1-C2 alkyl group or a C5-C14 aryl group and m = 4.

[0025] Advantageously, the NH2 groups are in the para position on the aromatic ring. In particular, R 1 and / or R 1 represent a hydrogen atom.

[0026] In an advantageous embodiment, the crosslinking agent is 4,4'-dithiodianiline.

[0027] 4,4'-Dithiodianiline is a hardener of the following general formula (2) which makes it possible to provide dynamic covalent disulfide bonds (SS), also called exchangeable bonds, to the epoxy resin.

[0028] [Chem.4] (2)

[0029] 4,4'-Dithiodianiline is not a hazardous substance in accordance with EC Regulation No. 1272 / 2008 unlike certain hardeners used for crosslinking conventional (non-vitrimeric) epoxy resins.

[0030] The crosslinking agent of general formula I used in the context of the present invention may advantageously be an aliphatic disulfide crosslinking agent such as cystamine of the following general formula (3):

[0031] [Chem. 5] NH2-(CH2)2-SS-(CH2)2-NH2 (3).

[0032] The crosslinking agent of general formula I used in the context of the present invention may also consist of a mixture of aromatic disulfide crosslinking agent of general formula II according to the invention and aliphatic disulfide crosslinking agent, in particular a mixture between 4,4'-dithiodianiline and cystamine. This mixture thus makes it possible to modulate the properties of the thermally conductive vitrimer composite materials obtained.

[0033] The crosslinking agent of general formula I according to the invention can be used alone or in a mixture with a multiamine crosslinking co-agent (f>l), in particular present in a content of between 1 and 40% by weight relative to the total weight of the crosslinking agent combination, more advantageously between 1 and 30% by weight. The multiamine crosslinking co-agent can be aliphatic or aromatic. As non-limiting examples of aliphatic multiamine crosslinking co-agent, mention may be made of the products following: Tris(2-aminoethyl)amine, triethylenetetramine, polyetheramines (for example marketed by Huntsman Corp. in the Jeffamine range), isophoronediamine, menthane diamine, 1,2-diaminocyclohexane, 1,3-diaminocyclohexane, 1,3-(diaminomethyl)cyclohexane, 4,4'-methylene dicyclohexylamine, 4,4'-diaminodicyclohexylmethane, 3,3'-dimethyl-4,4'-diaminodicyclohexylmethane and combinations thereof. As non-limiting examples of aromatic multiamine crosslinking co-agents, the following products may be mentioned: 3,3'-diaminodiphenylsulphone, 4,4'-diaminodiphenylsulphone, 4,4'-methylenebis(3-chloro-2,6-diethylaniline), 4,4'-methylenebis(2,6-diethylaniline), 4,4'-methylenedianiline, 3,4-dimethoxydianiline, m-xylenediamine and combinations thereof. This mixture of crosslinking agent with the crosslinking co-agent makes it possible to modulate the flow properties of the thermally conductive vitrimer composite material according to the invention, the stress relaxation properties and the thermomechanical properties.Advantageously, the crosslinking agent of general formula I according to the invention is used alone.

[0034] In the context of the present invention, the term "thermally conductive fillers" means any filler capable of diffusing heat in the media without macroscopic movement of matter. In particular, these fillers may be nanometric (i.e. having the largest of its dimensions in the order of a nanometer, in particular between 0.1 and 100 nm) or micrometric (i.e. having the largest of its dimensions in the order of a micrometer, in particular between 0.1 and 100 micrometers). They may be mineral or organic, advantageously mineral. They may have a thermal conductivity of at least 30 W / mK.Advantageously, the thermally conductive filler (b) according to the invention is chosen from zinc oxide (50 W / mK), aluminum oxide (30 W / mK), silicon carbide (120 W / mK), silicon nitride (100-200 W / mK), aluminum nitride (100-300 W / mK), carbon nanotubes (1000-4000 W / mK), graphene (2000-6000 W / mK), graphite (100-400 W / mK), diamond (1000 W / mK), hexagonal boron nitride (185-300 W / mK) and mixtures thereof, advantageously it is aluminum oxide, in particular micrometric aluminum oxide.

[0035] Advantageously, the thermally conductive filler according to the invention is not electrically conductive and is advantageously chosen from zinc oxide, aluminum oxide, silicon carbide, silicon nitride, aluminum nitride, diamond, hexagonal boron nitride and mixtures thereof.

[0036] The content of thermally conductive filler (b) of the thermally conductive vitrimer composite material according to the invention is between 70% and 90% by weight, advantageously between 70% and 87% by weight, more advantageously between 75% and 85% by weight, in particular between 82% and 85% by weight, relative to the total weight of the thermally conductive vitrimer composite material.

[0037] The thermal conductivity of the thermally conductive vitrimer composite material according to the invention, in particular measured at 23°C, is advantageously between 0.5 W / mK and 20 W / mK, more advantageously between 1 and 5 W / mK, in particular between 1.1 W / mK and 2 W / mK, more particularly between 1.2 W / mK and 1.6 W / mK.

[0038] The glass transition temperature (Tg) of the thermally conductive vitrimer composite material according to the invention is advantageously between 50°C and 250°C, advantageously between 90°C and 115°C, in particular between 100°C and 110°C. In particular, the Tg is measured by Differential Scanning Calorimetry (DSC) analysis, more particularly with the measuring device supplied by METTLER TOLEDO DSC FRS5 Star System on a temperature ramp at 10°C / min.

[0039] The present invention further relates to a process for preparing the thermally conductive vitrimer composite material according to the invention comprising the following successive steps: A)- mixture of the non-crosslinked epoxy resin comprising the thermally conductive fillers according to the invention with the crosslinking agent according to the invention, in particular 4,4'-dithiodianiline B)- heat treatment to obtain crosslinking of the epoxy resin C)- recovery of the thermally conductive vitrimer composite material.

[0040] The content of thermally conductive filler in the non-crosslinked epoxy resin according to the invention of step A) of the process according to the invention, before its mixing with 4,4'-dithiodianiline, is advantageously between 70% and 90% by weight, advantageously between 70% and 87% by weight, more advantageously between 75% and 85% by weight, relative to the total weight of the non-crosslinked epoxy resin comprising the thermally conductive fillers.

[0041] The stoichiometric ratio of non-crosslinked epoxy resin without thermally conductive fillers / crosslinking agent (in particular 4,4'-dithiodianiline) in the mixture of step A) of the process according to the invention is advantageously between 1 / 1.2, in particular it is 1 / 1.

[0042] In an advantageous embodiment, step B) according to the invention is carried out by pre-cooking the mixture at a temperature of 70°C to 90°C, advantageously 80°C, for a period of 50 min to 70 min, advantageously 1 hour, followed by cooking at a temperature of 150°C to 160°C, advantageously 150°C, for a period of 3 hours to 5 hours, advantageously 5 hours. Pre-cooking makes it possible to avoid settling and network heterogeneity phenomena.

[0043] The method according to the invention may further comprise, between steps A) and B), an intermediate step A1) of bringing the mixture into contact with a support to be impregnated so as to deposit the non-crosslinked mixture on said support. Advantageously, said support is a part of an electrical machine, in particular an electric motor, more specifically, it is the stator of an electric motor. Advantageously, said support is intended for aeronautics.

[0044] The non-crosslinked epoxy resin comprising the thermally conductive fillers according to the invention can be manufactured by conventional techniques well known to those skilled in the art and is in particular commercially available from LORD-PARKER under the name CoolTherm® EP-2000.

[0045] The present invention further relates to the non-crosslinked mixture of an epoxy resin comprising thermally conductive fillers and the crosslinking agent, the content of thermally conductive filler being between 70% and 90% by weight relative to the total weight of the mixture, said mixture advantageously being that obtained in step A) of the process comprising the following successive steps: A)- mixing of the non-crosslinked epoxy resin comprising the thermally conductive fillers with the crosslinking agent B)- heat treatment to obtain crosslinking of the epoxy resin C)- recovery of the thermally conductive vitrimer composite material, step B) being able to be implemented by pre-baking the mixture at a temperature of 70°C to 90°C, advantageously 80°C, for a duration of 50 to 70 min, advantageously 1 hour, followed by baking at a temperature of 150°C to 160°C, advantageously 150°C, for a duration of 3 to 5 hours, advantageously 5 hours.

[0046] The present invention further relates to the non-crosslinked mixture of an epoxy resin comprising, advantageously consisting essentially, in particular consisting, of thermally conductive fillers and 4,4'-dithiodianiline, the content of thermally conductive filler being between 70% and 90% by weight relative to the total weight of the mixture. Advantageously, the epoxy resin comprising thermally conductive fillers is as described above. In particular, said mixture is that obtained in step A) of the process according to the invention.

[0047] The present invention further relates to the use of the thermally conductive vitrimer composite material according to the invention for the insulation of electrical machines, advantageously for the insulation of electric motors, more advantageously for the encapsulation of stators of electric motors, in particular intended for aeronautics.

[0048] The present invention further relates to a kit for the preparation of a thermally conductive vitrimer composite material according to the invention comprising: - non-crosslinked epoxy resin comprising thermally conductive fillers and - the crosslinking agent according to the invention, in particular 4,4'-dithiodianiline.

[0049] The present invention finally relates to a method of insulating the stator of a motor. electrical by impregnation under vacuum and pressure, the process comprising the following successive steps: 1- placing the stator of the motor under vacuum placed in an autoclave in the presence of the non-crosslinked mixture according to the invention; 2- pressurizing the autoclave so as to cause said non-crosslinked mixture to penetrate into the interstices of said stator; 3- recovery of the stator impregnated with the mixture thus obtained 4- heat treatment so as to obtain crosslinking of the epoxy resin, 5- recovery of the stator insulated by the thermally conductive vitrimer composite material according to the invention.

[0050] Thus advantageously this method corresponds to steps A1), B) and C) of the method according to the invention as described above.

[0051] The thermally conductive vitrimer composite material according to the invention therefore allows the self-repair of microcracks and the possibility of recycling the resin of the composite material at the end of the life of the electrical machine. It also makes it possible to prevent the formation of microcracks which may appear during the encapsulation process (for example during the insulation process), that is to say during the implementation of the resin, and particularly during curing (steps B or 4 of the method according to the invention), due to the different expansion coefficients of the materials. The microcracks according to the invention may be up to a few millimeters wide and up to a few centimeters deep.

[0052] The present invention will be better understood in light of the description of the figures and examples which follow. The examples are given for information purposes only, without limitation. Brief description of the drawings

[0053] [Fig-1] [Fig.l] represents the measurement of the Relaxation Module under a force of 5N in shear at 1% at 1 rad / s at different temperatures (160°C to 190°C) of a composite material according to the invention obtained under the conditions of example 1.

[0054] [Fig.2] [Fig.2] represents the reshaping in a press at 170°C for 60 min for the composite material according to the invention obtained under the conditions of example 1.

[0055] [Fig.3] [Fig.3] represents the Arrhenius equation based on the relaxation times obtained in the relaxation measurements and giving an activation energy of 168 kJ / mol under the conditions of Example 1. EXAMPLE

[0056] Example 1: Preparation of a thermally conductive vitrimer composite material according to the invention by replacing the hardeners of a commercial two-component epoxy resin with 4,4'-dithiodianiline (crosslinking agent according to the invention).

[0057] LORD-PARKER's CoolTherm® EP-2000 commercial resin is an epoxy novolac resin comprising 82.1% by weight of thermally conductive fillers (alumina oxide). Normally the hardeners used in this commercial resin are MNA (Methyl Nadic Anhydride, CAS 25134-21-8) and 5-Norbornene-2,3-dicarboxylic anhydride (CAS 826-62-0) which are classified as hazardous substances according to EC Regulation No. 1272 / 2008.

[0058] These hardeners were replaced by 4,4'-dithiodianiline (4-AFD) with a stoichiometric ratio of uncrosslinked epoxy resin / 4,4'-dithiodianiline of 1 / 1.

[0059] After mixing the resin comprising the fillers with 4,4'-dithiodianiline, a 1-hour pre-cure at 80°C followed by a 5-hour curing at 150°C is applied. A perfectly homogeneous and crosslinked thermally conductive vitrimer composite material is obtained with a conversion greater than 95% and having a loading rate of 77% by weight in alumina and a glass transition temperature of approximately 105°C. The glass transition temperature was measured by Differential Scanning Calorimetry (DSC) analysis with the measuring device supplied by METTLER TOLEDO DSC FRS5 Star System on a temperature ramp at 10°C / min.

[0060] The synthesized composite material (16.4 g) could be reshaped under simple pressure (2 tons / m2) and thermal activation at 170°C for 1 h (which is normally impossible for a conventional thermosetting material) using a Model 4386 Bench Top Laboratory Manual Press with Electrically Heated Platens and a rectangular steel mold (7 cm x 4 cm x 4 mm). The material took the shape of the mold without any defects and with a homogeneous load distribution, which testifies to its dynamic capacity.

[0061] The crushed composite material could also be reshaped following the same protocol.

[0062] Furthermore, a crack is simulated on the synthesized composite material via a 2 cm long cutter cut on the completely crosslinked vitrimer composite material (16.4 g) of rectangular shape (7 cm x 4 cm x 4 mm), the repair is carried out in only 30 minutes at 170°C under pressure (0.75 ton / m2).

[0063] The composite material according to the invention can therefore be reshaped, repaired, and recycled, which clearly demonstrates its vitrimeric character.

[0064] In order to confirm these results, the composite material according to the invention was characterized by rheology tests at different temperatures, in order to measure the modulus and the relaxation time. All measurements are made with plane-plane geometries. The measurements are made on an ANTON PAAR device, model Modular Compact Rheometer (MCR) 302.

[0065] The samples of the composite material according to the invention are prepared using a punch on plates. The samples are then prepared from the crosslinked composite material and reshaped. The samples must be the size of the spindle used for measurement on the rheometer (8 mm diameter).

[0066] The measurement takes place at several temperatures: 190°C, 180°C, 170°C, and 160°C, under a force of 5N on a sample subjected to a shear of 1% at a speed of 1 rad / s with an air gap of 0.2 mm at a constant temperature.

[0067] The composite material according to the invention completely relaxes the stresses at all temperatures analyzed (from 160°C to 190°C), which confirms the vitrimeric character of the sample ([Fig.l]). The relaxation time r is measured at 0.37 according to the Arrhenius equation ([Fig.3]). The results obtained vary from 41s at 190°C to 875s at 160°C. It is therefore possible to conclude that the increase in temperature accelerates the relaxation just as the temperature accelerates the exchange kinetics.

[0068] From the relaxation times at different temperatures, it is possible to determine the activation energy using the Arrhenius equation which is as follows:

[0069] [Math.l] “S t,, exp — ' RT «• In g >3 KX ——r In 41 ; RT

[0070] The activation energy in the present case is equal to 168 kJ / mol.

[0071] A thermal conductivity analysis was carried out on the composite material according to the invention of Example 1. The thermal conductivity is estimated indirectly by measuring the thermal diffusivity. It is therefore necessary to use additional measurements: - Density at ambient temperature - Coefficient of thermal expansion between -60°C and +200°C - Specific heat between -60°C and +200°C.

[0072] The composite material according to the invention is in the form of a plate of dimensions (7cm x 4cm x 4mm) from which the samples for the different characterizations will be taken by water jet cutting.

[0073] The density of the composite material according to the invention is measured at room temperature (23°C) and atmospheric pressure noted p0 by double hydrostatic weighing in isopropanol. The density of the isopropanol is measured at the exact temperature of the test, i.e. 19°C. The measurement is carried out on 3 different samples taken from the mother plate. The density at room temperature of the sample is thus: p0 = 2587.72 ± 20.73 kg.m3.

[0074] The coefficient of linear expansion of the composite material according to the invention is measured using the LINSEIS L75 dilatometer. In this test, the sample in the form of a square with a side of 8 mm and a thickness of 2 mm is placed in pressure by a mobile quartz piston. The applied force is constant over time. A mobile furnace allows a temperature rise ramp to be imposed, during which the evolution of the elongation of the sample is measured by an inductive sensor with a resolution of 0.125 nm. A correction is applied to the measurement using a cycle on a quartz sample whose properties are perfectly known. The heating rate was set at 2 K.min 1 for all measurements, in order to avoid the appearance of thermal gradients in the sample. The measurements were carried out in the temperature range [-60°C - +200°C]. The results show a strong thermal expansion of the sample upon passing the Tg, characteristic of vitrimers. The coefficient of linear thermal expansion CTE of the material is then identified graphically over different temperature ranges. The following values ​​are thus obtained: - CTE = 3.28 x 105 K1 (-50°C <T<60°C) - CTE = 4.41 x 10 4 K 1 (75°C <T<95°C) - CTE = 4.99 x 105 K1 (100°C <T<160°C)

[0075] The specific heat measurements are carried out using a DSC TA Q200 calorimeter over the temperature range [-70°C - +210°C] at a heating rate of 10 K. min A sample of mass 17.71 mg was used. The specific heat is deduced from the heat flux from the mass of the sample and the test rate (10 K.min '). A linear regression is finally carried out before and after the glass transition temperature (up to 150°C) on the results. The analytical solutions are presented below: - Cp [J.kgAK *]=1013.21 + 3.60 x T [°C], for T <Tg - Cp [J.kgAK *]=1263.11 + 2.30 x T [°C], for T>Tg.

[0076] The method used for determining thermal diffusivity is the laser flash method. The device used is the LFA457 Netzsch available at the Laboratoire de Thermique et Energie in Nantes. The thermal diffusivity a expressed in m2.s 1 is given by the following relationship:

[0077] [Math.2] 1(7) a (T) = ---? , " p(T)fy(T)

[0078] where Cp is the specific heat in J.kgAKA p the density expressed in kg.m3 and X the thermal conductivity in Wm AK A

[0079] The samples for the measurement of thermal diffusivity are in the form of cylinders with a diameter of 12.7 mm and a thickness of approximately 2 mm. These are placed in a ceramic sample holder and a brief energy pulse is applied to the underside of the sample via a laser diode. An infrared detector is used to measure the temperature change on the front side, which is then used to determine the thermal diffusivity of the sample by minimizing the difference between the experimental temperature rise signal and a numerical model. The two surfaces of the sample are previously covered with a thin layer of graphite to maximize energy absorption. Finally, in order to determine the thermal diffusivity, it is necessary to know the thickness of the sample at each measurement temperature. At room temperature, the thickness is measured using a micrometer, and for other temperatures, the linear expansion (relative elongation) measurements, presented previously, are taken into consideration. The thermal diffusivity values ​​of the composite material according to the invention are presented in Table 1 for the different temperatures.Each value results from at least 3 measurements on the same sample.

[0080] [Tables 1] Temperature (°C) Thermal diffusivity a (mm2, s ') -60 0.66+0.06 -30 0.64+0.03 0 0.59+0.02 25 0.53+0.02 50 0.39+0.02 100 0.38+0.01 150 0.29+0.03 200 0.28+0.01

[0081] From the values ​​of thermal diffusivity, specific heat and density, the thermal conductivity of the composite material according to the invention is calculated using the previous formula. The obtained values ​​of thermal conductivity are presented in the following Table 2.

[0082] [Tables2] Temperature (°C) Thermal conductivity X (Wm *.K ') -60 1.22+0.14 -30 1.43+0.06 0 1.48+0.04 25 1.43+0.06 50 1.54+0.06 100 1.41+0.05 150 1.25+0.14 200 1.16+0.01

[0083] For comparison purposes, a sample (Comparative Example 1) is prepared from the same industrial resin and the non-dynamic reference hardener 4-MDA (4,4'-diaminodiphenylmethane). 4-MDA has the same chemical structure as 4-AFD without the dynamic disulfide bond of the following formula 4:

[0084] [Chem.6] C Jk JJ (4)

[0085] Theoretically, the formulated material cannot therefore be dynamic, which prevents any reshaping.

[0086] The sample is prepared under the same conditions as the composite material according to the invention. This sample is subjected to a reshaping test via a hot press and a mold, with an applied pressure of 1.5 ton / m2, and a temperature of 170°C for 30 min. The composite material obtained does not reshape. Indeed, the composite material breaks into pieces upon leaving the steel mold. With the pieces obtained, a second test is carried out for 1 h but no change is visible.

[0087] The comparative sample thus obtained therefore does not exhibit any vitrimeric property.

[0088] Example 2: Preparation of two other thermally conductive vitrimer composite materials according to the invention with different filler contents.

[0089] The process is identical to that of Example 1 with the exception of the epoxy resin used which is Poly[(phenyl glycidyl ether)-co-formaldehyde] with a mass molecular weight of 345 g / mol supplied by Sigma Aldrich and the quantity of thermally conductive fillers which was respectively 70% by weight relative to the total weight of the composite material (example 2) and 80% by weight relative to the total weight of the composite material (example 3).

[0090] The composite materials obtained all have the vitrimeric and thermally conductive character like the composite material of example 1.

Claims

Claims

1. Thermally conductive vitrimer composite material comprising: - (a) a crosslinked epoxy vitrimer resin matrix and - (b) thermally conductive fillers, the content of thermally conductive filler (b) being between 70% and 90% by weight relative to the total weight of the thermally conductive vitrimer composite material and said thermally conductive vitrimer composite material being obtainable by crosslinking an epoxy resin comprising said fillers with a crosslinking agent of the following general formula I: [Chem 1] H2N-ASSA-H2N (I) in which A represents a phenyl group or a linear or branched C1-C6 alkyl group, the phenyl or alkyl groups possibly being substituted by one or more substituents chosen from a C1-C20 alkyl group, a C5-C14 aryl group, an -OR2 group, a -(CO)R3 group, an -O(CO)R4 group, a -(SO)R5, a -NH-CO-R6 group, a -COOR7 group, a -NR8R9 group,a -NO2 group and a halogen atom and R2 to R9 independently represent a hydrogen atom, a C1-C2 alkyl group or a C5-C14 aryl group.,

2. Thermally conductive vitrimer composite material according to claim 1, characterized in that the epoxy resin of the matrix (a) is a glycidyl epoxy resin, advantageously a glycidyl epoxy ether resin, more advantageously a novolac epoxy resin.

3. Thermally conductive vitrimer composite material according to any one of claims 1 or 2, characterized in that the thermally conductive filler (b) is chosen from zinc oxide, aluminum oxide, silicon carbide, silicon nitride, aluminum nitride, carbon nanotubes, graphene, graphite, diamond, hexagonal boron nitride and mixtures thereof, advantageously it is aluminum oxide.

4. Thermally conductive vitrimer composite material according to any one of claims 1 or 2, characterized in that the crosslinking agent is 4,4'-dithiodianiline.

5. A method of preparing the thermally conductive vitrimer composite material according to any one of claims 1 to 4 comprising the following successive steps: A)- mixing of the non-crosslinked epoxy resin comprising the thermally conductive fillers with the crosslinking agent B)- heat treatment so as to obtain crosslinking of the epoxy resin C)- recovery of the thermally conductive vitrimer composite material.

6. Method according to claim 5, characterized in that step B) is carried out by pre-cooking the mixture at a temperature of 70°C to 90°C, advantageously 80°C, for a period of 50 to 70 min, advantageously 1 hour, followed by cooking at a temperature of 150°C to 160°C, advantageously 150°C, for a period of 3 to 5 hours, advantageously 5 hours.

7. Non-crosslinked mixture of an epoxy resin comprising thermally conductive fillers and crosslinking agent, the content of thermally conductive filler being between 70% and 90% by weight relative to the total weight of the mixture, said mixture advantageously being that obtained in step A) of the process according to any one of claims 5 or 6.

8. Use of the thermally conductive vitrimer composite material according to any one of claims 1 to 4 for the insulation of electrical and electronic equipment, advantageously for the insulation of electric motors, more advantageously for the encapsulation of stators of electric motors, in particular intended for aeronautics.

9. Kit for the preparation of a thermally conductive vitrimer composite material according to any one of claims 1 to 4 comprising: - the non-crosslinked epoxy resin comprising the thermally conductive fillers and - the crosslinking agent.

10. Method for insulating the stator of an electric motor by impregnation under vacuum and pressure, the method comprising the following successive steps: 1- placing the stator of the motor under vacuum placed in an autoclave in the presence of the non-crosslinked mixture according to claim 7; 2- pressurizing the autoclave so as to cause said non-crosslinked mixture to penetrate into the interstices of said stator; 3- recovering the stator impregnated with the mixture thus obtained 4- heat treatment so as to obtain crosslinking of the resin epoxy, 5- recovery of the stator insulated by the thermally conductive vitrimer composite material according to any one of claims 1 to 4.

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