Improved unidirectional prepregs

Abstract

A curable prepreg comprising a structural layer of unidirectionally conductive fibers with a gap therebetween, the structural layer having a first outer face and a substantially parallel second outer face, and the curable prepreg comprising a thermosetting resin impregnated within the structural layer and present within the gap, and a first layer of thermosetting resin in contact with a first outer surface of the structural layer, wherein a ratio of conductivity in an x-direction parallel to the conductive fibers to conductivity in a y-direction perpendicular to the conductive fibers is less than 1000.

Description

Technical Field

[0001] The present invention relates to a curable prepreg comprising a structural layer of unidirectional conductive fibers and a resin, which provides improved conductivity characteristics. Background Art

[0002] Composite materials have well-documented advantages over traditional building materials, particularly in terms of providing excellent mechanical properties at very low material densities. As such, the use of such materials is widespread, and their applications range from "industrial" and "sports and leisure" to high-performance aerospace components.

[0003] Prepregs (including fiber or fabric arrangements impregnated with a thermosetting resin such as epoxy resin) are widely used to produce such composite materials. The resin can be combined with the fibers or fabric in various ways. The resin can be pasted onto the surface of the fiber material, although more commonly it partially or completely impregnates the gaps between the fibers. In a common arrangement, discrete resin layers remain unimpregnated on the outer surface of the prepreg.

[0004] Once manufactured, typically a plurality of such prepreg layers are "laid up" as needed, and the resulting prepreg stack (i.e., laminate or preform) is typically cured by exposure to high temperatures to produce a cured composite structure. Curing can be carried out in a vacuum bag, which can be placed in a mold for curing. Alternatively, the stack can be formed and cured directly in the mold.

[0005] When such a laminate is made from a plurality of prepregs including discrete resin layers, this results in fiber layers sandwiching the discrete resin layers. Such an arrangement is known to provide desirable mechanical properties in any resulting cured composite.

[0006] However, aircraft skins composed of such composite materials may be damaged due to energy concentration when struck by lightning. Among the physical phenomena observed from lightning strikes, there is a phenomenon called "edge glow", which describes the condition in which a glow accompanied by particle or plasma jets appears at the tips or ends of carbon fibers in the exposed fiber surfaces of composite material components in a composite structure. Edge glow is caused by a voltage difference between conductive composite layers and typically occurs in regions of high current density caused by lightning strikes, where the voltage potential is at its maximum, such as the exposed fiber surfaces. When edge glow occurs in a region containing fuel or fuel vapor (such as near a fuel tank or fuel line, collectively referred to herein as the "fuel environment"), edge glow is a potential fuel ignition source. The phenomenon occurring at the edge is called "edge glow", and the phenomenon occurring on the surface is called "surface discharge". Both can be considered ignition hazards.

[0007] In addition, the presence of an electrically insulating interlayer results in low electrical conductivity in the direction orthogonal to the laminate surface (the so-called z-direction), which can exacerbate phenomena such as edge glow and is generally considered to contribute to the vulnerability of composite laminates to electromagnetic hazards such as lightning strikes. Lightning strikes can cause damage to composites, which can be quite significant and, if occurring on an aircraft structure in flight, can be catastrophic. Therefore, there are special problems for aerospace structures made of such composites.

[0008] Therefore, such edge glow phenomena may occur during lightning strike events, especially on composite laminates with low z-direction electrical conductivity. During a lightning strike event, due to fasteners connecting two composite parts, transient charges with high-intensity current travel through the skin and then into the wing substructure (e.g., structural spars or ribs). Thus typically, in a composite skin / spar assembly, the current travels partly on the skin and partly through the spar, where the spar represents one of the walls of the fuel tank. The current travels laterally from the fastener through the adjacent composite layers of the spar and tends to travel along the fibers due to higher electrical conductivity compared to the resin matrix. This path may produce a typical bright glow or spark at the spar / rib cap edge, thus providing the "edge glow" phenomenon.

[0009] In addition, composites used in aerospace applications must meet strict mechanical property standards. Therefore, any improvement in electrical conductivity must not have a negative impact on mechanical properties.

[0010] A wide range of techniques and methods have been proposed in the prior art to provide electrical conductivity in the z-direction of such composites.

[0011] WO 2008 / 056123 discloses how to improve electrical conductivity by adding hollow conductive particles in a resin interlayer such that they contact adjacent fiber layers and create electrical pathways in the z-direction. This relies on bridging across the electrically insulating interlayer to relatively conductive fiber layers (if they are made of, for example, carbon fibers).

[0012] WO 2010 / 150022 A1 teaches disrupting the interface between the structural layer and the interlayer in order to induce contact points between adjacent structural layers.

[0013] WO 2011 / 027160 A1 discloses the use of vitreous carbon particles in an interlayer with a maximum thickness of 50 μm.

[0014] WO 2013 / 186389 A1 and WO 2015 / 157486 A1 teach that the z-direction electrical conductivity can be improved by adding potato-shaped graphite in the interlayer.

[0015] WO 2016 / 048885 A1 discloses that the z-direction electrical conductivity can be increased by using conductive nano-sized particles and a lightweight carbon veil composed of randomly arranged carbon fibers located in the interlayer.

[0016] However, it has been found that, despite the improved z-direction electrical conductivity achieved by these techniques, prepregs including layers of unidirectional fibers (such as carbon fibers) may still exhibit vulnerability to electromagnetic hazards.

[0017] In particular, when a high current density flows through such a material, the electrical anisotropy of the prepreg can result in the generation of large potential differences, which can lead to electrical breakdown or hot spots at the edges (referred to as "edge glow") or on the inner surface of the panel (referred to as "surface discharge").

[0018] Therefore, there is still a need for unidirectional prepregs with improved electromagnetic properties. Summary of the Invention

[0019] The inventors of the present invention have found that while z-direction electrical conductivity is an important consideration in producing prepregs with good electromagnetic properties, it is a necessary but sometimes insufficient criterion. This is especially true in the case where the prepreg has a fiber layer composed of unidirectional fibers.

[0020] It has been unexpectedly found that improved electromagnetic properties can be achieved when the electrical conductivity of such prepregs in the y-direction (i.e., perpendicular to the direction of the unidirectional fibers) is higher than a certain critical value compared to the value of the electrical conductivity in the x-direction (i.e., parallel to the direction of the unidirectional fibers). This is unexpected because the electrical conductivity in the y-direction is generally much higher than the electrical conductivity in the z-direction.

[0021] Accordingly, in a first aspect, the present invention relates to a curable prepreg comprising a structural layer of unidirectional conductive fibers having a gap therebetween, the structural layer having a first outer surface and a second outer surface that is substantially parallel thereto, and the curable prepreg comprising a thermosetting resin impregnated within the structural layer and present within the gap, and a first layer of thermosetting resin in contact with the first outer surface of the structural layer, wherein the ratio of the electrical conductivity in the x-direction parallel to the conductive fibers to the electrical conductivity in the y-direction perpendicular to the conductive fibers is less than 1000.

[0022] Thus, the structural layer has a first outer surface and a second outer surface that is substantially parallel thereto, each of the surfaces defining an x-y plane, the x-y planes being separated from each other by a distance equal to the thickness of the structural layer in the z-direction orthogonal to the x-y plane; and a first layer of thermosetting resin in the x-y plane and in contact with the first outer surface of the structural layer, the first layer of thermosetting resin having a thickness in the z-direction.

[0023] Preferably, the ratio of the conductivity in the x - direction to the conductivity in the y - direction is less than 500, more preferably less than 350. However, it has been found that excellent conductivity characteristics can be achieved when the ratio of the conductivity in the x - direction to the conductivity in the y - direction is in the range of 10 to 1000, preferably in the range of 50 to 500.

[0024] When the fibers in the structural layer are unidirectional, impregnation of the resin into the gaps between them may cause the fibers to become spaced apart from each other. Therefore, if the resin content within the structural layer exceeds a critical value, the so - called y - direction conductivity significantly decreases due to the breakage of physical contact between adjacent conductive fibers. Although the y - direction conductivity can remain high relative to the z - direction conductivity (in fact, possibly higher by an order of magnitude), as it decreases relative to the x - direction conductivity (which is relatively unaffected by the resin content in the structural layer), it has been observed that this can lead to a reduction in the electromagnetic properties of the finally cured prepreg.

[0025] Without wishing to be bound by any particular theory, it is believed that this is due to an imbalance in the dissipation of electrical energy in the x - y plane, and thus affects the ability of the cured prepreg to handle three - dimensional electrical energy flow. Therefore, the present invention provides improved electromagnetic properties by ensuring that not only is the z - direction conductivity an acceptable minimum value, but also the dissipation characteristics in the x - y plane are optimized.

[0026] Therefore, preferably, before and after curing, the conductivity of the prepreg in the z - direction is greater than 6 S / m -1 and preferably greater than 15 S / m -1 . However, since the increase in the z - direction is typically achieved by a trade - off with other mechanical properties, the conductivity of the prepreg in the z - direction is not greater than 50 S / m -1 is usually sufficient. Additionally, when the conductivities in the x - y plane are arranged according to the requirements of the present invention, it has been found that excellent resistance characteristics can be achieved at these more moderate z - conductivity values.

[0027] Preferably, before and after curing, the conductivity of the prepreg in the x - direction is greater than 10,000 S / m -1 and more preferably greater than 20,000 S / m -1 .

[0028] Preferably, before and after curing, the conductivity of the prepreg in the y - direction is greater than 100 S / m -1 and more preferably greater than 300 S / m -1 .

[0029] In a preferred embodiment, the prepreg includes a thermosetting resin layer in contact with the second outer face of the structural layer. The second outer layer typically has the same composition as the first outer layer and preferably has the same or a similar thickness as the first outer layer. In this embodiment, when a plurality of such prepregs are stacked together, the first outer layer and the second outer layer combine to form a sandwich, as discussed below.

[0030] It is known that such a layered arrangement of a structural fiber layer sandwiching a resin layer provides a cured composite material with excellent mechanical properties. The thickness of the structural layer can vary depending on the intended application. However, generally, the thickness of the sandwich is proportional to the thickness of the structural layer. Thus, preferably, the ratio of the thickness of the structural layer to the total thickness of the first outer resin layer and the second outer resin layer if present is from 3:1 to 6:1.

[0031] Generally, the physical thickness of the structural layer will be determined by the areal weight of the fibers present. However, as mentioned, a thicker structural layer typically requires a thicker resin or sandwich in order to provide the desired mechanical properties.

[0032] However, it has been found that as the thickness of the sandwich increases, this has a tendency to reduce the z-direction conductivity, and prepregs with thicker sandwiches having acceptable electromagnetic properties are thus more difficult to produce. It is believed that this may be due to the increase in the size of the sandwich relative to the size of any conductive particles present therein, which reduces their ability to provide a conductive path across the resin sandwich.

[0033] The increased understanding of the importance of conductivity in the x-y plane provided by the present invention means that acceptable electromagnetic properties can thus be obtained with a thicker sandwich and thus also a thicker structural layer and thus a thicker prepreg than previously thought possible. Thus, thicker prepregs that are conductive in three dimensions become possible, which may be advantageous in certain applications.

[0034] Thus, the structural layer may include conductive fibers having an areal weight of 10 to 1200 g / m 2 . However, preferably, they have an areal weight greater than 200 g / m 2 , more preferably 500 to 1200 g / m 2 .

[0035] Thus, preferably, the structural layer may have a thickness of 10 to 1000 μm in a direction perpendicular to the first outer face and the second outer face. However, preferably, they have a thickness greater than 300 μm, preferably 350 to 1200 μm, more preferably 450 to 800 μm.

[0036] Accordingly, the total thickness of the first outer resin layer and, if present, the second outer resin layer is generally from 5 to 200 μm. However, preferably, they have a thickness greater than 50 μm, preferably from 60 to 200 μm, more preferably from 70 to 150 μm.

[0037] The thermosetting resin may be selected from those conventionally known in the art, such as phenolic resins, urea-formaldehyde resins, 1,3,5-triazine-2,4,6-triamine (melamine) resins, bismalemide resins, epoxy resins, vinyl ester resins, benzoxazine resins, polyester resins, unsaturated polyester resins, cyanate ester resins, or mixtures thereof. Epoxy resins are particularly preferred. A curing agent and optionally an accelerator may be included as needed.

[0038] The thermosetting resin is preferably an epoxy resin, and the epoxy resin used in the resin composition of the present invention and / or for preparing prepregs preferably has an epoxy equivalent weight (EEW) in the range of 10 to 1500, preferably it has an EEW in the range of 50 to 500. Suitable epoxy resins may include blends of two or more epoxy resins selected from monofunctional, difunctional, trifunctional, and / or tetrafunctional epoxy resins.

[0039] Suitable difunctional epoxy resins include those based on: diglycidyl ethers of bisphenol F (bisphenol F epoxy resins), such as Araldite GY281 and GY285 (Huntsman Advanced Materials), diglycidyl ethers of bisphenol A (bisphenol A epoxy resins), such as Epon 825 (DER 332 - Dow Chemical, Midland, MI), phenol and cresol epoxy novolacs, glycidyl ethers of phenol-formaldehyde adducts, glycidyl ethers of aliphatic diols, diglycidyl ethers, diethylene glycol diglycidyl ether, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidized olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imines and amides, glycidyl ethers, fluorinated epoxy resins, glycidyl esters, or any combination thereof. Suitable difunctional epoxy resins include GY 281 (also known as LY3581). The difunctional epoxy resin may be used alone or in any suitable combination with other difunctional epoxy resins.

[0040] The difunctional epoxy resin may be selected from diglycidyl ethers of bisphenol F, diglycidyl ethers of bisphenol A, diglycidyl dihydroxynaphthalene, or any combination thereof.

[0041] Suitable trifunctional epoxy resins can include those based on phenol and cresol epoxy novolacs, glycidyl ethers of phenol-formaldehyde adducts, aromatic epoxy resins, aliphatic triglycidyl ethers, di-aliphatic triglycidyl ethers, aliphatic polyglycidyl ethers, aliphatic polyglycidyl amines, heterocyclic glycidyl amidines and amides, glycidyl ethers, epoxidized olefins; brominated resins, aromatic glycidyl amines; fluorinated epoxy resins or any combination thereof. Suitable trifunctional epoxy resins are available under the trade names MY0500 and MY0510 (triglycidyl p-aminophenol) and MY0600 and MY0610 (triglycidyl m-aminophenol) from Huntsman Advanced Materials (Monthey, Switzerland). Triglycidyl m-aminophenol is also available under the trade name ELM-120 from Sumitomo Chemical Co. (Osaka, Japan).

[0042] Tetrafunctional epoxy resins are also preferred. It is contemplated that the benzene ring can be additionally substituted with other suitable non-epoxy substituents. By way of example, suitable substituents include hydrogen, hydroxyl, alkyl, alkenyl, alkynyl, alkoxy, aryl, aryloxy, aralkyloxy, aralkyl, halogen, nitro or cyano. Suitable non-epoxy substituents can be bonded to the benzene ring at the para or ortho positions, or at the meta positions not occupied by epoxy groups.

[0043] Suitable tetrafunctional epoxy resins include N,N,N',N'-tetraglycidyl-m-xylenediamine (commercially available from Mitsubishi Gas Chemical Company (Chiyoda-ku, Tokyo, Japan) under the name Tetrad-X) and Erisys GA-240 (from CVC Chemicals, Morrestown, New Jersey) and N,N,N',N'-tetraglycidyl methylene dianiline (e.g., MY720 and MY0721 from Huntsman Advanced Materials). Other suitable polyfunctional epoxy resins include DEN438 (from Dow Chemicals, Midland, Michigan), DEN439 (from Dow Chemicals), Araldite ECN 1273 (from Huntsman Advanced Materials), MY722 (from Huntsman Advanced Materials) and Araldite ECN 1299 (from Huntsman Advanced Materials).

[0044] Preferably, at least one polyfunctional epoxy resin has at least one meta-substituted benzene ring in its main chain. Preferred polyfunctional epoxy resins are those that are trifunctional or tetrafunctional. Most preferably, the polyfunctional epoxy resin will be a combination of trifunctional and polyfunctional epoxy resins. The polyfunctional epoxy resin can be saturated, unsaturated, cycloaliphatic, alicyclic, or heterocyclic.

[0045] The resin system can include a thermoplastic material soluble in the epoxy resin such as polyethersulfone to improve the toughness of the resin. Exemplary thermoplastic tougheners / particles include any of the following thermoplastics, alone or in combination: polyamide, copolyamide, polyimide, aromatic polyamide, polyketone, polyetheretherketone, polyester, polyurethane, polysulfone, polyethersulfone, high-performance hydrocarbon polymer, liquid crystal polymer, PTFE, elastomer, and segmented elastomer.

[0046] For example, a suitable toughener is PES particles sold under the trade name Sumikaexcel 5003P, which is commercially available from Sumitomo Chemicals. An alternative to 5003P is Solvay polyethersulfone 105RP or a non-hydroxyl-terminated grade such as Solvay 1054P.

[0047] The resin also suitably includes a curing agent and a curing agent accelerator. The curing accelerator is typically heat-activated and shortens the time required to cure the resin. Suitable curing agents are amines, including aromatic amines such as 1,3-diaminobenzene, 1,4-diaminobenzene, 4,4'-diamino-diphenylmethane; and polyaminosulfones such as 4,4'-diaminodiphenylsulfone (4,4'-DDS - available from Huntsman), 4-aminophenylsulfone, and 3,3'-diaminodiphenylsulfone (3,3'-DDS).

[0048] In a preferred embodiment, the resin can include a combination of one or more of the following components: triglycidyl aminophenol in the range of 8 to 34 wt% of the resin, bisphenol epoxy resin in the range of 20 to 28 wt%, tetraglycidylamine in the range of 25 to 35 wt%, a toughener in the form of polyethersulfone in the range of 10 to 25 wt%, and a curing agent in the form of diaminodiphenylsulfone (4,4’DDS or 3,3’DDS) in the range of 2 to 28 wt%.

[0049] In a preferred embodiment, the resin in the first outer resin layer and, if present, the second outer resin layer includes thermoplastic polymer particles. The thermoplastic polymer should be insoluble in the resin, typically epoxy resin, at room temperature and the elevated temperature at which the resin cures. Depending on the melting point of the thermoplastic polymer, the thermoplastic polymer can melt or soften to varying degrees during the curing of the resin at elevated temperature and re-solidify as the cured laminate cools.

[0050] Thermoplastic particles are polymers, which can be in the form of homopolymers, copolymers, block copolymers, graft copolymers or terpolymers. The thermoplastic particles can be thermoplastic resins having single or multiple bonds selected from carbon-carbon bonds, carbon-oxygen bonds, carbon-nitrogen bonds, silicon-oxygen bonds and carbon-sulfur bonds. One or more repeating units can be present in the polymer, which incorporate the following moieties into the main polymer backbone or are attached to the side chains of the main polymer backbone: amide moieties, imide moieties, ester moieties, ether moieties, carbonate moieties, urethane moieties, thioether moieties, sulfone moieties and carbonyl moieties. The thermoplastic particles can also have a partially crosslinked structure. The particles can be crystalline or amorphous or partially crystalline.

[0051] Suitable thermoplastics include polyamides, polycarbonates, polyacetals, polyphenylene ethers, polyphenylene sulfides, polyacrylates, polyethers, polyesters, polyimides, polyamide-imides, polyether-imides, polyurethanes. Polyamides are the preferred type of thermoplastic particles. The polyamide particles can be made from polyamide 6 (PA6), polyamide 12 (PA12), polyamide 11 (PA11) or any combination thereof. Preferred thermoplastic particles are polyamide particles having a melting point of about 140 °C to 240 °C. The particles suitably have a particle size of less than 100 μm. Preferably, the particle size ranges from 5 to 60 microns and more preferably from 10 to 30 microns. Preferably, the average particle size is about 20 microns. Suitable toughener particles include Orgasol 1002D NAT1 (PA6), Rilsan PA11 PC20HT (PA11) and Ultramid 4350 (PA6T), Vestamid 1010 and 610. The particles can be made by anionic polymerization according to PCT application WO2006 / 051222, by coextrusion, precipitation polymerization, emulsion polymerization or by cryogenic grinding. Preferably, the particles are made by direct polymerization rather than by grinding or precipitation.

[0052] Since the resin needs to be conductive and most need to be thermosetting resins, care needs to be taken to adjust the amount of toughener particles. It has been found desirable for the thermoplastic particles to be present at a level of 5 to 15 wt% based on the total resin in the prepreg.

[0053] Typically, the fibers in the structural layer will generally have a circular or nearly circular cross-section with a diameter range of 3 to 20 μm, preferably 5 to 12 μm.

[0054] An exemplary unidirectional fiber layer is made of HexTow TM carbon fibers available from Hexcel Corporation. Suitable HexTow for making many unidirectional fiber layers TMThe carbon fibers include: IM5 carbon fibers, which are available as 6,000, 12,000, and 24,000 filaments; IM7 carbon fibers, which are available as fibers containing 6,000 or 12,000 filaments and having weights of 0.223 g / m and 0.446 g / m respectively; IM8 - IM10 carbon fibers, which are available as fibers containing 12,000 filaments and having weights from 0.446 g / m to 0.324 g / m; and AS7 carbon fibers, which are available as fibers containing 12,000 and 24,000 filaments and having weights of 0.800 g / m and 1.600 g / m respectively. The tows typically have a width of 3 to 7 mm and are fed for impregnation in a device using a comb to hold the tow and keep it parallel and unidirectional, as discussed below.

[0055] The fibers can be any suitable conductive fibers, such as selected from carbon fibers, metallized glass fibers, graphite fibers, metallized polymers, and mixtures thereof, preferably carbon fibers.

[0056] In the context of the present invention, the z - direction conductivity can be provided by any suitable method. However, preferably, the first outer resin layer and, if present, the second outer resin layer include conductive particles in order to provide z - direction conductivity.

[0057] The conductive particles can include metal particles, carbon particles (such as vitreous carbon particles), or graphite particles (such as potato - shaped graphite). Preferably, the conductive particles are metal - coated particles, such as metal - coated glass or graphite particles. Preferred conductive particles are graphite coated with carbon by CVD (chemical vapor deposition). Preferred conductive particles include SG25 / 99.95SC from NGS Naturgraphit in Germany, which has an average particle size of 10 to 30 μm, and GHDR - 15 - 4 from Nippon Power Graphite Company in Japan, which has an average particle size of 10 to 30 μm and has a carbon coating deposited by carbon vapor deposition. Other preferred particles include graphite particles from Westwater with an average size of 10 to 30 μm and carbon microspheres from Sigratherm with an average size of 5 to 80 microns.

[0058] Preferably, the conductive particles are present at a level of 5 to 15 wt% based on the total resin in the prepreg.

[0059] Preferably, the conductive particles have a particle size of 10 to 80 μm.

[0060] Therefore, the sandwich is preferably thicker than the size (e.g., average diameter) of any conductive particles present by more than 10 μm.

[0061] Typically, by weight, the prepreg comprises from 15 to 70 wt%, preferably from 20 to 65 wt%, more preferably from 25 to 50 wt% and most preferably from 25 to 40 wt% of curable resin. By volume, typically the prepreg comprises from 15 to 70 vol% of curable resin, preferably from 20 to 60 vol%, more preferably from 30 to 50 vol% of curable resin.

[0062] However, it must be noted to ensure that the resin is distributed between the gaps between the fibers of the structural layer and the first and, if present, second outer resin layers. It has been found that as the resin content in the structural layer increases, the electrical conductivity in the y-direction decreases significantly. Therefore, preferably the resin content in the structural layer is less than 30 wt%, preferably less than 28.0 wt%, preferably less than 26.0 wt%.

[0063] It should also be noted that the gaps between the fibers are typically at least partially resin-free to provide an air discharge path so that air that may be present in the tows from the start or air that may be introduced during impregnation with liquid resin is not trapped within the structure. If there is no second layer of thermosetting resin in contact with the second outer surface, such air can escape along the length of the fibers and also from the second outer surface of the structural layer.

[0064] The degree of air in the structural layer is measured by a water absorption test that determines the degree of impregnation in the gaps between the fibers. In this test, first a sample of the prepreg material is weighed and clamped between two plates in such a way that a 5 mm wide strip protrudes. The arrangement is suspended in a water bath at room temperature (21 °C) for 5 minutes along the fiber direction. Then the sample is removed from the plates and weighed again, and the weight difference provides a value for the degree of impregnation within the sample. The smaller the amount of water absorbed, the higher the degree of impregnation. The prepreg according to the invention preferably has a water absorption of less than 6%, more preferably less than 5%, and typically from 2% to 5%.

[0065] Typically, by volume, the prepreg comprises from 45 to 75 vol%, preferably from 55 to 70 vol% of structural fibers.

[0066] The resin and fiber contents of uncured prepregs or prepreg stacks containing unidirectional carbon fibers are determined according to DIN EN 2559A (code A). The resin and fiber contents of cured composite materials containing carbon fiber materials are determined according to DIN EN 2564A.

[0067] The prepreg according to the invention can be manufactured in a known manner, for example by the method described and illustrated in WO 2010 / 150022, typically in a continuous process involving thousands of fibers (forming the structural layer of fibers) passing through a series of impregnation stages, which are typically guided by rollers for impregnating the resin into the structural layer.

[0068] Before the fibers contact the resin and reach the impregnation zone, they are typically arranged into a plurality of fiber tows, each tow comprising thousands of filaments, e.g., 12,000 filaments. These tows are mounted on bobbins and are first fed into a combing unit to ensure uniform separation of the fibers. The structural layer is typically formed from a plurality of fiber tows which are spread out and brought together on a spreading bar before being impregnated with resin.

[0069] To improve the handling of the resin, it is typically supported on a backing material such as paper. The resin is then typically fed from a roller such that it contacts the fibers, with the backing material remaining in place outside the resin and fiber contact area. During a subsequent impregnation process, the backing material provides a useful external material for applying pressure in order to achieve uniform impregnation of the resin. Each layer of resin applied is sometimes referred to as a film.

[0070] During this impregnation process, the resin passes between the gaps of the fibers. Optionally, as already mentioned, a second impregnation layer (i.e., a second film) comprising a thermosetting resin is provided, where the second side of the fiber layer is brought into contact with the second impregnation layer before compression. This can form a second layer of thermosetting resin in contact with the second outer surface of the structural layer. This can ultimately become a resin sandwich during the lay-up of multiple prepregs together with the first resin layer.

[0071] The method can be carried out in a single stage (i.e., one or two resin films are applied in a single stage), where the impregnation is carried out with an excess of resin such that a first resin layer (which includes any particulate material) is retained, which ultimately becomes the resin sandwich during the lay-up of multiple prepregs. Such a single-stage method tends to provide a degree of disruption to the structural layer, which can be advantageous, particularly for electrical conductivity.

[0072] Alternatively, the resin impregnation can be carried out in a two-stage method. This involves a first stage where one or two resin films are applied to the first side, typically the second side, of the fiber layer. The two resin films can consist only of resin and do not include any particulate materials such as toughening agents or conductive particles and are intended to impregnate into the gaps of the fibers in order to fully "wet out" the fiber layer. Then, following this impregnation is a second stage where contact is made with another resin which typically includes particulate materials, typically toughening agent particles, and is intended to lay down a resin layer which will ultimately become the sandwich when multiple such prepregs are stacked together. This second stage typically takes place after the prepreg has passed through a cooling stage. Sometimes this two-stage method is referred to in the art as "4-film forming".

[0073] To facilitate the impregnation of the resin into the fibers, this is typically carried out at an elevated temperature, such as from 60 °C to 170 °C, preferably from 100 °C to 150 °C, such that the resin viscosity is reduced, i.e. reduced to from 0.1 Pas to 100 Pas, preferably from 5 to 30 Pas, more preferably from 10 to 20 Pas, and even more preferably from 8 to 17 Pas. This is most conveniently achieved by heating the resin and the fibers to the desired temperature prior to impregnation, for example by passing them through an infrared heater.

[0074] Resin impregnation typically involves passing the resin and the fibers over rollers, which can be arranged in a number of ways. Two main arrangements are the "nip" arrangement and the "S-winding" arrangement.

[0075] The S-winding stage is where the resin and the fibers (both in sheet form) are passed around two separate rotating rollers in the shape of the letter "S". In the alternative "nip" arrangement, the fibers and the resin are nipped or squeezed together as they pass between the nip points between two adjacent or opposing rotating rollers. It should be understood that S-winding generally provides ideal conditions for reliable and reproducible impregnation of the resin between the fiber gaps, while also providing sufficient disruption. However, provided care is taken to control the pressure, for example by controlling the spacing between adjacent rollers, the nip stage can provide a more intense impregnation.

[0076] During this stage, a number of processes occur simultaneously, namely the impregnation of the resin into the gaps in the structural fiber layer, and any disruption of the structural layer.

[0077] As already discussed, the choice of impregnation conditions will require careful selection, for example in order to ensure that the desired degree of resin impregnation into the structural layer occurs such that some disruption of the structural layer takes place, but not so much that such disruption causes fiber separation, as this may induce a reduction in the y-direction conductivity. Thus, a balance must be found to find the optimum degree of structural layer disruption according to the particular conditions desired.

[0078] Parameters such as the separation between the rollers, the speed, the relative speed between the rollers and the resin and fibers, and the contact area of the rollers can be varied to achieve the desired degree of fiber disruption and also resin impregnation.

[0079] Alternatively, before impregnation, the structural fibres may first be subjected to fibre disruption means. This results in a portion of the fibres on the outside of the sheet becoming disrupted filaments. Depending on how the structural fibres are arranged, the disruption means may produce disrupted filaments in a variety of ways, such as by rupturing the adhesion points between the structural fibres and rupturing the structural fibres into shorter lengths, or by allowing free ends of the filaments to migrate into separate ruptures in the resin sandwich. This can be carried out by the method described in WO 2011 / 114140, where the disruption means involves passing the fibres over a wear surface, resulting in a portion of the fibres on the outside passing through in contact with the wear surface being ruptured, while the fibres not in contact with the wear surface remain unruptured.

[0080] As discussed above, unidirectional fibres are typically formed from a plurality of fibre tows, which are spread out and brought together before impregnation with resin. A common way of achieving this is to pass the fibres over a plurality of sequential spreading bars or rollers. Thus, it is convenient to incorporate the wear surface into the existing spreading bar arrangement. Thus, preferably, the wear surface is the surface of a spreading bar.

[0081] Multiple sets of S-winding rollers or pinch rollers may be used, with each set gradually increasing the pressure applied to the resin. Typical methods may also combine multiple sets of S-winding rollers and pinch rollers in the same production line.

[0082] After impregnation, there is typically a cooling step to reduce the tackiness of the resulting prepreg. This may be followed by further processing stages, such as lamination, slitting and separation.

[0083] Once prepared, the prepreg may be rolled up so that it can be stored for a period of time. Given the tackiness of such materials, a backing sheet is typically provided to enable the roll to be unwound during use. Thus, preferably, the prepreg according to the invention may include a backing sheet on the outside to facilitate handling of the material and / or rolling up of the material. It can then be unrolled and cut as required.

[0084] When it is desired to manufacture a composite material, typically a plurality of such prepregs are stacked together to produce a prepreg stack or preform.

[0085] Thus, in a second aspect, the invention relates to a plurality of prepregs as described therein, thereby comprising a structural layer of a plurality of conductive fibres and a resin sandwich formed by a plurality of first outer resin layers and, if present, second outer resin layers.

[0086] In a preferred arrangement, the fibres have a varying orientation throughout the prepreg stack, for example by arranging the fibres in adjacent layers to be orthogonal to each other in a so-called 0 / 90 arrangement, representing the angle between adjacent fibre layers. In many other arrangements, other arrangements such as 0 / +45 / -45 / 90 are also possible.

[0087] A preferred use of the prepreg of the present invention is as a tape; the prepreg can be prepared as a roll of material specifically for an automated tape laying apparatus. The prepreg is provided with a backing sheet which is removed when the prepreg is laid in a mold. Thus, the prepreg typically provided with a backing sheet is preferably flexible enough to be able to form a roll with a diameter less than 20 cm, preferably less than 10 cm. Known automated laying equipment requires the roll to meet specific dimensions. For example, the roll is wound onto a core with an inner diameter of 254 mm or 295 mm with a tolerance of ±0.5 mm. The roll can be cut into standard prepreg tape sizes, the widths of which include 600 mm (24"), 300 mm (12"), 150 mm (6"), 75 mm (3"), 50 mm (2"), 25 mm (1"), 6.34 mm (1 / 4") and 3.18 mm (1 / 8"), and is cut within a tolerance of ±0.050 mm, and then laid and cured as several layers of tape. Tapes are often used in this way in the production of aircraft components.

[0088] As discussed above, another advantage of being able to manufacture thicker conductive prepregs is that such automated tape laying methods can produce composites from fewer prepreg layers and thus produce composites in a shorter time.

[0089] In a third aspect, the present invention relates to a cured composite material which can be obtained by a method of exposing a prepreg or prepreg stack as described herein to high temperature and optionally high pressure to thermally cure a thermosetting resin and thereby produce a cured composite material.

[0090] Since the curing process described below basically does not affect the physical arrangement of the resin and the conductive fibers, the prepreg or prepreg stack preferably can have any of the technical features described for the individual prepreg when cured.

[0091] Such a composite material is typically subsequently cured by exposure to high temperature, wherein the thermosetting resin cures to provide the resulting cured composite material. Considering the reactivity of the resin and the amounts of resin and fibers used, the curing cycle for curing the prepreg and prepreg stack is a balance of temperature and time. This can be carried out in a known manner under high pressure such as autoclave technology. Alternatively or additionally, in the so-called vacuum bag technology, curing can be carried out at near atmospheric pressure.

[0092] As is known to those skilled in the art, such a curing process is typically exothermic, and therefore care must be taken to prevent excessive temperatures which can damage any mold or cause resin decomposition.

[0093] Typically, the cured resin has a glass transition temperature of 150°C to 200°C, more preferably 160°C to 200°C.

[0094] Once cured, the prepreg or prepreg stack becomes a composite material suitable for structural applications such as aerospace structures.

[0095] The present invention will now be illustrated by way of examples and with reference to the following drawings, in which:

[0096] Figure 1 is a schematic diagram of a typical current path during a lightning strike event on a composite wing box exhibiting an "edge glow" phenomenon.

[0097] Figure 2 is a schematic diagram of a method for manufacturing the prepreg of the present invention.

[0098] Figure 1 Shows the edge glow phenomenon when a lightning strike hits an aerospace component consisting of an outer skin and an inner spar made of composite material. As discussed, due to the typically low z-direction conductivity, current tends not to flow between adjacent layers, thereby establishing a potential difference inside. This establishment can cause electron surface ejection or plasma generation at the composite edge, and is typically manifested as a kind of resin explosion. Uncertainty about the nature of this phenomenon has led to concerns about fuel vapor ignition during lightning strike events.

[0099] Turning to Figure 2 , the prepreg manufacturing method proceeds from right to left, starting the method with a creel device 8 that can support 370 spools of carbon fiber tows (each tow having 12,000 individual carbon filaments). Each fiber bobbin on the creel is tightened by a tape and spring arrangement to provide uniform fiber tension between the tows to the machine. The fiber tows pass from the creel into a comber. Before they enter the comber, the individual tow tensions are measured at position 10 in Figure 2 . The individual 12k carbon fiber tow tensions are measured here using a hand-held fiber tension meter. The fiber break load from the tape and spring assembly on the creel is controlled to provide a fiber tension of approximately 250 g / tow at this point.

[0100] Ten tows randomly selected from each process web are measured for quality control and the nominal fiber tow tension is checked at a preferred individual tow tension of 250 g / tow. The fiber tows then pass through a comber 12. The fiber comber is used to separate the carbon fiber tows and align them into the fiber spreading bar area and control the total fiber web width such that the fiber areal weight of the prepreg is within the required tolerance. The fiber tows then pass over a load cell roller 14 that measures the total tension applied to the carbon fiber. The fiber then passes through spreading bars 16. These bars control fiber tensioning and spreading to control the final fiber tension and align the fiber before it contacts the resin-coated film at the feed point 22.

[0101] The two rods forming the pinch point 22 are locked so that they do not rotate, while the other rods do rotate before this. The first spreading rod 16 is a load cell roller to monitor the total overall fiber tension entering the spreading rod system. The fiber tows are heated in this spreading rod area by infrared heaters (not shown) in preparation for impregnation with the resin composition. The infrared heaters soften the fiber sizing agent to help promote good fiber impregnation. Fiber sizing agent is an epoxy resin solution applied to the carbon fibers at the point of manufacture to aid in fiber processing, but in some cases, the sizing agent may limit fiber spreading and impregnation.

[0102] Two pre-coated resin film rolls are loaded onto the prepreg machine unwind, one above the prepreg web 18 and one below the prepreg web 20. These film rolls provide resin that is fed through the top film unwind 18 and the bottom film unwind 20. The resin and fiber meet at the nip point 22. No significant impregnation occurs at this point.

[0103] For this 536gsm fiber area weight product, the pre-coated resin film is nominally 138gsm, so that a resin content of 34% by weight is achieved in the final product. Resin (including any particulate material) is applied to the right side of the super-calendered double-sided differential silicone coated release paper. The film roll braking tension in the unwinding device 18 and 20 is controlled and matched with the final fiber web tension so that the crease-free prepreg web passes through the hot S-wrap impregnation area 24, 28. Although a single impregnation stage is shown, including two resin films provided by the unwinding device 18, 20, a subsequent resin application stage can be included after the freezing plate 30, and the resin is not intended to enter the fiber gap, but to provide a resin layer, which will eventually become a resin interlayer when multiple such prepregs are stacked. This is sometimes referred to as a 4-film method.

[0104] The resin and fiber then pass through a first S-wrap compactor 24 and then through another infrared heating stage 26 for further heating. The prepreg is heated to 120°C to 130°C under the IR heater so that the resin viscosity decreases before the web enters the second, third and fourth heated S-wrap roll groups, as shown in FIG. Figure 2 , for impregnating the resin into a structural fiber layer of 12k carbon fiber tow. At this stage of the process, after the IR heater 26, the resin has a sufficiently low viscosity for impregnation into the fibers.

[0105] The resin and fibers pass through three additional S-winding compaction machines 28, where impregnation occurs to produce a fiber layer with reliable and sufficient impregnation failure. These S-winding roller sets are heated to 135°C to 160°C, have a diameter of 270 mm, and are separated to form a spacing of 350 to 450 mm between them. One or more of the S-winding compaction machines 28 can be replaced by a pair of pinch rolls forming the web, which are typically heated to 105°C to 125°C and have a spacing of 500 to 700 microns between them, such as a spacing of 625 microns (for a prepreg with a thickness of 500 microns).

[0106] The rotational speed on these rollers is controlled such that the web winding force is high, so that these forces act on the prepreg web to disrupt the fiber structure layer and allow the resin to enter the carbon fibers at a high flow rate to achieve good impregnation. It has been found that for low resistance values, disruption of the fiber structure layer by the winding force through S-winding is required, and impregnation is needed for successful automation of the prepreg tape laying operation in the customer method.

[0107] The fibers and resin then pass through a cold plate 30. The prepreg web is frozen on this cold plate to cool the prepreg to 20°C to 22°C, so that the process paper can be removed before further conventional processing stages of the prepreg, which are after the cold plate and not shown here but are known to those skilled in the art. Examples

[0108] Hextow TM IM5 carbon fibers are set on a plurality of spools holding fiber tows. Each fiber tow contains multiple carbon fiber filaments (12,000 filaments), each having a diameter of 7 microns. These are arranged to provide a structural layer with 536 gsm unidirectional carbon fiber filaments.

[0109] A resin system including a trifunctional epoxy resin, a bisphenol-F epoxy resin, and a 4,4’DDS curing agent was prepared. The resin includes 6.75 wt% of thermoplastic toughening agent particles (Orgasol 1002DNAT1) and 6.75 wt% of conductive particles (Nippon Graphite GHDR-15-4). This was provided as two layers of resin on a backing paper, supplied by spools 18, 20.

[0110] According to the steps outlined above regarding Figure 2 the 4-film method, prepregs were prepared. However, to vary the forces involved during impregnation, a variant was carried out in which the multiple S-winding stages were replaced by a pinch roll stage.

[0111] Measurement of conductivity

[0112] The electrical conductivity of the composite laminate in the z-direction was measured by the following method.

[0113] The panel is prepared from multiple unidirectional prepreg layers. The panel is cured in an autoclave at a temperature of 180 °C and a pressure of 0.7 MPa for 2 hours to form a cured panel with dimensions of 300 mm × 300 mm × 3 mm. A total of 9 samples are marked for testing in a 3×3 square grid arrangement and then cut from the panel such that the samples have dimensions of 40 mm × 40 mm × 3 mm. The square faces of the samples are sanded on a Linisher machine to expose the carbon fibers. Excessive sanding is prevented as this would penetrate through the first layer into the first interlayer. The square faces are then coated with a metal such as gold by thermal sputtering to a thickness of approximately 30 nm, or coated with tin - zinc by arc spraying to a thickness of at least 10 microns. Prior to testing, any metal on the sides of the samples is removed by sanding.

[0114] Each side of the sample is contacted with copper braid or wire to form electrodes that extend diagonally across the metal - coated surface. A power supply (TTz EL302P programmable 30V / 2A power supply unit, Thurlby Thandar Instruments, Cambridge, UK) capable of varying voltage and current is used to determine the resistance. Each sample can use two or four electrodes, with the latter being preferred as it is more reproducible. The power supply is contacted with the electrodes and held in place using a fixture. The fixture has a non - conductive coating or layer to prevent a circuit path from one braid to another. A current of one ampere is applied and the voltage is recorded. The resistance (R = V / I) can then be calculated using Ohm's law. Each cut sample is tested to give a series of values. To ensure the credibility of the test, each sample is tested twice. To verify the measurement results, a Flux Multimeter is also used to measure the resistivity by placing one electrode on one coated surface and the other electrode on the opposite coated surface.

[0115] Based on the calculated resistance [ohms], the conductivity [siemens / m] is calculated as conductivity (σ) = sample thickness (t) / {resistance (R) × sample area (A)}. The thickness is measured in meters and the area is the length (in meters) multiplied by the width (in meters).

[0116] The conductivity of the composite laminate in the x - y plane is measured by the following method.

[0117] Test at least three rectangular samples from the panel and obtain the average value. The sample size has a length of 100 mm and a width of 20 mm in the x-y plane, and a certain thickness in the z direction. Thus, the sample has two opposite rectangular ends defined by the width and thickness. The 0° reference direction in the x-y plane is parallel to the length of the sample. Place the panel in a metal holder and clamp it so that the brass plates are in contact with the two opposite rectangular ends, so that the sample is clamped between the brass plates, allowing current to pass through the sample along its length. Measure the resistance in the length direction. To ensure good and reproducible electrical contact, clean the ends of the sample, and metallization of each end of the sample is required. The opposite rectangular ends are coated with a metal such as tin-zinc with a thickness of approximately 100 microns (similar to z-conductivity measurement) by arc spraying.

[0118] Supply 1 A current using a TTi EL302P power supply unit (PSU) and measure the voltage using a TTi 1906 multimeter. Measure the resistance using a 4-probe measurement method.

[0119] Calculate the resistance (in ohms) according to Ohm's law (R = V / I), where I is the current in A, and calculate the conductivity using the following formula: Conductivity (σ) = sample length (l) / {width (w) × thickness (t) × resistance (R)}

[0120] Example 1

[0121] Prepare a prepreg stack using an eleven-layer prepreg with the following unidirectional fiber lay-up orientations: -45°, +45°, 90°, -45°, +45°, 0°, +45°, -45°, 90°, +45°, -45°, where the angle is the angle formed by the unidirectional fibers in the prepreg relative to an arbitrary 0° direction.

[0122] Cure the prepreg stack in an autoclave at 2 °C / min to 180 °C and 7 bar pressure for 2 hours.

[0123] Measure the value of z-conductivity by the above method. The conductivity value in the x-y plane is also carried out by the above method.

[0124] To measure the distribution of resin between the structural layer and the interlayer, measure the resin content in the fiber layer separately. This is done by analyzing images taken by microscopy of the central sample of nine samples. The thickness of the interlayer is also measured by analyzing images taken by microscopy of the central sample.

[0125] The ability of the resulting prepreg stack to pass or fail an “edge glow” test was also tested, which involves passing an electric current through the panel. Two aluminum fasteners were positioned through the thickness of the panel, offset from each other at an angle of 22.5° relative to the 0° direction, so as to prevent any current from passing directly along any carbon fibers between the fasteners. A digital camera was used to detect the presence or absence of edge glow (i.e., the test failed) or no edge glow (i.e., the test passed).

[0126] The results are shown in Table 1 below.

[0127] Table 1

[0128]

[0129] As can be expected, the conductivity in the 90 direction was always greater than that in the 0 direction. This is because two prepregs were aligned with the 90° direction, while only one prepreg was aligned with the 0° direction. However, due to the larger number of prepregs aligned at 45° to each measurement direction, their values were comparable, which tended to significantly homogenize the anisotropy in the x-y plane.

[0130] It can also be seen that the conductivity in the x-y plane varies depending on the type of impregnation method applied. This can be explained by the change in the y-direction conductivity of the prepreg, which is sensitive to the amount of resin present in the fiber gaps. It would be expected that the x-direction conductivity would remain unaffected. As can be seen, if the resin content reaches or exceeds 28 vol%, the x-y plane conductivity decreases significantly and fails the edge glow test.

[0131] Example 2

[0132] To further explore this, four of the prepared prepreg sets (Examples 1, 6, 9, and 10) were formed into eight-layer stacks, all aligned in the 0° direction. The prepreg stacks were cured in an autoclave at 2 °C / min to 180 °C and 7 bar pressure for 2 hours.

[0133] The results are shown in Table 2 below.

[0134] Table 2

[0135]

[0136] As can be seen, the conductivity in the x-direction (i.e., the 0° direction) is much greater than that in the y-direction (i.e., the 90° direction). It can also be seen that if the ratio of the conductivity in the x-direction parallel to the conductive fibers to the conductivity in the y-direction perpendicular to the conductive fibers is less than 1000, for example, 50 to 500, then even if the value of the z-direction conductivity is relatively low, the edge glow test can be passed.

Claims

1. A curable prepreg, the curable prepreg comprising a structural layer of unidirectional conductive fibers having a gap therebetween, the structural layer having a first outer surface and a second outer surface that is substantially parallel, and the curable prepreg comprising a thermosetting resin impregnated within the structural layer and present within the gap, and a first layer of thermosetting resin in contact with the first outer surface of the structural layer, wherein the ratio of the electrical conductivity in the x - direction parallel to the conductive fibers to the electrical conductivity in the y - direction perpendicular to the conductive fibers is less than 1000.

2. The curable prepreg according to claim 1, wherein the ratio of the electrical conductivity in the x - direction to the electrical conductivity in the y - direction is less than 500.

3. The prepreg according to claim 1 or claim 2, the prepreg comprising a second layer of thermosetting resin in contact with the second outer surface of the structural layer.

4. The prepreg according to any one of the preceding claims, wherein the ratio of the thickness of the structural layer to the total thickness of the first outer resin layer and the second outer resin layer if present is from 3:1 to 6:

1.

5. The prepreg according to any one of the preceding claims, wherein the areal weight of the fibers in the structural layer is greater than 200 g / m 2 , preferably 500 to 1200 g / m 2 .

6. The prepreg according to any one of the preceding claims, wherein the structural layer has a thickness greater than 300 μm, preferably from 350 to 1200 μm, in a direction perpendicular to the first outer surface and the second outer surface.

7. The prepreg according to any one of the preceding claims, wherein the total thickness of the first outer resin layer and the second outer resin layer if present is greater than 50 μm, preferably from 70 to 150 μm.

8. The prepreg according to any one of the preceding claims, wherein the resin in the first outer resin layer and the second outer resin layer if present comprises thermoplastic particles.

9. The prepreg according to claim 8, wherein the thermoplastic particles are present at a level of 5 to 15 wt% based on the total resin in the prepreg.

10. The prepreg according to any one of the preceding claims, wherein the conductive fibers are selected from carbon fibers, metallized glass fibers, graphite fibers, metallized polymers, and mixtures thereof, preferably carbon fibers.

11. The prepreg according to any one of the preceding claims, wherein the first outer resin layer and the second outer resin layer if present comprise conductive particles.

12. The prepreg according to claim 11, wherein the conductive particles comprise coated graphite particles.

13. The prepreg according to claim 11 or claim 12, wherein the conductive particles are present at a level of 5 to 15 wt% based on the total resin in the prepreg.

14. The prepreg according to any one of claims 11 to 13, wherein the conductive particles have a particle size of 10 to 80 μm.

15. The prepreg according to any one of claims 11 to 14, wherein the total thickness of the first outer resin layer and the second outer resin layer if present is more than 10 μm thicker than the size of the conductive particles.

16. The prepreg according to any one of the preceding claims, wherein the resin content in the structural layer is less than 30.0 wt%, preferably less than 28.0 wt%, more preferably less than 26 wt%.

17. A prepreg stack, the prepreg stack comprising a plurality of prepregs according to any one of the preceding claims, thus comprising a plurality of structural layers of conductive fibers and a plurality of resin interlayers, the plurality of resin interlayers being formed by the first outer resin layer and, if present, the second outer resin layer.

18. A cured composite material, the cured composite material obtainable by a method of exposing a prepreg or prepreg stack according to any one of the preceding claims to high temperature and optionally high pressure to thermally cure the thermosetting resin and thus produce the cured composite material.

19. The cured composite material according to claim 18, the cured composite material forming an aircraft component.

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