DE69107936T2 - Manufacturing method of a shield containing an electronic conductive polymer. - Google Patents

Description

The present invention relates to a process for manufacturing a screen based on electron-conducting polymers, intended to absorb high frequencies in a wide frequency range from 1 to 30 GHz.

It can be used as a lining for an anechoic (echo-free) room for measuring technology or as an absorbent on ships, in the aviation sector where lightweight composite materials are needed, and at airports.

At airports located near large cities and surrounded by large buildings, the very high frequency waves sent and received by the control towers are severely disrupted. Therefore, a low reflectivity of the airport buildings is important so that aircraft can be safely guided onto the runways. Currently, several large international airports are affected by this problem.

In the field of television, absorption of microwaves is also a critical problem, as is clearly shown in the document: Proceedings of the International Conference on Ferrites, September-October 1980, Japan, "Countermeasures against TV Ghost Interference Using Ferrite" by Keiichi Akita, pp. 885-889.

In the following text, microwaves are used to describe waves whose wavelength is between 30 cm and 1 cm.

Until now, the materials intended for absorbing microwaves have used the properties that certain compounds to be produced have dielectric and/or magnetic losses, related to their chemical nature.

These materials occur in the form of thin layers that are coated with dense magnetic materials, such as ferrite, or starting from the dispersion of a filler with increased magnetic permeability and/or dielectric constant (ferrite or iron powder) in a suitable organic binder (see documents EP-A-243 162, EP-A-90 432).

These solutions, which have resulted in a number of industrial and commercial applications, have limitations. For example, the use of dense magnetic compounds such as ferrite leads to increased masses, which are directly related to the high density of these compounds, which limits their field of application and excludes them in particular from the aeronautics sector.

Furthermore, the use of iron in the form of a powder dispersed in an insulating matrix is subject to certain limitations whenever it is necessary to maintain an insulating character for the final product.

Moreover, compounds with electrical losses such as those in which conductive inorganic fillers such as carbon black, graphite or metal powder are dispersed in a binder require the degree of filling and the aggregation phenomena of the particles to be controlled with an extremely high degree of reproducibility. In fact, the functioning of these dielectric materials is linked to the fact that the curve of the dielectric and therefore absorbing properties is in the area of penetration (continuity of a dielectric property, conductivity or dielectric constant), where small changes in the concentration of the filler can have a major impact on the property (or ε) under study (S-shaped curve).

The absorbent composite materials of the type with inorganic filler dispersed in a binder are obtained by mixing a powder of a magnetic or dielectric compound and a binder powder and then pressing the mixture. In such a technique, the threshold for the penetration of the filler is between 30 and 40% by mass. This increased amount of filler is essential for the homogeneity and therefore for the Strength of the material is detrimental and often results in heavy materials, which excludes them from space technology.

In addition, the microwave absorbing materials with inorganic dielectric filler show a relatively narrow bandwidth (absorption band), which further limits their application range.

There are also electrically conductive organic polymers, which form a class of materials with numerous applications, both due to their electronic (good electrical conductivity) and their electrochemical properties (reversible doping). Well-known electrically conductive organic polymers include polyacetylenes, polypyrroles and polyanilines.

To date, conductive polymers are used primarily because of their electrochemical properties in electrochemical generators (batteries) or in electrochromic cells.

These electrically conductive polymers have the disadvantage that they lack mechanical resistance and that they have a very high electrical conductivity of the order of 10 to 100 X⁻¹.cm⁻¹.

Therefore, to date, numerous studies have been carried out to try to improve the mechanical strength of electrically conductive organic polymers; these have led to the production of composite materials in which the mechanical strength of the polymers is improved by combining them with another material with 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, No. 1, 1984, "Some properties of polypyrrole-paper composites".

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

From document EP-A-0 357 059 it is also known to produce electromagnetic shields by either dissolving an insulating polymer and a monomer or by expanding an insulating polymer and then exposing the arrangement to vapors of an agent which oxidizes the monomer. This results in a surface deposition of conductive polymer.

The invention relates to a process for producing a new microwave-absorbing screen based on an electron-conducting polymer, which makes it possible to remedy the above-mentioned disadvantages. In particular, the screen obtained is relatively light, which allows it to be used in the aeronautical sector. In addition, it has a much wider bandwidth than conventional screens with an inorganic conductive filler, which allows it to be used in areas not accessible to the current technology.

More specifically, the invention relates to a process for manufacturing a microwave-absorbing screen comprising at least one layer of an electron-conducting organic polymer doped with an anion and of an electrically insulating thermoplastic organic polymer which penetrate each other, the process consisting in, for each layer:

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

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

c) - the arrangement until complete oxidation of the monomers and thus achieve its polymerization in situ;

(d) - co-precipitating the insulating polymer and the conductive polymer to form a powder from a penetration of the insulating polymer with the conductive polymer having an electrical conductivity of less than 0.15 S/cm;

e) - to shape this penetration powder to form a thick penetration layer of 1 to 10 mm thickness.

In the screen obtained, the conductive polymer has a concentration of between 1 and 25% by weight of the alloy layer. The amount of conductive polymer dispersed in the insulating polymer is therefore much lower than that used in screens with inorganic filler, which facilitates the manufacture of the screen and improves its mechanical properties. Furthermore, by varying this amount of conductive polymer, it is possible to change the electrical conductivity of the screen depending on the intended application.

After step d) of the process of the invention, a powder of composite material is obtained which is perfectly homogeneous and can be used directly for the manufacture of the absorbent screen, which makes it possible to avoid the operations of mixing the conductive polymer with the powder of the insulating polymer according to the classic technique of absorbent screens with inorganic filler, a technique which leads to much too high concentrations of conductive phase.

In particular, the insulating polymer is present in the solution or suspension in the form of microparticles having an average particle size of 0.1 to 1 µm.

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

Injection molding is an attractive layered manufacturing technique that allows different shapes to be obtained in a short time (1 to 3 minutes), thus limiting any possible degradation of the material.

The applied pressures are selected between 1 and 300 MPa.

By varying the number of rinses and the nature of the agent used for washing, one can also change the high frequency properties of the screen.

For a screen to be absorbent, it must have a low electrical conductivity ε and a sufficiently large thickness e so that the reflection coefficient R satisfies the equation

where Zs is the impedance at the surface of the absorbing material with Zs = (u/e).tanh(-j(2πe/λ₀) (εu)) (e is the thickness of the material, λ₀ is the incident wavelength, j² is the complex number with value -1 and µ is the magnetic permeability of the material).

This expression shows that for R to tend to 0, it is necessary that Zs tends to 1-jo. However, tanh(x) tends to 0 when x tends to 0. This explains why it is advantageous not to let the ratio e/λ0 fall too much, otherwise Zs tends to 0. Orders of magnitude for e of 1 to 10 mm are generally accepted for the microwave frequencies considered.

Furthermore, this equation (which is universally valid, whatever frequency range is used) shows that a tuning condition for the absorption for arbitrary values of (ε, m) cannot be found.

In the case of a material with dielectric losses, there is only one pair (ε', ε'') for a given ratio e/λ₀ that leads to a tuning of the screen. In the case of electrical shielding, it is therefore necessary to On the one hand, it is important to use only a small thickness of the material (a few tens of μm) and, on the other hand, to use as high a value of conductivity as possible, since only the ratio between the transmitted energy and the incident energy is measured.

An absorbing screen therefore has nothing to do with an electromagnetic shield, which must be conductive and of small thickness in order to reflect the incident wave. Moreover, the manufacture of these shields of small thickness is not compatible with the manufacture of absorbing screens of large thickness.

The method of the invention makes it possible to produce a multilayer screen. In this case, several layers of penetration are formed by repeating each time steps a) to e) and changing each time the type and/or concentration of the monomer and/or the type of insulating polymer to obtain layers with different electrical conductivities, and superimposing these layers. In particular, the following steps can be carried out: preparing a first powder of a given conductivity and cold-pressing this powder to form the first layer, preparing a second powder as previously described, of a different conductivity to the first, spreading this second powder on the first layer, then cold-pressing 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 superimposed layers with different electrical conductivities, the electrical conductivity of this sequence of layers decreasing from the first layer to the last. The bottom layer is generally supported by an electrically conductive base and the top layer is generally in contact with the air. This provides impedance matching in the thickness of the screen, which further increases its bandwidth (or absorption band).

The conductive support can be a metal plate, a fabric of carbon fibers, a metallized polymer film or finally a conductive paint. In the case of the paint and the fabric, the dimensional stability of the arrangement is based on the polymer layers based on conductive polymers.

For monomer concentrations in the solution or suspension below 1 part by weight per 100 parts by weight of the insulating polymer (corresponding to concentrations below 1 wt% of the conductive polymer in the alloy) the screen is transparent to microwaves, and for monomer concentrations above 30 parts by weight per 100 parts by weight of the insulating polymer (corresponding to concentrations above 25 wt% of the conductive polymer in the alloy) the absorption properties deteriorate and the screen now plays the role of a microwave reflector.

Furthermore, in contrast to the previous technique, it is possible to vary the dielectric properties of the absorbing screen of the invention by varying the amount of dopant in the solution or suspension and thus the degree of doping of the electron-conducting polymer by the anion, by changing the shape of the conductive polymer obtained (different type and different macromolecular structure of the polymers) or by varying its degree of oxidation for a predetermined concentration of the conductive polymer in the absorbing layer.

The electron-conducting organic polymers usable in the invention can be copolymers or homopolymers which can be prepared from one or more monomers in a liquid medium, and in particular in an aqueous medium.

Examples of such polymers include polypyrroles, polyanilines and polythiophenes, which are prepared by chemical oxidation of the corresponding monomers in a solvent medium. For the avoidance of doubt, the term "polypyrroles" here designates homopolymers and copolymers of pyrrole and/or of pyrrole derivatives, such as the derivatives mentioned in documents FR-A-2 616 790 and EP-A-0 105 768.

The same applies to the terms "polyanilines" and "polythiophenes".

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

The threshold for penetration of the conductive polymer into the insulating polymer depends on the nature of the conductive polymer.

To obtain polypyrrole chemically, oxidizing agents are used to polymerize the pyrrole in a solution whose redox potential is close to that of pyrrole.

Examples of such oxidizing agents include iron(III) perchlorate, iron(III) sulfate, iron or copper chloride, the double nitrate of ammonium and cerium, ammonium persulfate and salts of organic cations or organic radical cations, for example methyl-10-phenothiazinium perchlorate.

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

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

According to the invention, the electrically conductive organic polymer can also be a nitrogen-containing polymer such as the polymers described in FR-A-2 588 007, which can be prepared in the same way by oxidation by chemical means, preferably in an acidic medium containing fluoride ions.

In contrast to those with inorganic fillers, the electrically conductive organic polymers are compatible with all common thermoplastics. They can therefore be processed using the classic techniques of pressing, injection molding, extrusion etc., which minimizes the manufacturing costs. In addition, these electron-conducting polymers have a high film-forming ability, which allows the production of multi-layered sheet structures.

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

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

It is also possible to use polymethyl methacrylate (PMMA), polystyrene, polyvinyl chloride (PVC), polycarbonates, polyphenylene oxides, polysulfones and all the insulating polymers listed in document US-A-4 617 353, which are soluble in most organic solvents.

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

Preferably, the insulating polymer is a perhalogenated polymer, 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 containing at least one cyclic carbon chain with conjugated double bonds with an oxidizing agent in a suspension of microparticles of the insulating polymer.

Other characteristics and advantages of the invention will become more apparent from the following description, which is given by way of illustration and not by way of limitation, with reference to the accompanying drawings.

- Fig. 1 shows the variation of the electrical conductivity ( ), expressed in X⁻¹.cm⁻¹, as a function of the concentration of polypyrrole doped with perchlorate ions (PoPy), expressed in % by weight, in polytetrafluoroethylene;

- Fig. 2 represents the variations in the reflection coefficient R, expressed in dB, of an absorbing screen as a function of the frequency F of the incident wave, expressed in GHz: curve a relates to a screen obtained according to the invention, consisting of a layer of approximately 5 mm of polypyrrole-Teflon containing 5.57% by weight of polypyrrole, curve b relates to an absorbing screen according to the prior art, approximately 5 mm thick, consisting of a mixture of epoxy resin and 30% by weight of carbon black, and curve c relates to an absorbing screen according to the prior art, approximately 5 mm thick, consisting of a mixture of polychloroprene and ferrite-spinel containing 70% by weight of ferrite;

- Figure 3 represents an absorbent multilayer screen obtained by the process according to the invention;

- Figure 4 represents the variations in the reflection coefficient R, expressed in dB, of an absorbent multilayer screen obtained according to the invention as a function of the frequency F of the incident wave, expressed in GHz;

- Fig. 5 and 6 show the variations of ε' and ε'' as a function of the frequency F of the incident wave (GHz) for two different manufacturing methods (pressing and injection molding) of the same screen, and

- Fig. 7 and 8 show the variations of the reflection coefficient R(dB) as a function of the frequency F of the incident wave (GHz) for the two manufacturing methods corresponding to Fig. 5 and 6.

Fig. 1 shows the change in conductivity as a function of the concentration (mass %) of polypyrrole doped with ClO₄⊃min; ions of a single-layer screen according to the invention, where the insulating polymer is polytetrafluoroethylene is.

The penetration threshold of the polypyrrole in a screen according to the invention is lower than 2% by mass. This property is very characteristic of materials in which the growth of the conductive phase takes place in situ, in contrast to materials obtained by simply mixing a conductive powder in an insulating phase, for which the penetration threshold is 30 to 40% by mass.

This curve shows that samples can be obtained whose electrical conductivity can be varied in a range of values below 1 X⁻¹.cm⁻¹, which allows use at very high frequencies. It also shows that a conductivity of less than 1 X⁻¹.cm⁻¹ is obtained for a polypyrrole concentration of less than 10% by weight, the polypyrrole being doped with ClO₄⁻ ions at a concentration of approximately 33 mol%.

Curve a of Figure 2 represents the variations of the reflection coefficient R as a function of the incident frequency F for an absorbing screen according to the invention consisting of a plate of approximately 5 mm thickness of a polypyrrole-Teflon alloy containing 5.57% by weight of polypyrrole doped with 33 mol% of ClO₄⊃min; ions.

Curve b represents the variations of the reflection coefficient as a function of the incident frequency for a dielectric composite material of approximately 5 mm consisting of a mixture of epoxy resin and carbon black containing 30 wt% carbon black.

Curve c represents the variations of the reflection coefficient as a function of frequency of a magnetic composite material of approximately 5 mm consisting of a mixture of polychloroprene and ferrite spinel containing 30 vol% ferrite spinel.

It can be seen from these curves that the absorbing screen of the invention has bandwidth characteristics close to those of the magnetic material, which may seem surprising a priori for a dielectric material. Such behavior is explained by the nature of the Frequency dependence of the dielectric constant, which shows a greater decrease in the case of conductive polymers than in the case of inorganic compounds. This results in a broadening of the bandwidth, as can be clearly seen from curves a and b.

This broadening of the bandwidth allows an extension of the application area of the absorbing screens according to the invention compared to those of the previous technology.

Examples of the manufacture of an absorbent single-layer screen based on doped polypyrrole according to the invention are given below.

example 1

Absorbent screen based on polypyrrole and polytetrafluoroethylene.

First, 500 ml of a solution of distilled water containing 0.2 mol/l of iron(III) perchlorate, which serves as an oxidizing agent for the polymerization, are prepared. Then, 84 g of an aqueous suspension of polytetrafluoroethylene (PTFE) with an average particle size of 0.3 μm and 60% by weight are added to this solution. The solution obtained therefore has a PTFE concentration of 8.6% by weight and is freed from oxygen by passing argon through it.

Then add 1.95 ml of pyrrole, i.e. 3.7 parts by weight of pyrrole to 100 parts by weight of dry Teflon, while stirring vigorously.

The polymerization of the pyrrole is allowed to proceed in the presence of PTFE for one hour under argon according to the following reaction:

C₄H₅N + 2.3FeIII(ClO₄)₃ + kPTFE

T&lsqbstr;C&sub4;H&sub3;N 0.3ClO&sub4;/kPTFE&rsqbstr; + 2HClO4; + 2.3FeII(ClO₄)₂ ,

where k is the molar PTFE concentration in the composite material. This results in a black powdery precipitate, which is obtained by adding 500 ml of ethanol to the reaction medium.

The resulting powdered product is then filtered, rinse it with a 50/50 water-ethanol mixture and then dry it in a drying oven at 40ºC. Then, by pressing the dry powder in the cold under a pressure of 100 MPa, a homogeneous plate with a surface area of approximately 300 mm² and a thickness of 5 mm is formed.

The quantitative chemical analysis of the screen obtained gives a nitrogen content of 1.2% and a fluorine content of 65.6%, i.e. a polypyrrole concentration of 5.57% by mass in the alloy. The variations of its reflection coefficient as a function of frequency are those of curve a in Fig. 2.

The electrical conductivity of this shield is 6.5.10⁻⁵ X⁻¹.cm⁻¹, and the real and imaginary parts ε' and ε'' at 4 GHz have the values 10.0 and 8.0 respectively.

The conductivity was measured on a disk 50 mm in diameter and 3 mm thick, obtained by pressing the dry powder in the cold above 100 MPa and then metallized by vapor deposition of gold between the two surfaces using a Keithley 617 electrometer. This measurement was perfectly reproducible.

Examples 2 to 7

Six single-layer screens were prepared as in Example 1 for different amounts of polypyrrole. Table I below gives the electrical conductivity and the high-frequency properties of the screens obtained, in particular the real and imaginary parts ε', ε'' of their dielectric constant, as a function of the polypyrrole concentration in the screen, given in mass%, and of the amount of pyrrole in the solution, given in parts by weight per 100 parts by weight of Teflon. This table also gives the thickness for tuning these screens as a function of frequency. The polypyrrole concentration in the alloy is given for a doping level with perchlorate ions of approximately 33 mol%.

It will be noted that the mass fraction of polypyrrole in the alloy is always greater than the initial pyrrole content in the solution due to the fixation of one ClO₄⊃min; on four pyrroles.

The conductivity was measured as in Example 1, The polypyrrole concentration was determined from the ratio of the nitrogen content of the alloy to the nitrogen content of pyrrole.

The study of this Table I allows us to draw the following conclusions:

- In contrast to the results obtained with carbon black, this type of synthesis leads to excellent uniformity both within the same batch (batches 2 and 3) and between two different batches (batches 4 and 5). This property is particularly remarkable since the reproducibility in ε is of the order of a few percent between two batches.

- The rinsing conditions allow the high frequency properties to be modified 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 prepared, the high-frequency properties and electrical conductivities of which, as previously determined, are given in the table.

These absorbing layers can only be used in multilayer screens; since, in contrast to examples 1 to 7, ε'' is larger than ε', there is no thickness tuning.

These examples also show that the procedure described leads to a similarity of properties when the quantities synthesized are increased fivefold (samples 8 and 9).

Example 10

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

First, prepare a solution of 100 ml of nitrobenzene containing 5 g of high molecular weight PVC. Add 6 g of anhydrous FeCl3 to the solution and stir for 10 minutes. Add 0.4 ml of pyrrole to the mixture, which leads to immediate polymerization of the pyrrole. Continue polymerization for 45 minutes under a nitrogen flow.

The resulting reaction mixture is then added dropwise for Drops are poured into 1000 ml of ethanol. First, the polypyrrole-PVC alloy precipitates, which is then filtered and rinsed with ethanol over a frit. The powder is dried under vacuum and processed by cold pressing. The polypyrrole concentration in the product obtained is approximately 8% by weight.

The high frequency characterization measurements yield 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 - j 0.4.

In this case, we have a material that has an absorbent character up to 10 GHz and a transparent character for electromagnetic waves from 10 GHz onwards.

One can of course increase the polypyrrole concentration in the mixture and obtain higher values of the dielectric constant.

The microwave-absorbing screen according to the invention can be a single-layer screen, as described in Examples 1 to 7 and 10, or a multilayer screen, as shown in Fig. 3. This multilayer screen comprises, optionally on a conductive carrier 12, several superimposed layers 1, 2, 3, ..., n, the electrical conductivity of which decreases from layer 1 to layer n.

The following example concerns such a multi-layer screen.

Example 11

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

Curves d, e and f of Fig. 4 refer to the dependence of the reflection coefficient R (expressed in dB) on the frequency F (expressed in GHz) for the first, second and third layers alone, respectively, and curve g refers This dependency applies to the multilayer shield consisting of these three layers. Such a layer sequence produces a shield that provides an attenuation of -10 dB between 4.8 and 20 GHz.

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

Example 12

Absorbent screen based on polycarbonate and polypyrrole.

200 g of polycarbonate are dissolved in 5 l of chloroform at ambient temperature. The oxidizing agent (324 g of anhydrous ferric chloride) is added to the previous mixture, dissolved in ethyl ether. The pyrrole (60 ml) is then 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 takes place under an inert atmosphere for 1 hour with stirring.

Precipitation is carried out in 30 l of methanol at a flow rate of 50 ml/min. After filtration and washing with methanol, the powder is dried under vacuum at 20ºC. The powder has a particle size of 120 µm and a conductivity of 3.4.10-2 X-1.cm-1 at ambient temperature.

The high frequency measurements carried out on the powder pressed at 200ºC under 18.10⁷ Pa (1800 bar) for 50 minutes give the value ε of 23-j 47 at 4 GHz.

Examples 13 and 14

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

200 g of powdered PVC are dissolved in 5 l of nitrobenzene at ambient temperature. 300 g of FeCl₃ 6H₂O are added to the mixture while stirring.

Once the solution has become clear, 32 ml of pyrrole (i.e. 15.4 parts by weight of pyrrole per 100 parts by weight of PVC) are added at once to the reactor and the reaction is allowed to proceed with stirring for 1 hour.

The mixture is then transferred to a second reactor, containing 30 l of ethanol. The transfer is carried out at a flow rate of 50 ml/min under vigorous stirring, which allows the size of the particles formed during the precipitation to be controlled.

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

The powder is dried under vacuum at 20ºC, then crushed to obtain a mobile powder consisting of spheres of 150 µm diameter showing penetration of polypyrrole grains into the PVC spheres.

To obtain a powder that has optimal application properties, it is stabilized using a commercial antioxidant (2%), Irgastab M 70 from Ciba Geigy (organotin compound), and it contains a commercial processing aid, Paraloid 4M 355 from Rohm & Haas, which allows to lower the viscosity in the molten state (4%). The measured concentration of polypyrrole in the alloy is 10 vol.%.

Two types of application were investigated and completely different high frequency behavior was demonstrated.

Example 13: Uniaxial pressing of the powder

The powder is introduced into a mould previously heated to 170ºC, a mould of dimensions which allow, after cooling, to extract a coaxial test piece compatible with the measuring system used.

The mold is then pressurized at a pressure increase rate of 3.8 x 10⁷ Pa/min to a maximum pressure of 18 x 10⁷ Pa (1800 bar). The pressure is then maintained for 3 minutes, after which the mold is removed from the machine to cool. The sample is then ready for high frequency measurement.

Example 14: Injection molding of the powder

The powder is introduced into the hopper of an injection molding press with the screw temperature ranged between 150 and 180ºC and the mold preheated to 60ºC. The molding pressure is of the order of 1.2.10⁸ Pa with a total cycle time of 3 minutes.

The properties obtained for an alloy pressed or injection-molded according to Examples 13 and 14 are given in Table II below.

Fig. 5 and 6 refer to the screen of Fig. 13 and 14 respectively. Curves h and j of Fig. 5 and 6 give the frequency dependence of ε', curves i and k of Fig. 5 and 6 give the frequency dependence of ε''.

- Pressed material (Fig. 5)

The microstructure created at the time of synthesis is not destroyed (one finds that of polypyrrole-Teflon) because a classical behavior is observed, i.e. a variation of ε'' with frequency F according to the law ε'' = ( 0 / ε 0 )(2 π F)-S, where 0 is the static conductivity of the material and ε 0 is the dielectric constant of the vacuum (the exponent S here has the value -0.48).

- Injection molded material (Fig. 6)

In parallel with an overall reduction in the value of ε' (curve j), which can be attributed to a slight degradation of the alloy during the injection molding, the appearance of a new phenomenon is noted. At 5 GHz, a dielectric relaxation is present. This is apparently due to a change in the microstructure of the material. The aggregates formed during the growth process are destroyed and polystyrene grains rearrange themselves to penetrate with a dimension smaller than that obtained in Example 13.

With one and the same type of PVC-polypyrrole alloy, we therefore have two families of materials whose properties are exclusively related to the type of processing.

The curves in Fig. 7 and 8 show the variation 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 layers of 1 mm, curves m and r to layers of 3 mm, curves o and t to layers of 4 mm and curves p and u to layers of 5 mm.

It can be seen that the pressed material (Fig. 7) in a single layer is difficult to tune; such a material offers an advantage mainly in a conductivity gradient structure with a three-layer structure of the type:

- inner layer with ε'< ε'' in the entire frequency range,

- middle layer with ε'> ε'' in a part of the frequency range as in Example 13 (Fig. 5),

- outer layer of flat type, transparent, made of the insulating polymer of the alloy (here PVC, for which ε has a value of 2.7).

The injection-molded material (Example 14) shows a very remarkable behavior because, on the one hand, it remains in resonance over a very wide frequency range (4-16 GHz), which allows it to be used in a single layer over a wide range of thicknesses, and, on the other hand, it has very wide bandwidths (35% at 10 dB), while maintaining a few dB up to 20 GHz (Fig. 8). In fact, it is observed that a conventional absorber shows a relatively symmetrical absorption peak, whereas the injection-molded material, for example with a thickness of 2 mm (curve r), shows a slope that does not exceed -6 dB, which makes it advantageous in applications that combine it with insulating layers that no longer function in this frequency range. TABLE I Concentration of polypyrrole (in mass %) Pyrrole (in parts by weight) Lot No. Synthesized quantity Matching thickness (mm) Remarks Similarity of a lot rinsed once twice three times TABLE II FREQUENCY (GHz) PRESSED MATERIAL (Example 13) INJECTION MOLDED MATERIAL (Example 14)

Claims (10)

1. A method for producing a microwave-absorbing screen (10) comprising at least one layer (1, n) of an electron-conductive organic polymer doped with an anion and an electrically insulating thermoplastic organic polymer which penetrate each other, which method consists in that for each layer: a) forming a solution or colloidal suspension of the insulating organic polymer in a solvent compatible with the chemical reaction to produce the doped conductive polymer, the solution or suspension including an anionic oxidizing dopant; b) at least one liquid monomer which is polymerizable by oxidation in the form of the doped, conductive polymer is added to the solution or suspension, the monomer comprising at least one ring-shaped, carbon-containing chain with conjugated double bonds and is added in an amount of 1 to 30 parts by weight per 100 parts by weight of the insulating polymer, c) the whole is allowed to react until the monomer is completely oxidised, thus obtaining its polymerisation in situ; (d) the insulating polymer and the conductive polymer are coprecipitated to form a powder of interpenetrating insulating polymer-conductive polymer material having an electrical conductivity below 0.1 S/cm; (e) this powder of interpenetrating material is moulded to form a thick layer of interpenetrating material of 1 to 10 mm thickness. 2. Method according to claim 1, characterized in that step e) consists of compaction or injection molding. 3. Process according to claim 1 or 2, characterized in that the insulating polymer is a water-emulsifiable polymer. 4. Process according to one of claims 1 to 3, characterized in that the insulating polymer is a perhalogenated polymer. 5. Process according to claim 1 or 2, characterized in that the insulating polymer is selected from polyvinyl chloride, polytetrafluoroethylene and polycarbonates. 6. Process according to one of claims 1 to 5, characterized in that the monomer comprises a heteroatom in the ring-shaped chain. 7. Process according to one of claims 1 to 6, characterized in that the monomer is selected from pyrrole, thiophene, bithiophene and an alkyl-3-thiophene. 8. Method according to one of claims 1 to 7, for manufacturing a microwave-absorbing multilayer screen, characterized in that several layers of interpenetrating material are formed by repeating steps a) to e) each time and changing each time the type and/or concentration of the monomer and/or the type of insulating polymer in such a way that layers with different electrical conductivities are obtained and by stacking these different layers on top of each other. 9. Method according to claim 8, characterized in that the different layers are stacked one on top of the other in such a way that the electrical conductivity of the layer stack decreases from the first layer (1) to the last layer (n). 10. Method according to one of claims 1 to 9, characterized in that a conductive carrier (12) is brought into contact with the layer or layers of mutually penetrating material in order to support the layer or layers.