Three-dimensional conductive knitted mat used as a lightning-resistant barrier

Abstract

The present invention relates to a three-dimensional conductive mat, which is formed of a conductive knit fabric capable of distributing an electric charge homogeneously over its surface, characterised in that the knit fabric comprises at least one conductive metal filament yarn; to such a mat and to a composite material comprising 40-95% by volume of a thermoplastic and / or thermosetting polymer material; and to the use of such a three-dimensional conductive mat or such a composite material as a lightning resistant barrier.

Description

[Technical field]

[0001] The present invention relates to a lightning-resistant wall / surface, in particular a lightning-resistant wall / surface that is exposed to lightning, and therefore for example to aircraft fuselage parts in this respect. [Background technology]

[0002] The advantages of composites, especially carbon / epoxy composites, over aluminum are now evident due to their mechanical performance and their lightness. However, creating lightning exposed parts made of composites requires ensuring their resistance to lightning strikes and their ability to disperse electric charges along the aircraft fuselage, for example, without damaging the part, while the conductivity of aluminum is sufficient to perform this function.

[0003] This lightning protection function is typically addressed in carbon / epoxy composites in a number of different ways, which are not mutually exclusive and are optionally cumulative: carbon is a good conductor, but when struck by lightning it is damaged, which degrades the composite's performance, especially its mechanical performance.

[0004] The first method consists in adding a surface layer, which is made of, for example, generally very low basis weight (50-300, in particular about 80 g / m 2 ), commonly called "copper mesh" (copper / aluminum / bronze), made of expanded metal, perforated foil (notably available from 3M), intended to distribute the charge evenly over its surface.

[0005] The second method consists in adding a non-perforated foil, 1-15 cm wide and 0.05-1 mm thick, which may have the function of collecting the charges from the copper fabric and discharging them to other parts intended for the rear of the aircraft. When the use of a conductive layer is not possible, for example when it is necessary to be transparent to radio waves, as in the case of a radome, a diverter is used, which may take the form of a foil. The foil has a lightning conductor function, attracting lightning directly and discharging the charge. In some embodiments, the foil is placed at the junction between the two parts, constituting an equidistant equipotential zone, and the screw creates conductivity between the two parts.

[0006] A third method involves using a composite material having a conductive component, in either of the two forms cited above, within a thermosetting matrix.

[0007] These solutions are not satisfactory.

[0008] First, the use of fabrics, especially fabrics pre-impregnated with polymeric material (or "prepregs"), is particularly common. These fabrics are traditionally made of orthogonally arranged weft and warp threads and traditionally have a flat structure. To obtain a volumetric (or 3D, three-dimensional) article, the fabric is generally cut and placed in a mold, the overall shape of which corresponds to the overall shape of the part to be made. A polymeric material (or resin) is then injected into the mold and polymerized, especially to obtain a rigid part. Draping the textile reinforcement onto the mold is a time-consuming and difficult task. It requires the use of several "prepreg" layers, which need to be carefully cut and placed according to the shape of the mold to ensure sufficient thickness while avoiding over-coating. Cutting metallic fabrics, pre-impregnated or not, involves article losses that can amount to 30% of the material. Metallic conductive fabrics are even more difficult to drape, since the shape of the part is three-dimensional.

[0009] Several pieces of metal fabric can be sewn together to create complex surfaces: their implementation is complex and the continuity of the fibers is not ensured, reducing the homogeneity of the distribution of the charge over the surface.

[0010] On the other hand, the use of non-perforated foil requires relatively complicated cutting and results in the generation of waste that must be scrapped.

[0011] Finally, the use of a thermosetting matrix in a conductive composite has the disadvantage that the composite is prone to absorbing thermal energy, degrading, and forming holes.

[0012] US Patent Application Publication No. 2020 / 290296 describes a three-dimensional conductive mat made of conductive carbon knit, but this has too much resistance to be used as a lightning-resistant barrier.

[0013] US Pat. No. 4,755,904 describes a conductive mat made of a conductive knitted fabric: the mat is planar and non-stereoscopic. Summary of the Invention [Problem to be solved by the invention]

[0014] The object of the present invention is to provide a lightning protection or lightning resistant part, the surface of which may have a complex three-dimensional geometric shape, the manufacture and implementation of which can be easily scaled up industrially, and which does not have the above-mentioned drawbacks. [Means for solving the problem]

[0015] For this reason, the present invention relates to a three-dimensional conductive mat, which consists of a conductive knitted fabric capable of distributing an electric charge homogeneously over its entire surface, characterized in that the knitted fabric contains at least one conductive metal filament yarn. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Conductive knitted fabrics are obtained from at least one filament yarn made of conductive material, which may be monofilament or multifilament and / or formed from staple fibers that are bonded, for example by twisting or winding or any other textile process. In the sense of the present invention, a knitted fabric comprises one or more knitted yarns, which, in terms of their shape, may consist of one or more mesh yarns (loops), one or more filler yarns (wavy), one or more float yarns, but not necessarily of weft yarns (unidirectional). Various knitting techniques, in particular circular knitting or weft knitting, make it possible to obtain knitted fabrics, in particular forming stitch-free, integral 2D or 3D parts. From a technical point of view, conductive knitted fabrics can be obtained by weft knitting techniques: this is the preferred direction of the yarns, which, regardless of their shape, is similar to a fabric, the weft direction forming rows, as opposed to the longitudinal direction forming columns.

[0017] These knitted structures have many advantages compared to woven structures. In fact, in addition to the possibility of initially creating 3D structures in one piece without stitches, knitting can be done from a single spool of thread, if appropriate, for stitching threads, while the fabric still requires several different spools. Furthermore, draping the woven structure onto the mold is a lengthy and delicate operation, especially if the desired shape is complex, and requires the use of several layers of fabric, which need to be cut (item losses can amount to 30% of the material) and carefully placed according to the shape of the mold, thereby ensuring a sufficient thickness while avoiding excessive overlap, and requires the addition of reinforcement parts locally to ensure the retention of mechanical strength. This retention is imperfect; because the fibers are not continuous. 2D or 3D knitting makes it possible to create complex articles, which may, if appropriate, be directly draped in 2D or 3D shapes, ensuring the continuity of the threads throughout the resulting article, and the knitting already has a shape suitable for obtaining the desired article, and does not have to be placed around a flexible substrate, for example around a silicone bladder, etc. The entire assembly is then placed in a mold, where consolidation in vacuum is achieved making it possible to obtain the finished article.

[0018] Moreover, textile structures also have to be handled delicately when they are pre-impregnated with the most commonly used polymeric materials (e.g. gelled ones), these structures are sticky when the protective film is removed and are usable only for a limited time at room temperature. Conversely, knitting makes it possible to integrate thermoplastic polymeric materials in the form of threads or fibers, mixed, if appropriate, with conductive threads or fibers, to obtain preforms (intermediate / temporary form before the final form) called "dry", which contain both the conductive material or materials and the matrix.

[0019] Thus, the woven mats of the present invention are advantageously made in the form of final parts, including 3D composites. The present invention provides easy implementation and continuity of the conductive fibers, improving the conductivity and homogeneity of the charge distribution.

[0020] Preferably, the knitted fabric comprises at least one conductive filament thread, in particular 1 to 4 threads, for example 4 copper threads with a diameter of 0.1 mm.

[0021] And preferably, at least one conductive thread is metallic, such as copper, bronze, aluminum, brass, titanium, silver, gold, or alloys thereof.

[0022] And, preferably, the knitted fabric comprises a single metal filament yarn, such as copper, having a diameter of 0.01 to 1 mm.

[0023] Preferably, the conductive knitted fabric has at least one conductive unidirectional (UD) yarn, which is capable of transferring-discharging an electric charge in the direction of the UD yarn. Each UD yarn is a weft yarn.

[0024] And preferably, the conductive UD yarn or yarns are metallic, such as copper, bronze, or aluminum.

[0025] Preferably, the metallic UD yarns consist of a bundle of 12 copper yarns with a diameter of 0.02-2 mm, or have a conductivity comparable to that of such a bundle, so that these UD yarns have the ability to discharge large amounts of charge, corresponding to lightning strikes, possibly repeated.

[0026] In an interesting alternative, the conductive knit comprises at least two different conductive materials.

[0027] In another interesting alternative, the conductive knitted fabric comprises 0-40% by volume of one or more reinforcing threads, such as carbon fiber, glass or aramid, which or these reinforcing threads may be present, for example, in the form of one or more mesh threads, filler threads and / or float threads, and / or one or more weft threads added into the knitted fabric in the form of one or more unidirectional threads.

[0028] Another object of the invention consists of a composite material, characterized in that it comprises a mat as described above and 40-95% by volume of a thermoplastic and / or thermosetting polymeric material. The composite material (final article) is obtained from several components, which will be explained in more detail below, including the mat (intermediate article) described above, optionally containing 0-60% by volume of a thermoplastic and / or thermosetting polymeric material, preferably exclusively thermoplastic. The polymeric material can be exclusively thermoplastic or exclusively thermosetting. The thermoplastic polymeric material can be incorporated in the metal knit structure of the mat, for example in the form of one or more mesh threads, filler threads and / or float threads, and / or one or more weft threads added in the knit in the form of one or more unidirectional threads. Examples of thermoplastic polymers include those made of polycarbonate (PC), polyetherimide (PEI), polypropylene (PP), polyamide (PA), poly(methyl methacrylate) (PMMA), poly(ethylene terephthalate) (PET), poly(phenylene sulfide) (PPS), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), either alone or as mixtures or copolymers of several thereof. Thermosetting polymer materials can be integrated into the conductive knit of the mat by subsequent impregnation. Thermosetting polymer materials can include those made of polyurethane (PU), epoxy resin, cyanate ester, phenolic resin, unsaturated polyester.

[0029] In this composite material, the polymeric material advantageously comprises 100-5% by volume of thermoplastic material and 0-95% by volume of thermosetting resin. Since the metal knitted structure has fiber continuity that improves electrical conductivity and distribution and discharge of electric charge, it heats up less when struck by lightning and can form the polymeric matrix only with thermoplastic material without thermosetting resin. As already clearly stated, it is possible to use no thermoplastic material, but this is less preferred. In fact, the proportion of thermoplastic polymer in the polymeric material that is mainly thermosetting, which is less than half, makes the polymeric material weldable. On the other hand, this thermosetting material is relatively resistant to punctures due to the relatively low heating when struck by lightning as described above. Preferably, the thermoplasticity is obtained at a relatively high glass transition temperature Tg by using a thermoplastic polymer having a relatively higher glass transition temperature than the glass transition temperature of the thermosetting resin, in particular a Tg of more than 120°C, thereby ensuring the heat resistance of the polymeric matrix.

[0030] It is possible that no thermosetting polymeric material is used. If a thermosetting polymer is present, its volume fraction is preferably greater than that of the thermoplastic polymeric material.

[0031] Preferably, the composite material of the invention is obtained by combining reinforcing fibers with the above-mentioned knitted conductive mat. The reinforcing fibers may thus be associated with the thermoplastic polymer material in the form of woven threads, mats, optionally themselves associated with the thermoplastic polymer material and / or pre-impregnated with the thermosetting polymer material.

[0032] However, in a preferred variant of this embodiment, the composite material is obtained by superimposing a knitted conductive mat according to the invention with one or more braids of one or more reinforcing threads, each braid of one or more reinforcing threads being preassociated with a thermoplastic polymer material and / or pre-impregnated with a thermosetting polymer material.

[0033] The invention also relates to the use of the above-mentioned three-dimensional conductive mat or composite material for constructing a lightning-resistant barrier of a land, water or air vehicle or a building, in particular a train body, an airplane fuselage or a space vehicle.

[0034] The present invention will be better understood in light of the following examples. EXAMPLES

[0035] Comparative Example 1

[0036] The composite is intended to distribute the charge homogeneously across its surface, 80 g / m 2 and copper foil, 10 cm wide and a few tenths of a mm thick, whose function is to collect the charges from the copper fabric and to discharge them towards the rear of the aircraft, and then to overlay the assembly thus obtained, part of whose surface consists of the copper mesh fabric and other parts of the copper foil, a mat of woven carbon fibre pre-impregnated with epoxy resin.

[0037] This material is very difficult to drape, especially given its complex three-dimensional shape. The material punctured and tore apart the first time it was struck by lightning.

[0038] Example 1

[0039] A conductive knit is made with one or more mesh threads, filler threads and / or float threads, each consisting of copper threads with a diameter of 0.1 mm, and a thermoplastic polymer material integrated in the metal knit structure in the form of one or more mesh threads, filler threads and / or float threads and / or one or more weft threads added to the knit in the form of one or more unidirectional threads. This knit is directly produced in the desired three-dimensional shape, despite its complexity. It has the continuity of its conductive threads / fibers.

[0040] This three-dimensional conductive knit is superimposed with one or more reinforcing mats of the same three-dimensional shape, which consist of a woven, matted or knitted fabric of reinforcing fibers, such as carbon, glass or aramid, associated with a thermoplastic polymer material. A first example of a reinforcing knit is Kevlar (Aramid) and thermoplastic knit, i.e. with one or more mesh threads, filler threads and / or float threads, one of which consists of aramid and the other of thermoplastic, in which a number of unidirectional (UD) carbon threads and a number of unidirectional UD threads are inserted as weft threads. A second example of a reinforcing knit is glass and thermoplastic knit. A third example of a reinforcing knit is carbon and thermoplastic knit.

[0041] Composite materials can be obtained in any desired three-dimensional complex shape in a single piece, preserving the continuity of the fibers, after sintering at a temperature above the Tg of the thermoplastic and cooling.

[0042] Example 2

[0043] The conductive knit of Example 1 is modified by inserting 12 parallel unidirectional (UD) copper yarns with a diameter of 0.2 mm as the weft of the knit. This three-dimensional conductive knit is overlaid with the same woven fabric, mat, and knit as in Example 1.

[0044] Examples 3 and 4

[0045] Examples 1 and 2 are reproduced except that the reinforcing knits, mats, and woven fabrics are pre-impregnated with liquid thermosetting resin in an amount such that the polymeric material of the composites constitutes at least 40% by volume of the composites, divided into a majority of thermosetting polymer and a minority of thermoplastic polymer.

[0046] Examples 5 and 6

[0047] Examples 1 and 2 are reproduced, but without the reinforcing mat(s), and instead the reinforcing function in the copper braid is incorporated by one or more mesh, filler and / or float yarns of reinforcing fibers, such as carbon, glass or aramid, and / or one or more unidirectional (UD) yarns as weft yarns.

[0048] Examples 7 and 8

[0049] Examples 5 and 6 are reproduced, impregnating the reinforcing copper braid with liquid thermosetting resin in an amount such that the polymeric materials of the composite constitute at least 40% by volume of the composite, divided into a majority of thermosetting polymer and a minority of thermoplastic polymer.

[0050] The homogeneous distribution of the filler over the entire surface due to the copper braid is extremely effective: the paint burns homogeneously despite at least four lightning strikes without destroying the copper braid, which always conducts current homogeneously even after these strikes.

[0051] The charge displacement / discharge function by unidirectional copper (UD) yarns, which have a relatively large cross section and conductivity, is still very efficient; UD yarns are conductive enough to drain the charge without burning the paint and therefore heating it up.

[0052] The mechanical functionality provided by the reinforcing fibers / yarns of the fabrics, mats and knits remains intact even after repeated lightning strikes and is not structurally degraded by the shock waves, which are absorbed by the very tough material and do not penetrate it, whereas the composite of Comparative Example 1 was penetrated and delaminated during the first strike.

Claims

1. A three-dimensional conductive mat, which is made of a conductive knitted fabric capable of uniformly distributing electric charges over the entire surface thereof, characterized in that the knitted fabric has at least one conductive metal filament yarn.

2. The mat according to claim 1, characterized in that the at least one conductive yarn is made of copper, bronze, aluminum, brass, titanium, silver, gold, or an alloy thereof.

3. The mat according to claim 2, characterized in that the knitted fabric has a single metal filament yarn, for example, a single metal filament yarn such as copper with a diameter of 0.01 to 1 mm.

4. The mat according to claim 1 or claim 2, characterized in that the conductive knitted fabric has at least one conductive unidirectional (UD) yarn, and the electric charges can be moved, that is, discharged, in the direction of the UD yarn.

5. The mat according to claim 4, characterized in that one or more of the conductive (UD) yarns are made of one or more metals, such as copper, bronze, or aluminum.

6. The mat according to claim 5, characterized in that the metal UD yarn is composed of a bundle of 12 copper yarns with a diameter of 0.02 to 2 mm, or has a conductivity comparable to that of such a bundle.

7. The mat according to any one of claims 1 to 3, characterized in that the conductive knitted fabric has at least two different conductive materials.

8. The mat according to any one of claims 1 to 3, characterized in that the conductive knitted fabric has 0 to 40% by volume of one or more reinforcing yarns, such as carbon fiber, glass, or aramid.

9. A composite material, characterized by having the mat according to any one of claims 1 to 3, and 40 to 95% by volume of a thermoplastic polymer material and / or a thermosetting polymer material.

10. The composite material according to claim 9, characterized in that the polymer material contains 100 to 5% by volume of a thermoplastic material and 0 to 95% by volume of a thermosetting resin.

11. The composite material according to claim 10, characterized in that the volume ratio of the thermosetting polymer material is larger than the volume ratio of the thermoplastic polymer material. **Claim 12**: A composite material, comprising the mat according to any one of Claims 1 to 3, 40 to 95% by volume of a thermoplastic polymer material and / or a thermosetting polymer material, and reinforcing fibers obtained by combining the reinforcing fibers with the mat according to any one of Claims 1 to 3. **Claim 13**: A composite material, comprising the mat according to any one of Claims 1 to 3, 40 to 95% by volume of a thermoplastic polymer material and / or a thermosetting polymer material, and reinforcing fibers obtained by combining the reinforcing fibers with the mat according to any one of Claims 1 to 3, or a composite material obtained by superimposing one or more knitted fabrics of the mat according to any one of Claims 1 to 3 and one or more reinforcing yarns. **Claim 14** Use of the three-dimensional electrically conductive mat according to any one of Claims 1 to 3, or a composite material comprising the mat according to any one of Claims 1 to 3 and 40 to 95% by volume of a thermoplastic polymer material and / or a thermosetting polymer material, for constructing a lightning protection wall of a land, water or air vehicle, or a building, particularly a train car body part, an aircraft fuselage or a spacecraft.