Electromagnetic wave-absorbing composite material, method for manufacturing same and uses thereof

Abstract

The invention relates to an electromagnetic wave-absorbing composite material, said composite material consisting of a support matrix formed by cork granules which are coated with an electromagnetic wave-absorbing material and which are bonded to one another by a resin, characterized in that tri-hydrate alumina (THA) is trapped in the resin.

Description

Composite material absorbing electromagnetic waves, its manufacturing process and its uses

[0001] The present invention belongs to the field of electromagnetic wave absorbers, and in particular to microwave electromagnetic waves.

[0002] Composite materials consisting of a polymer foam-based matrix and filled with materials with specific properties are of interest for attenuating electromagnetic waves in the microwave range and are now widely used, particularly in anechoic chambers.

[0003] Pyramidal absorbers made of polyurethane (PU) foam loaded with carbon black are today the absorbers giving the best performance over a wide frequency band in radio frequencies and microwave frequencies, and are by far the most used electromagnetic absorbers in the construction of anechoic chambers.

[0004] However, these materials have a major drawback: their manufacturing process results in inhomogeneity in composition, and machining PU foam is difficult. Indeed, producing absorbents from PU foam involves impregnating unfilled polyurethane foam with an aqueous solution containing carbon to provide the absorbent material. This manufacturing process leads to inhomogeneity in composition, which is exacerbated in the case of pyramidal parts. Furthermore, machining this PU foam is quite challenging due to its poor mechanical properties, ultimately resulting in inconsistencies in the dimensions of the final part and, similarly, in the properties of the finished product.

[0005] A new class of electromagnetic absorbers was then developed using a new support material (or matrix) to replace PU foam, while having reflectivity performance in normal and oblique incidence identical to, or even superior to, that of pyramidal absorbers made of PU foam for an absorber height less than or equivalent to that of current absorbers.

[0006] He is known to the person in the trade of electromagnetic absorption devices using cork.

[0007] Thus, Japanese patent application JP4371426 describes a woody panel coated with magnetic paint to achieve its absorbent properties. It specifies that the wood used can be cork. The wood is therefore coated only on the surface.

[0008] Furthermore, a PU foam is known in which perforations are made, the walls of which are coated with a conductive material. This PU foam can be replaced by a cork sheet. In this case, the material is present only on the walls of the perforations.

[0009] Therefore, the plates described in the patent documents above, which are only surface-loaded, act as shielding and cannot be considered electromagnetically absorbing. Furthermore, they have a very specific shape. However, the intended applications require a variety of shapes such as pyramids, dihedral angles, and parallelepipeds.

[0010] To resolve the aforementioned defects and drawbacks, the applicant has developed the electromagnetic wave-absorbing composite material according to French patent 3048974. This composite material comprises a cork matrix and an electromagnetic wave-absorbing material, the cork matrix being formed of particles whose equivalent diameter is between 1 µm and 20 mm, and said composite material having a real permittivity greater than 1.2 and a dielectric loss tangent tan δ greater than 0.1.

[0011] The advantage of cork for solving the technical problem lies in its following properties: it is a naturally occurring product found in the bark of certain trees. The carbon footprint of using such a material is therefore very low compared to that of a matrix made from a petrochemical-derived material. Furthermore, the use of cork complies with evolving environmental standards; it is a material with very high fire resistance because it does not propagate flames and does not release toxic fumes. Its use thus allows compliance with the standards applicable to anechoic chambers (particularly REACH regulations); it has the advantage of having a low density, on the order of 240 kg / m³. 3This results in a lightweight composite material. It possesses good dimensional stability and mechanical strength, eliminating the machinability problems commonly encountered with PU foam matrices. These characteristics ensure good reproducibility of the final part.

[0012] Advantageously, the cork particles are coated with the filler. This coating is possible because cork naturally has numerous pores opening onto its surface, these pores providing a multitude of points of attachment for the filler. Furthermore, the small particle size of the cork particles allows them to present a large specific surface area for coating.

[0013] The material absorbing electromagnetic waves makes the composite material according to the invention electromagnetically absorbent.

[0014] The applicant continued their studies and research to further improve the mechanical and dielectric properties of the absorbent material. During this process, they discovered that incorporating trihydrated alumina into the resin binder made it possible to achieve this goal, and that heat treatment of the resulting material further improved these properties.

[0015] The present invention therefore relates firstly to a composite material absorbing electromagnetic waves, said composite material consisting of a support matrix formed of cork grains to which is associated a material absorbing electromagnetic waves and which are linked to each other by a resin, characterized by the fact that trihydrated alumina (ATH) is trapped in said resin.

[0016] In a first embodiment, the cork grains can be coated with the material absorbing electromagnetic waves, the said coated cork grains being bound together by the resin incorporating the ATH.

[0017] The composite material may also include at least one bonding agent for the material that absorbs electromagnetic waves to the cork grains.

[0018] The bonding agent for the material absorbing electromagnetic waves can be gum arabic.

[0019] In a second embodiment, the resin can incorporate both the electromagnetic wave-absorbing material and the HTA.

[0020] Cork can be natural cork or expanded cork. Expanded cork has the advantage of having a density of around 70 kg / m³ 3 , which is less than the density value of unexpanded cork.

[0021] Cork grains can have an equivalent diameter ranging from 1 µm to 20 mm. Examples of cork grains include cork granules with an equivalent diameter between 1 mm and 5 mm, and preferably between 2 mm and 3 mm, or cork powder with a particle size ranging from 1 µm to 1000 µm. The equivalent diameter was measured by optical microscopy and scanning electron microscopy.

[0022] In this application, the equivalent diameter corresponds to the equivalent diameter in volume, namely the diameter of the equivalent sphere which has the same volume as the particle in question.

[0023] The material absorbing electromagnetic waves can be chosen from magnetic materials, dielectric materials, electrically conductive materials and mixtures thereof.

[0024] Electrically conductive materials can be chosen from metallic particles, carbon particles, carbon fibers, carbon nanotubes, graphene and mixtures thereof.

[0025] The material that absorbs electromagnetic waves can be in the form of microparticles or nanoparticles with an equivalent diameter between 1 nm and 1000 µm.

[0026] The resin binder can be chosen from thermoplastic polymer resins, such as polypropylene, polyethylene, polystyrene; thermosetting polymer resins, such as polyester, polyvinyl esters, epoxy, polyurethane; and their mixtures, the resins being able to be non-bio-based, such as epoxy, polyurethane and polyester resins, or bio-based such as resins derived mainly from plant-based raw materials, such as polylactic acid which is a derivative from corn starch, or polyamide 11, which is a derivative of castor oil.

[0027] The mass ratio of the material absorbing electromagnetic waves to the cork grains can be from 1:100 to 2:1, in particular from 1:10 to 5:10.

[0028] The mass ratio of resin to cork grains can be from 1:10 to 10:1, in particular from 1:2 to 2:1.

[0029] ATH can be incorporated into the resin at a rate of 1 to 1000 parts by weight, in particular 20 to 200 parts by weight, for 100 parts of resin.

[0030] The bonding agent(s) may be present at a rate of 1 to 200 parts by weight, in particular 10 to 50 parts by weight, for 100 parts by weight of cork granules.

[0031] The composite material according to the present invention can be in the form of molded or machined parts, such as the shape of plates, formed plates, pyramids, truncated pyramids, honeycomb pyramids, convoluted or rounded parts, corners or coatings such as thin or thick layers.

[0032] The present invention also relates to a method for manufacturing a composite material as defined above according to a first embodiment, characterized in that it comprises the successive steps of: placing in aqueous solution the material absorbing electromagnetic waves and where applicable the bonding agent(s) of the material absorbing electromagnetic waves to the cork grains; mixing until a homogeneous preparation is obtained the cork grains and said aqueous solution obtained in step (1); drying the homogeneous preparation thus obtained in open air or in an oven at a temperature between 20°C and 150°C until complete evaporation of the aqueous phase to obtain cork grains coated with the material absorbing electromagnetic waves; mixing the coated cork grains thus obtained with the resin into which the ATH has been incorporated to obtain a bonded preparation;mold the said bonded preparation thus obtained to obtain the desired composite material.

[0033] The bonding agent(s) can also be dispersants of the material absorbing electromagnetic waves in the aqueous solution of the latter, including gum arabic, the quantity of bonding agent(s) in the aqueous solution being in particular from 0.1 to 200% by weight, in particular from 5 to 40% by weight.

[0034] The present invention also relates to a method for manufacturing a composite material as defined above according to a second embodiment, characterized in that it comprises the successive steps of: mixing the cork grains with the resin into which the ATH and the material absorbing electromagnetic waves have been incorporated; molding the mixture thus obtained to obtain the desired composite material.

[0035] According to a particular embodiment of the process according to the invention, in an additional step, the molded composite material thus obtained is heated to a temperature of 30 to 350°C, in particular 90°C, for 1 to 100 days, in particular 28 days.

[0036] The present invention also relates to the use of the composite material as defined in one of above or obtained by the process as defined above, as an absorber, in particular in an anechoic chamber; as a radar absorber in stealth devices, or to improve the electromagnetic compatibility of electronic devices, such as embedded systems and measuring instruments.

[0037] The following examples illustrate the present invention without, however, limiting its scope.

[0038] Example 1 (comparative) and Examples 2 and 3 (of the invention)

[0039] (1) 1000 mL of water, 180 g of gum arabic and 150 g of carbon black powder of equivalent nanometric diameter marketed by the company Univar under the name VULCAN XC72R CABOT were mixed.

[0040] (2) 5 g of cork grains with an equivalent diameter of 3 to 4 mm (measured by optical microscopy) and 8 g of the solution prepared in step (1) were mixed until a homogeneous preparation was obtained.

[0041] (3) The coated grains prepared in step (2) were dried in an oven at 50°C for 4 days.

[0042] (4) xg of a polyurethane resin marketed by Composites Distribution under the name Axon F180 and yg of ATH were mixed together, then 3g of dried coated grains from step (3) were added and mixed together.

[0043] We have thus prepared composite materials absorbing electromagnetic waves with the formulations indicated in Table 1:

[0044] Example 1 (comparative) Example 2 Example 3 x443y033

[0045] (5) The mixtures prepared in step (4) were then molded and allowed to polymerize.

[0046] Example 4: Deformation test with respect to stress

[0047] The test is a three-point bending test and the stress is evaluated in MPa as a function of the deformation.

[0048] The deformation response to the applied stress is illustrated by the graph which gives the bending curves obtained for the samples according to Examples 1 to 3.

[0049] The comparison between Examples 1 (comparative) and 2 shows, on the one hand, that the maximum level of constraint that can be applied to the sample in Example 2 is greater than the maximum level of constraint that can be applied to the sample in Example 1 (comparative).

[0050] Therefore, in terms of use in an anechoic chamber, the risk of rupture of an absorbing material following accidental contact with an operator is lower for the absorbing material of the invention.

[0051] The same conclusion can be reached with regard to Example 3.

[0052] Example 5: Comparative evaluation of the loss tangent

[0053] One parameter of interest is the loss tangent (tan δ), which defines the energy losses of a medium - more rigorously the amount of electromagnetic energy converted into heat energy - that an electromagnetic wave passing through this medium will undergo.

[0054] This quantity was evaluated as a function of frequency for the samples of Example 1 (comparative) and Examples 2 and 3 (of the invention).

[0055] Laillustre les curves obtained for the evaluation of the loss tangent.

[0056] The material losses according to Examples 2 and 3 are higher than those of the material according to Example 1 (comparative).

[0057] Example 6: Heat treatment

[0058] A second optimization method is shown here, which also aims to improve the mechanical behavior and dielectric properties of the electromagnetic absorbing material made of cork, this time by applying a heat treatment.

[0059] This is a method traditionally used to "simulate" the aging of a material based on the Arrhenius law according to which kinetic reactions are accelerated by a factor of 2 to 3 when the temperature increases by 10°C.

[0060] A temperature of 90°C is applied to the samples in Example 2 for 28 days. Note that this parameterization would simulate 10 years of aging, according to the Arrhenius law.

[0061] The mechanical and dielectric properties are evaluated and compared with those of a second set of samples that have not undergone heat treatment.

[0062] Example 7: Improvement of the flexural modulus (MPa)

[0063] The mechanical properties are evaluated using the same method as that employed for the addition of ATH, and the flexural modulus is derived from this. This value is shown for all samples before and after treatment. The average value for each of the two sample categories is represented by a horizontal, dashed line. Measurement uncertainties are indicated by vertical gauges centered on a single point.

[0064] As shown in the figure, comparing the average values ​​illustrates an improvement of over 30% in the flexural modulus of the samples after heat treatment. Similar to the observation made following the inclusion of ATH, the material obtained after heat treatment will be less likely to break following an accidental impact.

[0065] Example 8: Improvement of the real permittivity and loss tangent properties

[0066] The applied heat treatment also aims to improve the actual permittivity and loss tangent properties of the material.

[0067] Recall that the actual permittivity of a material characterizes the ease with which an electromagnetic wave passes through it. A significant difference in permittivity between two media will result in a significant reflection of the electromagnetic wave at the interface between these two media. In the case of an electromagnetic absorber, the closer this permittivity value is to that of air (or a vacuum, i.e., ε' = 1), the lower the risk of electromagnetic wave reflection.

[0068] As the curves show, the permittivity (ε') is significantly improved by heat treatment up to a frequency of 12 GHz, thus reducing once again the gap between the value of the material and that of air or vacuum.

[0069] The loss tangent (tan δ) increases after treatment and for the entire frequency range considered (1-18 GHz) ( ), thus promoting the conversion of electromagnetic energy into heat energy within the material.

Claims

– Electromagnetic wave-absorbing composite material, said composite material consisting of a support matrix formed of cork grains to which is associated a material absorbing electromagnetic waves and which are linked to each other by a resin, characterized in that trihydrated alumina (ATH) is trapped in said resin. - Composite material according to claim 1, characterized in that the cork grains are coated by the material absorbing electromagnetic waves, said coated cork grains being bound by the resin incorporating the ATH. - Composite material according to claim 2, characterized in that the electromagnetic wave-absorbing material coating the cork grains is associated with the latter by at least one bonding agent. - Composite material according to claim 3, characterized in that the bonding agent of the material absorbing electromagnetic waves is gum arabic. - Composite material according to claim 1, characterized in that the resin incorporates both the electromagnetic wave-absorbing material and the HWA. - Composite material according to any one of claims 1 to 5, characterized in that the cork is natural cork or expanded cork. - Composite material according to any one of claims 1 to 6, characterized in that the cork grains have an equivalent diameter between 1 µm and 20 mm. - Composite material according to any one of claims 1 to 7, characterized in that the material absorbing electromagnetic waves is selected from magnetic materials, dielectric materials, electrically conductive materials and mixtures thereof. - Composite material according to claim 8, characterized in that the electrically conductive materials are selected from metallic particles, carbon particles, carbon fibers, carbon nanotubes, graphene and mixtures thereof. - Composite material according to any one of claims 1 to 9, characterized in that the material absorbing electromagnetic waves is in the form of microparticles or nanoparticles having an equivalent diameter between 1 nm and 1000 µm. - Composite material according to any one of claims 1 to 9, characterized in that the resin binder is selected from thermoplastic polymer resins, such as polypropylene, polyethylene, polystyrene; thermosetting polymer resins, such as polyester, polyvinyl esters, epoxies, polyurethane; and mixtures thereof, the resins being able to be non-bio-based, such as epoxy, polyurethane and polyester resins, or bio-based such as resins derived essentially from plant-based raw materials, such as polylactic acid which is a derivative from corn starch, or polyamide 11, which is a derivative from castor oil. - Composite material according to any one of claims 1 to 11, characterized in that the mass ratio of the material absorbing electromagnetic waves to the cork grains is from 1:100 to 2:1, in particular from 1:10 to 5:

10. - Composite material according to any one of claims 1 to 12, characterized in that the mass ratio of the resin to the cork grains is from 1:10 to 10:1, in particular from 1:2 to 2:

1. - Composite material according to any one of claims 1 to 13, characterized in that the ATH is incorporated into the resin at a rate of 1 to 1000 parts by weight, in particular 20 to 200 parts by weight, for 100 parts of resin. - Composite material according to any one of claims 3 to 14, characterized in that the bonding agent(s) are present at a rate of 1 to 200 parts by weight, in particular 10 to 50 parts by weight, for 100 parts by weight of cork granules. - Composite material according to any one of claims 1 to 15, characterized in that it is in the form of molded or machined parts, such as the form of plates, formed plates, pyramids, truncated pyramids, honeycomb pyramids, convoluted or rounded parts, corners or coatings such as thin or thick layers. – A method for manufacturing a composite material as defined in any one of claims 2 to 4 and 6 to 16, characterized in that it comprises the successive steps of: placing in aqueous solution the material absorbing electromagnetic waves and where applicable the bonding agent(s) of the material absorbing electromagnetic waves to the cork grains; mixing until a homogeneous preparation is obtained the cork grains and said aqueous solution obtained in step (1); drying the homogeneous preparation thus obtained in open air or in an oven at a temperature between 20°C and 150°C until complete evaporation of the aqueous phase to obtain cork grains coated with the material absorbing electromagnetic waves; mixing the coated cork grains thus obtained with the resin into which the ATH has been incorporated to obtain a bonded preparation; molding said bonded preparation thus obtained to obtain the desired composite material. - A method according to claim 17, characterized in that the bonding agent(s) are also dispersants of the material absorbing electromagnetic waves in the aqueous solution of the latter, being in particular gum arabic, the quantity of bonding agent(s) in the aqueous solution being in particular from 0.1 to 200% by weight, in particular from 5 to 40% by weight. - A method for manufacturing a composite material as defined in any one of claims 5 to 16, characterized in that it comprises the successive steps of: mixing the cork grains with the resin into which the ATH and the material absorbing electromagnetic waves have been incorporated; molding the mixture thus obtained to obtain the desired composite material. – A process according to any one of claims 17 and 18 or claim 19, characterized in that the molded composite material thus obtained is heated to a temperature of 30 to 350°C, in particular 90°C, for 1 to 100 days, in particular 28 days. – Use of the composite material as defined in any one of claims 1 to 16 or obtained by the process as defined in any one of claims 17 to 20, as an absorber, in particular in an anechoic chamber; as a radar absorber in stealth devices, or to improve the electromagnetic compatibility of electronic devices, such as embedded systems and measuring instruments.

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