EP0473515B1 - Method for making a absorbing shield based on electronic conductive polymer - Google Patents

Description

The present invention relates to a method of manufacturing a screen based on electronic conductive polymer intended to absorb hyper-frequencies in a large frequency range from 1 to 30GHz.

It can be used as an anechoid chamber coating (chamber without echo) for experimentation or as absorbent on ships, in the aeronautical field using light composite materials, as well as in airports.

In airports near large cities and surrounded by large buildings, the microwave frequencies emitted and received by control towers are highly disturbed. Also, the low reflectivity of airport buildings is essential in order to be able to steer planes safely on runways and take-offs. Currently, several major international airports are affected by this problem.

In the television domain, microwave absorption is also a crucial problem, as is clear from the document Proceedings of the International Conference on Ferrites, September-October 1980, Japan, "Countermeasures against TV Ghost Interference Using Ferrite" by Keiichi Akita, p. 885-889.

In the following text, we will speak of microwaves to indicate waves whose wavelength is between 30cm and 1cm.

Materials intended to absorb microwaves had, until now, called upon the properties which certain compounds had to generate dielectric and / or magnetic losses related to their chemical nature.

These materials are in the form of thin layers produced with dense magnetic materials such as ferrite or from the dispersion of a charge of high magnetic permeability and / or electrical permittivity (ferrite powder or iron) in an appropriate organic binder (see documents EP-A-243 162, EP-A-90 432).

These solutions, which have led to a certain number of industrial and commercial applications, however have limitations. Thus, the use of dense magnetic compounds such as ferrite generates high masses directly linked to the high density of these compounds, limiting their field of application and excluding them in particular from the aeronautical field.

In addition, the use of iron in the form of a powder dispersed in an insulating matrix has certain limitations insofar as it is always necessary to maintain an insulating character to the final material.

Furthermore, compounds with dielectric losses, such as systems where mineral conductive fillers such as carbon black, graphite, metallic powders in a binder are dispersed, require controlling the charge rate and the phenomena of particle aggregation with an extremely high level of reproducibility. Indeed, the functioning of these dielectric materials is linked to the fact that the curve of (dielectric) and therefore absorbent properties is located in the percolation zone (continuity of a dielectric property, conductivity or permittivity) where small variations in the concentration of the charge can produce large effects on the property (σ or ε) studied ("S" curve).

The composite absorbent materials of the mineral filler type dispersed in a binder are obtained by mixing a powder of a magnetic or dielectric compound and a binder powder then compression of the mixture. In such a technique, the load percolation threshold is generally between 30 and 40% by mass. This high amount of charge is detrimental to the homogeneity and therefore the solidity of the material and often leads to heavy materials excluding them from the space domain.

In addition, microwave absorbing materials with a mineral dielectric charge have a relatively narrow "pass" band (absorption band), which further limits their field of application.

Furthermore, there are electrically conductive organic polymers forming a class of materials having many applications, both because of their electronic property (good electrical conductivity) and their electrochemical property (reversible doping). As known electrically conductive organic polymers, mention may be made of polyacetylenes, polypyrroles and polyanilines.

To date, conductive polymers are mainly used for their electrochemical property in electrochemical generators (batteries) or electrochromic cells.

These electrically conductive polymers have the drawback of lacking mechanical strength and of having a very high electrical conductivity, of the order of 10 to 100 ohms ⁻¹ .cm ⁻¹.

Also, numerous studies have been done to date in an attempt to improve the mechanical strength of organic polymers that conduct electricity; they have led to the production of composite materials in which the mechanical strength of the polymers is improved by combining them with another material having better mechanical properties. This material can consist of a support such as paper on which the electrically conductive polymer is deposited, as described in the document Journal of Electronic Materials, vol. 13, n ° 1, 1984, "Some properties of polypyrrole-paper composites".

It is also possible to disperse the electrically conductive polymer in an insulating polyvinyl chloride (PVC) matrix as described in document US-A-4 617 353 or of a styrene-vinylsulfonate, styrene-acid copolymer. acrylic or styrene-butadiene-acrylic acid as described in document FR-A-2 616 790.

By document EP-A-0 357 059, it is also known to produce electromagnetic shielding, either by dissolving an insulating polymer and a monomer, or by swelling of the insulating polymer, then by exposure of the assembly. to vapors of an oxidizing agent of the monomer. A deposit of conductive polymer is thus obtained on the surface.

The subject of the invention is a method of manufacturing a new microwave absorbing screen based on electronic conductive polymer, in particular making it possible to remedy the above drawbacks. In particular, the screen obtained is relatively light, which allows its use in the aeronautical field. In addition, it has a much wider bandwidth than that of conventional screens with an inorganic conductive charge, which which allows its use in fields not accessible to the prior art.

• a) - à former une solution ou une suspension colloïdale du polymère organique isolant dans un solvant compatible avec la réaction chimique d'obtention du polymère conducteur dopé, cette solution ou suspension renfermant un dopant anionique oxydant ;
• b) - à ajouter à la solution ou suspension au moins un monomère liquide polymérisable par oxydation sous forme dudit polymère conducteur dopé, ce monomère comportant au moins une chaîne carbonée cyclique à doubles liaisons conjuguées et étant ajouté à raison de 1 à 30 parties en poids pour 100 parties en poids de polymère isolant ;
• c) - à laisser réagir l'ensemble jusqu'à oxydation complète du monomère et obtenir ainsi sa polymérisation in situ ;
• d) - à coprécipiter le polymère isolant et le polymère conducteur pour former une poudre d'un interpénétré polymère isolant-polymère conducteur ayant une conductivité électrique inférieure à 0,1S/cm ;
• e) - à mouler cette poudre d'interpénétré pour former une couche épaisse d'interpénétré de 1 à 10mm d'épaisseur.

More specifically, the subject of the invention is a method for manufacturing a microwave absorbing screen comprising at least one layer of an electronic conductive organic polymer, doped with an anion, and of an insulating thermoplastic organic polymer electrical interpenetrating, consisting, for each layer:

• a) - forming a colloidal solution or suspension of the insulating organic polymer in a solvent compatible with the chemical reaction for obtaining the doped conductive polymer, this solution or suspension containing an anionic oxidizing dopant;
• b) - to add to the solution or suspension at least one oxidizable polymerizable liquid monomer in the form of said doped conductive polymer, this monomer comprising at least one cyclic carbon chain with conjugated double bonds and being added in an amount of 1 to 30 parts by weight per 100 parts by weight of insulating polymer;
• c) - allowing the assembly to react until complete oxidation of the monomer and thus obtaining its polymerization in situ;
• d) - coprecipitating the insulating polymer and the conductive polymer to form a powder of an interpenetrating insulating polymer-conductive polymer having an electrical conductivity of less than 0.1 S / cm;
• e) - molding this interpenetrating powder to form a thick layer of interpenetrating 1 to 10mm thick.

In the screen obtained, the conductive polymer has a concentration of between 1 and 25% by weight of the alloy layer. Thus, the amount of conductive polymer dispersed in the insulating polymer is much lower than that used in mineral-loaded screens, which facilitates the manufacture of the screen and improves its mechanical properties. In addition, by playing on this quantity of conductive polymer, it is possible to modulate the electrical conductivity of the screen according to the envisaged application.

After step d) of the process of the invention, a powder of composite material is obtained, perfectly homogeneous and directly usable for the preparation of the absorbent screen, which makes it possible to avoid the operations of mixing the conductive polymer with the insulating polymer powder according to the conventional technology of absorbent screens with mineral charge which leads to concentrations in the conductive phase which are much too high.

In particular, the insulating polymer in the solution or suspension is in the form of micro-particles having an average particle size of 0.1 to 1 »m.

The emulsion or solution, formed during step c), is broken by adding a co-surfactant such as primary alcohols having from 1 to 10 carbon atoms such as methanol, ethanol, n-propanol, n-butanol, etc. After filtering, rinsing and drying the powder obtained can be compacted cold or hot, injected or extruded to form the absorbent layer or layers of the screen.

Injection is a layer manufacturing technique, attractive since it makes it possible to obtain various shapes, in a short time (1 to 3 minutes), making it possible to limit any degradation of the material.

The pressures applied are chosen from 1 to 300 MPa.

In addition, by varying the number of rinses and the nature of the washing agent, the radio characteristics of the screen can be modulated.

où Zs est l'impédance de surface du matériau absorbant avec Zs = $\sqrt{\text{»/ε}}\text{.tanh(-j(2πe/λo)}\sqrt{\text{ε »}}$

) où e est l'épaisseur du matériau, λo est la longueur d'onde incidente, j² est le nombre complexe valant -1 et » est la perméabilité magnétique du matériau.For a screen to be absorbent, it must have a low electrical conductivity ε and a thickness e sufficiently high so that the reflection coefficient R satisfies the equation:

where Zs is the surface impedance of the absorbent material with Zs = $\sqrt{\text{"/ Ε}}\text{.tanh (-j (2πe / λo)}\sqrt{\text{ε »}}$

) where e is the thickness of the material, λo is the incident wavelength, j² is the complex number equal to -1 and "is the magnetic permeability of the material.

This expression shows that for that | R | goes to 0, Zs has to go to 1-jo. Now tanh (x) tends to 0 when x tends to 0. This explains why it is advisable not to decrease the ratio e / λo too much, otherwise Zs will tend to 0. Orders of magnitude of e from 1 to 10mm are commonly accepted for the microwave frequencies considered.

Furthermore, this equation (universal whatever the frequency range used) indicates that an absorption agreement condition cannot be found whatever the level of (ε, »).

In the case of a material with dielectric losses, there exists only one pair (ε ′, ε˝), for a given ratio e / λo, leading to a tuning of the screen. Thus, in the case of electrical shielding, it is necessary to use only a small thickness of material (a few tens of »m) on the one hand and on the other hand a conductivity level as high as possible because we only measure the ratio between the transmitted energy and the incident energy.

An absorbent screen therefore has nothing to do with an electromagnetic shielding which must be conductive and thin in order to reflect the wave. incident. In addition, the manufacture of these thin shields is not compatible with the manufacture of very thick absorbent screens.

With the method of the invention, it is possible to produce a multilayer screen. In this case, several layers of interpenetrate are formed each time repeating steps a) to e) and each time changing the nature and / or the concentration of the monomer and / or the nature of the insulating polymer so as to obtain layers of different electrical conductivities and these different layers are stacked. In particular, the following steps can be carried out: formation of a first powder of given conductivity and cold compression of this powder to form the first layer; formation of a second powder, as described above, of conductivity different from the first, spreading of this second powder on the first layer then cold compression of the assembly and so on.

It is also possible to produce a multilayer screen with an electrical conductivity gradient. In other words, the screen comprises several stacked layers of different electrical conductivities, the electrical conductivity of this stack of layers decreasing from the first layer to the last layer. The lower layer is generally supported by an electrically conductive support and the upper layer is generally in contact with air. An impedance matching in the thickness of the screen is thus obtained, which further increases its "pass" band (or absorption band).

The conductive support can be a metal plate, a carbon fiber textile, a metallized polymer film or even a conductive paint. In the case of paint and textiles, the rigidity of the assembly is due to the polymer layers based on conductive polymer.

For concentrations of monomer in the solution or suspension, less than 1 part by weight per 100 parts of insulating polymer (which corresponds to concentrations less than 1% by weight of conductive polymer in the alloy), the screen is transparent to microwaves and for monomer concentrations greater than 30 parts by weight per 100 parts by weight of insulating polymer (which corresponds to concentrations greater than 25% by weight of conductive polymer in the alloy), the properties d absorption is degraded and the screen then plays the role of a microwave reflector.

It is also possible, unlike the prior art, to modulate the dielectric properties of the absorbent screen of the invention by varying the amount of dopant in the solution or suspension and therefore the doping rate of the electronic conductive polymer. by the anion, on the morphology of the conductive polymer obtained (nature and macromolecular structure different from the polymers) or on its degree of oxidation, for a determined concentration of conductive polymer in the absorbent layer.

The electronic conductive organic polymers which can be used in the invention can be copolymers or homopolymers which can be produced in a liquid medium and in particular in an aqueous medium from one or more monomers.

By way of example of such polymers, mention may be made of polypyrroles, polyanilines and polythiophenes, which are produced by chemical oxidation of the corresponding monomers in a solvent medium. It is specified that the term “polypyrroles” designates here the homopolymers and the copolymers of pyrrole and / or pyrrole derivatives such as those mentioned in documents FR-A-2 616 790 and EP-A-0 105 768.

The same is true for the term "polyanilines" and the term "polythiophenes".

In particular, the monomer comprises a heteroatom in the cyclic chain and preferably is chosen from pyrrole, thiophene, bithiophene or an alkyl-3-thiophene having from 1 to 12 carbon atoms.

The percolation threshold of the conductive polymer in the insulating polymer depends on the nature of the conductive polymer.

To obtain the polypyrrole by chemical means, oxidizing agents are used to polymerize the pyrrole in solution whose Redox potential is close to that of pyrrole.

By way of example of such oxidizing agents, mention may be made of ferric perchlorate, ferric sulfate, iron or copper chloride, double ammonium and cerium nitrate, ammonium persulfate and cation salts or organic cation radicals, for example methyl-10-phenothiazinium perchlorate.

The solvents used can be very diverse. Thus, water, aqueous acid solutions such as sulfuric acid, perchloric acid and organic solvents such as acetonitrile, nitrobenzene, chloroform and dichloromethane can be used. It is also possible to use the solvents mentioned in document US-A-4 617 353. These solvents must be compatible with the polymerization reaction of the chosen monomer.

Preferably, water or acidic aqueous solutions are used for the polymerization of pyrrole and aniline, and acetonitrile, chloroform or nitrobenzene is used for the polymerization of thiophene, bithiophene and 3-alkylthiophenes.

According to the invention, the electrically conductive organic polymer can also be a nitrogenous polymer such as those described in FR-A-2 588 007, which can also be prepared by oxidation by chemical route, preferably in an acid medium containing fluoride ions.

Electrically conductive organic polymers are, unlike mineral fillers, compatible with all conventional thermoplastics. Thus, they can be implemented by conventional pressing, injection, extrusion, etc. techniques. thus minimizing their manufacturing cost. In addition, these electronic conductive polymers have a strong film-forming ability, thus making it possible to produce multilayer laminates.

The insulating polymers which can be used in the invention must have good thermomechanical characteristics and must be emulsifiable, for example in water, since the polymerization reaction of the electronic conductive polymer is carried out in an aqueous medium. These insulating polymers can be homopolymers or copolymers.

Examples of insulating polymers that may be mentioned include polytetrafluoroethylene (PTFE) commonly known as teflon, polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC) which are insoluble in most organic solvents and which can be used. suspended in water in the form of very fine particles or emulsion.

Polymethyl methacrylate (PMMA), polystyrene, polyvinyl chloride (PVC), polycarbonates, polyphenylene oxides, polysulfones, and all the insulating polymers cited in document US Pat. No. 4,617,353 can also be used. , which are soluble in most organic solvents.

A third family of insulating polymers, both soluble in usual solvents and emulsifiable in water, such as the usual elastomer latexes (chloroprene, styrene-butadiene, isobutylene-isoprene copolymer, natural rubber, butadiene-acrylonitrile ,. ..) can be used.

Preferably, the insulating polymer is a perhalogenated polymer and in particular a perfluorinated and / or perchlorinated polymer.

The manufacture of a microwave absorbing monolayer screen according to the invention consists in reacting a monomer comprising at least one cyclic carbon chain with double bonds conjugated with an oxidant in a suspension of microparticles of the insulating polymer.

• la figure 1 représente l'évolution de la conductivité électrique (σ), exprimée en ohm⁻¹.cm⁻¹ en fonction de la concentration de polypyrrole (PoPy), exprimée en pourcentage en poids, dopé aux ions perchlorate, dans du polytétrafluoroéthylène ;
• la figure 2 représente les variations du coefficient de réflexion R, exprimé en décibels, d'un écran absorbant en fonction de la fréquence F de l'onde incidente, exprimée en GHz : la courbe a est relative à un écran obtenu selon l'invention constitué d'une couche de 5mm environ de polypyrrole-téflon à 5,57% en poids de polypyrrole, la courbe b est relative à un écran absorbant selon l'art antérieur de 5mm d'épaisseur environ, constitué d'un mélange de résine époxyde et de noir de carbone à 30% en poids, et la courbe c d'un écran absorbant selon l'art antérieur de 5mm d'épaisseur environ constitué d'un mélange de polychloroprène et de ferrite spinelle à 70% en poids de ferrite ;
• la figure 3 représente un écran absorbant multicouche obtenu par le procédé conforme à l'invention ;
• la figure 4 représente les variations du coefficient de réflexion R, exprimé en décibels, d'un écran absorbant multicouche obtenu selon l'invention en fonction de la fréquence F de l'onde incidente, exprimée en GHz ;
• les figures 5 et 6 donnent les variations de ε′ et ε˝ en fonction de la fréquence F de l'onde incidente (GHz) pour deux modes de fabrication différents (compression et injection) d'un même écran, et
• les figures 7 et 8 donnent les variations du coefficient de réflexion R(dB) en fonction de la fréquence F de l'onde incidente (GHz) pour les deux modes de fabrication correspondant aux figures 5 et 6.

Other characteristics and advantages of the invention will become more apparent from the description which follows, given by way of illustration and not limitation, with reference to the appended drawings in which:

• FIG. 1 represents the evolution of the electrical conductivity (σ), expressed in ohm⁻¹.cm⁻¹ as a function of the concentration of polypyrrole (PoPy), expressed in percentage by weight, doped with perchlorate ions, in polytetrafluoroethylene;
• FIG. 2 represents the variations of the reflection coefficient R, expressed in decibels, of an absorbing screen as a function of the frequency F of the incident wave, expressed in GHz: the curve a relates to a screen obtained according to the invention made up of a layer of approximately 5mm of polypyrrole-teflon at 5.57% by weight of polypyrrole, curve b relates to an absorbent screen according to the prior art of approximately 5mm in thickness, consisting of a mixture of epoxy resin and black of carbon at 30% by weight, and the curve c of an absorbent screen according to the prior art of approximately 5mm thickness made up of a mixture of polychloroprene and spinel ferrite at 70% by weight of ferrite;
• FIG. 3 represents a multilayer absorbent screen obtained by the method according to the invention;
• FIG. 4 represents the variations in the reflection coefficient R, expressed in decibels, of a multilayer absorbent screen obtained according to the invention as a function of the frequency F of the incident wave, expressed in GHz;
• FIGS. 5 and 6 give the variations of ε ′ and ε˝ as a function of the frequency F of the incident wave (GHz) for two different manufacturing modes (compression and injection) of the same screen, and
• FIGS. 7 and 8 give the variations of the reflection coefficient R (dB) as a function of the frequency F of the incident wave (GHz) for the two manufacturing modes corresponding to FIGS. 5 and 6.

d'un écran monocouche conforme à l'invention, le polymère isolant étant du polytétrafluoroéthylène.FIG. 1 represents the evolution of the conductivity σ as a function of the concentration,% by mass, of polypyrrole doped with ClO ions $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$

a monolayer screen according to the invention, the insulating polymer being polytetrafluoroethylene.

The percolation threshold for polypyrrole in a screen according to the invention is less than 2% by mass. This property is quite characteristic of materials where the growth of the conductive phase unlike those obtained by simple mixing of a conductive powder in an insulating phase for which the percolation threshold is around 30 to 40% by mass.

à une concentration de 33% en mole environ.This curve shows that one can obtain samples whose electrical conductivity is adjustable in a range of values less than 1 ohm ⁻¹ .cm ⁻¹, allowing use in microwave. It also shows that a conductivity of less than 1 ohm⁻¹.cm⁻¹ is obtained for a polypyrrole concentration of less than 10% by weight, the polypyrrole being doped with ClO ions. $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$

at a concentration of approximately 33 mol%.

.The curve a in FIG. 2 represents the variations of the reflection coefficient R as a function of the incident frequency F of an absorbent screen according to the invention, consisting of a plate 5mm thick approximately of polypyrrole-teflon alloy with 5.57% of polypyrrole by weight, doped with 33% by mole in ClO ions $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$

.

Curve b represents the variations in the reflection coefficient as a function of the incident frequency for a dielectric aggregate material of approximately 5 mm, consisting of a mixture of epoxy resin and carbon black at 30% by weight of carbon black.

Curve c represents the variations in the reflection coefficient as a function of the frequency of a magnetic aggregate material of approximately 5 mm, consisting of a mixture of polychloroprene and spinel ferrite at 30% by volume of spinel ferrite.

It can be seen from these curves that the absorbent screen of the invention has bandwidth characteristics close to those of the magnetic material, which may seem a priori surprising for a dielectric material. Such behavior is explained by the nature of the frequency variation of the electrical permittivity which has, in the case of conductive polymers, a greater decrease than that of the mineral compounds, this results in a widening of the bandwidth as is clear from curves a and b.

This widening of the bandwidth allows a widening of the field of application of absorbent screens according to the invention compared to those of the prior art.

Examples of the preparation of a monolayer absorbent screen based on doped polypyrrole, according to the invention, are given below.

EXAMPLE 1

Absorbent screen based on polypyrrole and polytetrafluroethylene.

First of all, 500 ml of a distilled water solution containing 0.2 mol / l of ferric perchlorate is prepared, which will be used as an oxidizing agent for the polymerization. Then added to this solution 84 g of an aqueous suspension of polytetrafluoroethylene (PTFE) at 60% by weight, having an average granulation of 0.3 »m. The solution obtained thus has a concentration of PTFE of 8.6% by weight and is deoxygenated by an argon sweep.

Then added thereto, while vigorously stirring, 1.95 ml of pyrrole, or 3.7 parts by weight of pyrrole per 100 parts by weight of dry teflon.

où k désigne la concentration molaire en PTFE dans le composite ; ceci donne un précipité noir pulvérulent obtenu par ajout au milieu réactionnel de 500ml d'éthanol.The polymerization of the pyrrole is allowed to continue in the presence of PTFE for one hour under argon, according to the following reaction: $${\text{C₄H₅N + 2,3Fe}}^{\text{III}}{\text{(ClO₄) ₃ + kPTFE → [C₄H₃N, 0.3ClO₄ / kPTFE] + 2HClO₄ + 2.3Fe}}^{\text{II}}\text{(ClO₄) ₂,}$$

where k denotes the molar concentration of PTFE in the composite; this gives a pulverulent black precipitate obtained by adding 500 ml of ethanol to the reaction medium.

The pulverulent product obtained is then filtered, rinsed with a 50/50 water-ethanol mixture and finally it is dried in an oven at 40 ° C. A homogeneous plate with a surface area of 300mm² and a thickness of approximately 5mm is then formed by cold compression of the dry powder under a pressure of 100MPa.

The quantitative chemical analysis of the screen obtained gives% N = 1.2 and% F = 65.6, ie a polypyrrole concentration of 5.57% by mass in the alloy. The variations in its reflection coefficient as a function of the frequency are those of curve a in FIG. 2.

The electrical conductivity σ of this screen is 6.5x10 ⁻⁵ohm ⁻¹.cm ⁻¹ and the real and imaginary parts ε ′ and ε˝ at 4GHz are respectively 10.0 and 8.0.

The conductivity measurement was made on a disc 50mm in diameter and 3mm thick, obtained by cold compression of the above dry powder at 100MPa, then metallized with gold by evaporation, using a Keithley 617 electrometer between each face. This measurement was perfectly reproducible.

EXAMPLES 2 to 7

Six monolayer screens were manufactured as in Example 1 for different amounts of polypyrrole. Table I, below, gives the electrical conductivity σ and the radioelectric characteristics of the screens obtained and in particular the real and imaginary parts ε ′, ε˝ of their electrical permittivity, as a function of the polypyrrole concentration in the screen, given in% by mass and the quantity of pyrrole in the solution, given in parts by weight per 100 parts by weight of teflon. This table also gives the thickness of tuning of these screens as a function of the frequency. The polypyrrole concentration in the alloy is given for a doping rate in perchlorate ions of around 33% by mole.

pour 4 pyrroles.Note that the mass rate of polypyrrole in the alloy is always higher than the initial rate of pyrrole in solution due to the fixation of 1ClO $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$

for 4 pyrroles.

The conductivity was measured as in Example 1, the polypyrrole concentration was determined from the% N alloy /% N pyrrole ratio.

• à l'inverse des résultats obtenus avec les noirs de carbone, ce mode de synthèse conduit à une excellente homogénéité que ce soit à l'intérieur d'un même lot (lots 2 et 3) ou sur deux lots différents (lots 4 et 5). Cette propriété est particulièrement remarquable puisque la reproductibilité sur ε est de l'ordre de quelques % entre deux lots.
• les conditions de rinçage permettent de moduler les caractéristiques radioélectriques de manière extrêmement fine pour une composition donnée (lots 5 à 7).

Examination of this table I makes it possible to draw the following conclusions:

• Unlike the results obtained with carbon blacks, this method of synthesis leads to excellent homogeneity, whether within a single batch (lots 2 and 3) or over two different lots (lots 4 and 5 ). This property is particularly remarkable since the reproducibility on ε is of the order of a few% between two batches.
• the rinsing conditions make it possible to modulate the radioelectric characteristics in an extremely fine manner for a given composition (lots 5 to 7).

EXAMPLES 8 and 9

As in Example 1, absorbent films based on polypyrrole were produced, the radioelectric characteristics and the electrical conductivities of which were determined as above, are given in the table.

These absorbent layers cannot be used only in multi-layer screens; ε˝ being greater than ε ′, unlike examples 1 to 7, there is no agreement thickness.

These examples also make it possible to show that the procedure described leads to a similarity of properties as soon as the quantities synthesized are multiplied by 5 (lots 8 and 9).

EXAMPLE 10

Preparation of an absorbent screen based on polypyrrole doped with anhydrous FeCl₃ and PVC.

First, a 100 ml solution of nitrobenzene containing 5 g of high molecular weight PVC is prepared. 6g of anhydrous FeCl₃ are added to the solution which is stirred for ten minutes. 0.4 ml of pyrrole is introduced into the mixture, which leads to instant polymerization of the pyrrole. The polymerization is continued for 45 minutes under a stream of nitrogen.

The reaction mixture obtained is then poured dropwise into 1000 ml of ethanol. There is then precipitation of the polypyrrole-PVC alloy which is then filtered and rinsed with ethanol on sintered glass. The powder is dried under vacuum and is implemented by cold pressing. The polypyrrole concentration in the product obtained is approximately 8% by weight.

• f = 130 MHz : ε = 6,5-j8,8
• f = 1 GHz : ε = 4,5-j1,9
• f = 4 GHz : ε = 3,9-j0,8
• f = 10 GHz :ε = 3,7-j0,4.

The radioelectric characterizations give the following values:

• f = 130 MHz: ε = 6.5-j8.8
• f = 1 GHz: ε = 4.5-j1.9
• f = 4 GHz: ε = 3.9-j0.8
• f = 10 GHz: ε = 3.7-j0.4.

There is therefore in this case a material having an absorbent character up to 10 GHz and a transparent character to radio waves from 10 GHz.

One can of course increase the polypyrrole concentration in the mixture and obtain higher permittivity levels.

The microwave absorbing screen according to the invention can be a single-layer screen as described in Examples 1 to 7 and 10 or else a multi-layer screen as shown in FIG. 3. This multi-layer screen has several layers 1, 2, 3, ..., n stacked, possibly on a conductive support 12 having decreasing electrical conductivities from layer 1 to layer n.

The following example relates to such a multilayer screen.

EXAMPLE 11 :

The lower layer or first layer may consist of a layer 2 mm thick according to Example 8, the second layer may consist of a layer 1.3 mm thick according to Example 4 and the third layer consist of a 2.7mm thick layer according to Example 10.

The curves d, e and f of FIG. 4 relate to the evolution of the reflection coefficient R (expressed in decibels) with the frequency F (expressed in GHz) for the first, second and third layers alone and the curve g relates to this same development for the multilayer screen made up of these three layers. Such a stack gives a screen which has attenuation at -10dB between 4.8 and 20 GHz.

In general, the number of layers and the thickness of each of the layers depends on the application envisaged, the total thickness of the screen can vary from 1 to 10 mm.

EXAMPLE 12

Absorbent screen based on polycarbonate and polypyrrole.

200g of polycarbonate are dissolved in 5 liters of chloroform at room temperature. The oxidizing agent (324 g of anhydrous ferric chloride) is added in solution in ethyl ether to the above mixture. The pyrrole (60 ml) is added all at once to the reactor. The solution obtained contains 29 parts by weight of pyrrole per 100 parts by weight of polycarbonate.

The polymerization of the pyrrole is carried out with stirring, under an inert atmosphere for 1 hour.

The precipitation takes place in 30 liters of methanol at a flow rate of 50 ml / min. After filtration and washing with methanol, the powder is dried under vacuum at 20 ° C. The powder has a particle size of 120 »m and a conductivity of 3.4.10⁻² ohms ⁻¹.cm⁻¹ at room temperature.

The radioelectric measurements carried out on the powder pressed at 200 ° C under 18.10⁷PA (1800 bars) for 50 minutes give a value of ε at 4GHz = 23-j47.

EXAMPLES 13 and 14

Absorbent screen based on PVC and polypyrrole doped with FeCl₃ hexahydrate.

200g of powdered PVC are dissolved in 5 liters of nitrobenzene at room temperature. 300g of FeCl₃, 6H₂O are added to the mixture with stirring.

Once the solution has become clear, 32 ml of pyrrole (ie 15.4 parts by weight of pyrrole per 100 parts by weight of PVC) are then added suddenly to the reactor and the reaction continues with stirring for 1 hour.

The mixture is then transferred to a second reactor containing 30 liters of ethanol. The transfer is carried out at a flow rate of 50 ml / min, with vigorous stirring, which makes it possible to control the size of the particles formed during precipitation.

The mixture is then transferred to a pressure filter and rinsed with ethanol until a colorless rinsing solution is obtained.

The powder is dried under vacuum at 20 ° C., then deagglomerated to form a fluid powder consisting of spheres with a diameter of 150 μm having an interpenetration of polypyrrole grains in the PVC spheres.

In order to obtain a powder having optimal processing characteristics, it is stabilized using a commercial antioxidant (2%) (organometallic tin compound) Irgastab M 70 from Cyba Geigy and contains a commercial processing agent Paraloid 4M 355 from Rohm and Haas making it possible to lower the viscosity in the molten state (4%). The polypyrrole concentration measured in the alloy is 10% by volume.

Two types of implementation have been tried and have shown quite different radio behaviors.

Example 13 Uniaxial pressing of the powder

The powder is introduced into a mold previously heated to 170 ° C., of dimensions allowing, after cooling, to extract a coaxial test piece compatible with the measurement system used.

The mold is then pressurized with a pressure build-up speed of 3.8.10⁷ Pa / min up to a maximum pressure of 18.10⁷Pa (1800 bar). The pressure is then maintained for 3 minutes then the mold is extracted from the machine to be cooled. The sample is then ready for radio measurement.

Example 14 : Injection of the powder.

The powder is introduced into the hopper of a injection molding machine, the temperature of the screw being adjusted by zone between 150 and 180 ° C and the mold being preheated to 60 ° C. The molding pressure is around 1.2.10⁸Pa with a total cycle time of 3 minutes.

In Table II below, the characteristics obtained on a pressed or injected alloy according to Examples 13 and 14 are given.

• Matériau pressé, (figure 5)
  La microstructure créée au moment de la synthèse n'est pas détruite ; (on retrouve celle du polypyrrole téflon) puisqu'on observe un comportement classique, c'est-à-dire une variation de ε˝ avec la fréquence F selon la loi ε˝= (σo/εo)(2πF)^-S, où σ o est la conductivité statique du matériau et εo est la permittivité du vide (avec ici l'exposant s valant -0,48).
• Matériau injecté, (figure 6)
  Parallèlement à une diminution globale du niveau de ε′ (courbe j) imputable à une légère dégradation de l'alliage au cours de l'injection, on note l'apparition d'un phénomène nouveau. Il y a présence vers 5 GHz d'une relaxation diélectrique. Celle-ci semble imputable à une modification de la microstructure du matériau. Les agrégats formés au cours du procédé de croissance sont détruits et des grains de polypyrrole se réarrangent pour percoler avec une taille inférieure à celle obtenue dans l'exemple 13.

Figures 5 and 6 are respectively related to the screen of Examples 13 and 14. The curves h and j of Figures 5 and 6 give the evolution of ε ′ with the frequency and the curves i and k of Figures 5 and 6 give the evolution of ε˝ with frequency.

• Pressed material, (figure 5)
  The microstructure created at the time of synthesis is not destroyed; (we find that of the teflon polypyrrole) since we observe a classic behavior, that is to say a variation of ε˝ with the frequency F according to the law ε˝ = (σo / εo) (2πF) ^-S , where σ o is the static conductivity of the material and εo is the permittivity of the vacuum (with here the exponent s being -0.48).
• Injected material, (Figure 6)
  In parallel with an overall decrease in the level of ε ′ (curve j) due to a slight degradation of the alloy during injection, there is the appearance of a new phenomenon. There is a dielectric relaxation around 5 GHz. This seems to be due to a modification of the microstructure of the material. The aggregates formed during the growth process are destroyed and polypyrrole grains rearrange themselves to percolate with a size smaller than that obtained in Example 13.

We therefore have, with the same type of alloy PVC-polypyrrole, of two families of materials whose properties are exclusively linked to the mode of implementation.

The curves in FIGS. 7 and 8 give the evolution of the reflection coefficient R (in dB) as a function of the frequency of the incident wave (in GHz) for the screens of Examples 13 and 14 respectively and for different thicknesses. Curves l and q relate to 1mm layers; the curves m and r of the 2mm layers; the n and s curves of the 3mm layers; the curves o and t of the 4mm layers and the curves p and u of the 5mm layers.

• couche interne avec ε′< ε˝ dans tout le domaine de fréquence,
• couche médiane avec ε′> ε˝ dans une partie du domaine de fréquence comme dans l'exemple 13 (figure 5),
• couche externe du type plan transparente en polymère isolant de l'alliage (ici le PVC, dont ε vaut 2,7).

Note that the pressed material (Figure 7) is difficult to tune in a single layer; such a material is above all of interest in a structure with a conductivity gradient with a three-layer structure of the type:

• internal layer with ε ′ <ε˝ in the whole frequency domain,
• middle layer with ε ′> ε˝ in part of the frequency domain as in Example 13 (Figure 5),
• outer layer of the transparent planar type of insulating polymer of the alloy (here PVC, of which ε is 2.7).

The injected material (example 14) exhibits a quite remarkable behavior since on the one hand it remains resonant in a very large frequency range (4-16 GHz), which allows use in monolayer in a wide range of thicknesses and on the other hand, quite significant bandwidths (35% at 10 dB) with conservation of a few dB up to 20 GHz (Figure 8). Indeed, it is noted that a conventional absorbent has a relatively symmetrical absorption peak while the injected material, in thickness 2mm for example (curve r) has a rise not exceeding -6dB, which makes it interesting in applications putting it in association with insulating layers no longer working in this frequency range.

Claims (10)

1. Method of manufacturing a microwave-absorbing screen (10) composed of at least one layer (1, n) of an electronically conductive organic polymer, doped by an anion, and of a thermoplastic organic polymer, electrically insulating, these polymers being interpenetrating networks, consisting, for each layer:

   a) - in forming a solution or a colloidal suspension of the insulating organic polymer in a solvent compatible with the chemical reaction for obtaining the doped conductive polymer, this solution or suspension containing an oxidizing anionic dopant;

   b) - in adding to the solution or suspension at least one liquid monomer which can be polymerized by oxidation into the form of the said doped conductive polymer, this monomer including at least one cyclic carbon-containing chain with conjugated double bonds and being added in a proportion of 1 to 30 parts by weight per 100 parts by weight of insulating polymer;

   c) - in letting the system react until complete oxidation of the monomer is obtained and thus polymerizing it in-situ;

   d) - in coprecipitating the insulating polymer and the conductive polymer in order to form a powder of an insulating-polymer/conductive-polymerinterpenetrating network having an electrical conductivity less than 0.1 S/cm;

   e) - in moulding this powder of interpenetrating network in order to form a thick layer of interpenetrating network of 1 to 10 mm in thickness.
2. Method according to Claim 1, characterized in that step e) consists of a compacting operation or an injection operation.
3. Method according to Claim 1 or 2, characterized in that the insulating polymer is a polymer which can be emulsified in water.
4. Method according to any one of Claims 1 to 3, characterized in that the insulating polymer is a perhalogenated polymer.
5. Method according to Claim 1 or 2, characterized in that the insulating polymer is chosen from polyvinyl chloride, polytetrafluoroethylene and polycarbonates.
6. Method according to any one of Claims 1 to 5, characterized in that the monomer includes a heteroatom in the cyclic chain.
7. Method according to any one of Claims 1 to 6, characterized in that the monomer is chosen from pyrrole, thiophene, bithiophene and an alkyl-3-thiophene.
8. Method, according to any one of Claims 1 to 7, for manufacturing a microwave-absorbing multilayer screen, characterized in that several layers of interpenetrating network are formed by repeating each time steps a) to e) and by changing each time the nature and/or the concentration of the monomer and/or the nature of the insulating polymer so as to obtain layers of different electrical conductivities and in that these various layers are stacked.
9. Method according to Claim 8, characterized in that the various layers are stacked so that the electrical conductivity of the stack of layers decreases from the first layer (1) to the last layer (n).
10. Method according to any one of Claims 1 to 9, characterized in that a conductive support (12) is placed in contact with the layer or layers of interpenetrating network in order to support this or these layers.

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