Improved unidirectional prepreg

Abstract

A curable prepreg comprising a structural layer of unidirectional conductive fibers having gaps between unidirectional conductive fibers, having a first outer surface and a second outer surface essentially parallel to it, and containing a thermosetting resin impregnated within the structural layer and present in the gaps, and a first layer of thermosetting resin in contact with the first outer surface of the structural layer, wherein the ratio of conductivity in the x-direction parallel to the unidirectional conductive fibers to conductivity in the y-direction perpendicular to the unidirectional 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 resin that provides improved conductive properties. [Background technology]

[0002] Composite materials offer well-established advantages over conventional structural materials, particularly in their ability to achieve excellent mechanical properties at very low material densities. As a result, the applications of such materials are expanding, ranging from industrial and sports / leisure applications to high-performance aerospace components.

[0003] Prepregs, which consist of fibers or fabrics impregnated with thermosetting resins such as epoxy resins, are widely used in the production of such composite materials. The resin can be combined with the fibers or fabric in various ways. The resin may be adhered to the surface of the fibrous material, but more commonly, it partially or completely impregnates the gaps between the fibers. In common configurations, a separate resin layer remains unimpregnated on the outer surface of the prepreg.

[0004] Once manufactured, typically several plies of such a prepreg are "laid-up" as desired, and the resulting prepreg stack, i.e., laminate or preform, is cured, typically by exposure to elevated temperatures, resulting in a cured composite structure. Curing may be carried out in a vacuum bag that can be placed inside a mold for curing. Alternatively, the stack may be formed and cured directly in the mold.

[0005] When such a laminate is fabricated from multiple prepregs, each containing a separate resin layer, this results in a fiber layer with the separate resin layers interposed between them. Such a configuration is known to achieve desirable mechanical properties in the resulting cured composite material.

[0006] However, a lightning strike on the skin of an aircraft made of such composite materials can cause damage due to energy concentration. Among the physical phenomena observed from lightning strikes is a phenomenon known as “edge glow,” which describes a state in which a glow of light, combined with particle or plasma emission, appears at the tips or ends of carbon fibers on the exposed fiber surfaces of composite components within a composite structure. Edge glow is caused by a voltage difference between conductive composite layers and typically occurs in high-current-density areas resulting from the lightning strike where the potential is maximum, such as on exposed fiber surfaces. When edge glow occurs in areas containing fuel or fuel vapor (collectively referred to herein as the “fuel environment”), such as in a fuel tank or near a fuel line, it is a potential fuel ignition source. The phenomenon occurring at an edge is called “edge glow,” and the phenomenon occurring on a surface is called “surface discharge.” Both can be considered ignition hazards.

[0007] Furthermore, the presence of an electrically insulating interleaf layer can reduce conductivity in the direction perpendicular to the surface of the laminate, the so-called z-direction, potentially exacerbating 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 extremely severe damage to composite materials, and can be catastrophic if they occur on an aircraft structure in flight. Therefore, this is a particular problem for aerospace structures made from such composite materials.

[0008] Therefore, this edge glow phenomenon can occur during lightning events, particularly in composite laminates with low z-direction conductivity. During a lightning event, a transient charge with a high-intensity current travels through the skin due to the fasteners connecting two composite parts, and then enters the wing's substructure (e.g., structural spars or ribs). Thus, typically in a composite skin / spar assembly, the current travels partly over the skin and partly through the spar representing one of the fuel tank walls. The current travels laterally from the fastener through the adjacent composite ply of the spar and tends to travel along the fibers due to their higher conductivity compared to the resin matrix. This path can generate a typical bright glow or spark at the spar / rib cap edge, resulting in the "edge glow" phenomenon.

[0009] Furthermore, composite materials intended for aerospace applications must meet stringent standards regarding mechanical properties. Therefore, any improvement in conductivity must not adversely affect the mechanical properties.

[0010] To achieve conductivity in the z-direction of such composite materials, a wide range of techniques and methods have been proposed in the prior art.

[0011] International Publication No. 2008 / 056123 discloses how conductivity is improved by adding hollow conductive particles to a resin interleaf layer such that the hollow conductive particles come into contact with adjacent fiber layers and create electrical pathways in the z direction. This is achieved by crosslinking a relatively conductive fiber layer (e.g., made from carbon fibers) across an electrically insulating interleaf layer.

[0012] International Publication No. 2010 / 150022 teaches how to break the interface between a structural layer and an interleaf layer in order to induce contact points between adjacent structural layers.

[0013] International Publication No. 2011 / 027160 discloses the use of glassy carbon particles in an intermediate layer having a maximum thickness of 50 μm.

[0014] International Publication Nos. 2013 / 186389 and 2015 / 157486 teach that the conductivity in the z-direction can be improved by adding potato-shaped graphite to the interleaf layer.

[0015] International Publication No. 2016 / 048885 discloses that conductivity in the z-direction can be enhanced by using conductive nano-sized particles located in an interleaf layer and a lightweight carbon veil composed of randomly arranged carbon fibers. [Overview of the project] [Problems that the invention aims to solve]

[0016] However, it has been found that prepregs containing unidirectional fibers, such as layers of carbon fibers, can still exhibit vulnerability to electromagnetic hazards, despite the improved z-direction conductivity achieved by these technologies.

[0017] In particular, when high current densities flow through such materials, the electrical anisotropy of the prepreg can create a large potential difference, 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. [Means for solving the problem]

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

[0020] Surprisingly, it has been found that when such a prepreg has a conductivity in the y direction (i.e., perpendicular to the direction of the unidirectional fibers) exceeding a certain critical value with respect to a much larger conductivity value in the x direction (i.e., parallel to the direction of the unidirectional fibers), improved electromagnetic properties can be achieved. This is surprising because the conductivity in the y direction is generally much higher than the conductivity in the z direction.

[0021] Thus, in a first aspect, the present invention relates to a curable prepreg comprising a structural layer of unidirectional conductive fibers having gaps between the unidirectional conductive fibers, having a first outer surface and a second outer surface that is essentially parallel, and being impregnated within a structural layer and containing a thermosetting resin present within the gaps, and a first layer of thermosetting resin in contact with the first outer surface of the structural layer, wherein 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.

[0022] Thus, the structural layer has a first outer surface and a second outer surface that is essentially parallel, and each surface defines an x-y plane 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 having a thickness in the z direction and in contact with the first outer surface of the structural layer.

[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 conductive properties 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 50 to 500.

[0024] When the fibers in the structural layer are in one direction, the impregnation of resin into the gaps between them may cause the fibers to separate from each other. Therefore, when the resin content in the structural layer exceeds the critical value, the physical contact between adjacent conductive fibers is broken, so that the so-called conductivity in the y direction is significantly reduced. The conductivity in the y direction can remain high relative to the conductivity in the z direction (in fact, it may be several orders of magnitude higher), but it decreases relative to the conductivity in the x direction (which is not significantly affected by the resin content in the structural layer). As a result, it has been observed that this may reduce the electromagnetic properties of the finally cured prepreg.

[0025] While not wishing to be bound by any particular theory, this is due to the imbalance in the dissipation of electrical energy in the x-y plane, and thus the cured prepreg is thought to affect the ability to handle three-dimensional electrical energy flow. Therefore, the present invention achieves improved electromagnetic properties by ensuring that not only is the conductivity in the z direction the minimum acceptable 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 generally achieved by a trade-off with other mechanical properties, the conductivity of the prepreg in the z direction is generally sufficient if it is 50 S / m -1 or less. Furthermore, it has been found that excellent electrical resistance properties can be achieved with these more moderate z conductivity values when the conductivity in the x-y plane is adjusted according to the requirements of the present invention.

[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 and even more preferably greater than 20,000 S / m.

[0028] Preferably, before and after curing, the conductivity of the prepreg in the y-direction is 100 Sm. -1 Larger, more preferably 300Sm -1 Larger.

[0029] In a preferred embodiment, the prepreg includes a layer of thermosetting resin that contacts the second outer surface of the structural layer. The second outer layer is typically the same composition as the first outer layer and preferably the same or similar thickness as the first outer layer. In this embodiment, the first and second outer layers combine to form an interleaf layer when multiple such prepregs are laminated together, as will be described later.

[0030] Such a laminated configuration, including a structural fiber layer with resin layers interposed between them, is known to result in a cured composite material with excellent mechanical properties. The thickness of the structural layer may vary depending on the intended application. However, generally, the thickness of the interleaf layer is proportional to the thickness of the structural layer. Therefore, preferably, the ratio of the thickness of the structural layer to the total thickness of the first outer resin layer and, if present, the second outer resin layer is 3:1 to 6:1.

[0031] Generally, the physical thickness of a structural layer is governed by the basis weight of the fibers present. However, as mentioned above, thicker structural layers generally require thicker resin or interleaf layers to achieve the desired mechanical properties.

[0032] On the other hand, it was found that as the thickness of the interleaf layer increases, the conductivity in the z direction tends to decrease, and therefore, it is more difficult to produce prepregs with thicker interleaf layers with acceptable electromagnetic properties. This is thought to be because the size of the interleaf layer becomes larger relative to the size of any conductive particles present, reducing its ability to provide conductive paths across the resin interleaf layer.

[0033] Therefore, deepening the understanding of the importance of conductivity in the x-y plane provided by the present invention means that acceptable electromagnetic properties can be obtained even with a thicker interleaving layer than previously considered, thus a thicker structural layer, and thus a thicker prepreg. Therefore, a thicker prepreg that is three-dimensionally conductive becomes possible, which may be advantageous in certain applications.

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

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

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

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

[0038] The thermosetting resin is preferably an epoxy resin. Preferably, the epoxy resin used in the preparation of the resin composition and / or prepreg of the present invention has an epoxy equivalent weight (EEW) in the range of 10 to 1500, and preferably 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 bifunctional 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 Epon825 (DER 332-Dow Chemical, Midland, MI), phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, glycidyl ethers of aliphatic diols, diglycidyl ethers, diethylene glycol diglycidyl ethers, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidized olefins, brominated resins, aromatic glycidylamines, heterocyclic glycidylimidines and amides, glycidyl ethers, fluorinated epoxy resins, glycidyl esters, or any combination thereof. A suitable bifunctional epoxy resin is GY281 (also known as LY3581). The bifunctional epoxy resin can be used alone or in any suitable combination with other bifunctional epoxy resins.

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

[0041] Suitable trifunctional epoxy resins may include those based on phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, aromatic epoxy resins, aliphatic triglycidyl ethers, dialiphatic triglycidyl ethers, aliphatic polyglycidyl ethers, aliphatic polyglycidylamines, heterocyclic glycidylimidines and amides, glycidyl ethers, epoxidized olefins, brominated resins, aromatic glycidylamines, fluorinated epoxy resins, or any combination thereof. Suitable trifunctional epoxy resins are available from Huntsman Advanced Materials (Monthey, Switzerland) under the trade names MY0500 and MY0510 (triglycidyl para-aminophenol) and MY0600 and MY0610 (triglycidyl meta-aminophenol). Triglycidyl meta-aminophenol is also available from Sumitomo Chemical Co., Ltd. (Osaka, Japan) under the trade name ELM-120.

[0042] Tetrafunctional epoxy resins are also preferred. The phenyl ring may be further substituted with other suitable non-epoxy substituents. Suitable substituents include, for example, hydrogen, hydroxyl, alkyl, alkenyl, alkynyl, alkoxyl, aryl, aryloxyl, aralkyloxyl, aralkyl, halo, nitro, or cyano groups. The suitable non-epoxy substituents may be bonded to the phenyl ring at the para or ortho position, or at the meta position not occupied by the epoxy group.

[0043] Suitable tetrafunctional epoxy resins include N,N,N',N'-tetraglycidyl-m-xylenediamine (commercially available from Mitsubishi Gas Chemical Company, Inc. (Chiyoda-ku, Tokyo, Japan) under the name Tetrad-X), Erisys GA-240 (from CVC Chemicals, Morrestown, NJ), and N,N,N',N'-tetraglycidylmethylenedianiline (e.g., MY720 and MY0721 from Huntsman Advanced Materials). Other suitable polyfunctional epoxy resins include DEN438 (from Dow Chemicals, Midland, MI), DEN439 (from Dow Chemicals), Araldite ECN1273 (from Huntsman Advanced Materials), MY722 (from Huntsman Advanced Materials), and Araldite ECN 1299 (from Huntsman Advanced Materials).

[0044] Preferably, at least one of the polyfunctional epoxys has at least one metasubstituted phenyl ring in its skeleton. Preferred polyfunctional epoxy resins are trifunctional or tetrafunctional. Most preferably, the polyfunctional epoxy resin is a combination of a trifunctional epoxy and a polyfunctional epoxy. The polyfunctional epoxy resin may be saturated, unsaturated, cylcoaliphatic, alicyclic, or heterocyclic.

[0045] The resin system may contain thermoplastic materials soluble in epoxy resins, such as polyethersulfone, to improve the toughness of the resin. Exemplary thermoplastic reinforcing agents / particles include, alone or in combination, the following thermoplastic materials: polyamides, copolyamides, polyimides, aramids, polyketones, polyetheretherketones, polyesters, polyurethanes, polysulfones, polyethersulfones, high-performance hydrocarbon polymers, liquid crystal polymers, PTFE, elastomers, and segmented elastomers.

[0046] A suitable reinforcing agent is, for example, PES particles sold under the trade name Sumikaexcel 5003P, which is commercially available from Sumitomo Chemical Co., Ltd. Alternatives to 5003P include non-hydroxyl-terminated grades such as Solvay polyethersulfone 105RP or Solvay 1054P.

[0047] The resin also preferably contains a curing agent and a curing accelerator. The curing accelerator is usually thermally activated and reduces the time required to cure the resin. Suitable curing agents are aromatic amines, such as amines including 1,3-diaminobenzene, 1,4-diaminobenzene, and 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 preferred embodiments, the resin may comprise one or more combinations of the following components: a reinforcing agent in the form of triglycidylaminophenol in an amount of 8 to 34% by weight relative to the resin, a bisphenol epoxy in an amount of 20 to 28% by weight relative to the resin, a tetraglycidylamine in an amount of 25 to 35% by weight relative to the resin, a reinforcing agent in the form of polyethersulfone in an amount of 10 to 25% by weight relative to the resin, and a curing agent in the form of diaminodiphenylsulfone (4,4'-DDS or 3,3'-DDS) in an amount of 2 to 28% by weight relative to the resin.

[0049] In preferred embodiments, the resin of the first outer resin layer and, if present, the second outer resin layer, comprises thermoplastic polymer particles. The thermoplastic polymer must be insoluble in the resin, typically epoxy resin, at room temperature and at the elevated temperature at which the resin cures. Depending on the melting point of the thermoplastic polymer, it may melt or soften to varying degrees during the curing of the resin at the elevated temperature and solidify again as the cured laminate cools.

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

[0051] Suitable thermoplastic materials include polyamides, polycarbonates, polyacetals, polyphenylene oxides, polyphenylene sulfides, polyacrylates, polyethers, polyesters, polyimides, polyamideimides, polyetherimides, and polyurethanes. Polyamides are a preferred type of thermoplastic particle. 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 preferably have a particle size of less than 100 μm. The particle size is preferably in the range of 5 to 60 microns, more preferably 10 to 30 microns. The average particle size is preferably about 20 microns. Suitable reinforcing particles include Orgasol 1002 D NAT1 (PA6), Rilsan PA11 P C20HT (PA11), Ultramid 4350 (PA6T), Vestamid 1010, and 610. The particles can be produced by anionic polymerization in accordance with PCT application international publication 2006 / 051222, by co-extrusion, precipitation polymerization, emulsion polymerization, or by cryogenic pulverization. It is preferable that the particles be produced by direct polymerization rather than by pulverization or precipitation.

[0052] Since the resin needs to be conductive and mostly thermosetting, the amount of reinforcing particles must be carefully adjusted. It was found that thermoplastic particles should ideally be present at a level of 5-15% by weight based on the total resin in the prepreg.

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

[0054] Exemplary unidirectional fiber layers are made from HexTow® carbon fibers available from Hexcel Corporation. Suitable HexTow® carbon fibers for use in the production of many unidirectional fiber layers include IM5 carbon fibers, available as 6,000, 12,000, and 24,000 filaments; IM7 carbon fibers, available as fibers containing 6,000 or 12,000 filaments with weights of 0.223 g / m and 0.446 g / m, respectively; IM8-IM10 carbon fibers, available as fibers containing 12,000 filaments with weights of 0.446 g / m to 0.324 g / m; and AS7 carbon fibers, available as fibers containing 12,000 and 24,000 filaments with 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 supplied for impregnation in equipment that uses combs to hold the tows and keep them parallel and unidirectional, as described later.

[0055] The fibers may be any suitable conductive fibers selected from the list consisting of, for example, carbon fibers, metallized glass fibers, graphite fibers, metallized polymers and mixtures thereof, preferably carbon fibers.

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

[0057] Examples of conductive particles include metal particles, carbon particles such as glassy 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 CVD (chemical vapor deposition) carbon-coated graphite. Preferred conductive particles include SG25 / 99.95SC from NGS Naturgraphit GmbH in Germany, which has an average particle size of 10-30 μm, and GHDR-15-4 from Nippon Power Graphite Co., Ltd., which has an average particle size of 10-30 μm and a carbon coating deposited by carbon vapor deposition. Other preferred particles include graphite particles from Westwater GmbH, which have an average size of 10-30 μm, and carbon microballoons from Sigratherm GmbH, which have an average size of 5-80 microns.

[0058] Preferably, the conductive particles are present at a level of 5 to 15% by weight 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 interleaf layer is preferably more than 10 μm thicker than the size (e.g., average diameter) of the conductive particles present.

[0061] Typically, on a weight basis, the prepreg contains 15 to 70% by weight of curable resin, preferably 20 to 65% by weight, more preferably 25 to 50% by weight, and most preferably 25 to 40% by weight. On a volume basis, typically, the prepreg contains 15 to 70% by volume of curable resin, preferably 20 to 60% by volume, and more preferably 30 to 50% by volume.

[0062] However, care must be taken to ensure that the resin is distributed in the gaps between the fibers of the structural layer and the first outer resin layer and, if present, the second outer resin layer. It is known that the higher the resin content in the structural layer, the more significantly the conductivity in the y-direction decreases. Therefore, preferably, the resin content in the structural layer is less than 30% by weight, preferably less than 28.0% by weight, and preferably less than 26.0% by weight.

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

[0064] The degree of air in the structural layer is measured by a water pickup test (water absorption test) which determines the degree of impregnation into the gaps between fibers. In this test, a test specimen of the prepreg material is first weighed and clamped between two plates so that a 5 mm wide strip protrudes. This configuration is suspended in a water bath at room temperature (21°C) for 5 minutes in the direction of the fibers. The test specimen is then removed from the plates and weighed again, and the difference in weight gives the value of the degree of impregnation in the test specimen. The less water picked up, the higher the degree of impregnation. The prepreg according to the present invention preferably has less than 6%, more preferably less than 5%, and typically 2-5% water pickup.

[0065] By volume, a prepreg typically contains 45–75 volume% structural fibers, preferably 55–70 volume% structural fibers.

[0066] The resin and fiber content of uncured prepregs or prepreg stacks containing unidirectional carbon fibers is measured according to DIN EN 2559 A (code A). The resin and fiber content of cured composites containing carbon fiber materials is measured according to DIN EN 2564 A.

[0067] The prepreg according to the present invention may be manufactured in a known form, for example, by a process described and illustrated in International Publication No. 2010 / 150022, in a continuous process that typically involves passing thousands of fibers, typically guided by rollers, through a series of impregnation steps to form a fibrous structural layer, which acts to impregnate a resin into the structural layer.

[0068] Before the fibers come into contact with the resin and reach the impregnation zone, they are typically aligned into tows of multiple fibers, each tow containing thousands of filaments, e.g., 12,000. These tows are mounted on a bobbin and first fed into a combing unit to ensure uniform separation of the fibers. The structural layer is typically formed from these tows of multiple fibers, spread out on a spreader bar to integrate together before being impregnated with resin.

[0069] To improve the handling of the resin, it is customary to support the resin on a backing material such as paper. The resin is then typically supplied from a roll so that it contacts the fibers, while the backing material remains in place outside the contact area between the resin and the fibers. During the subsequent impregnation process, the backing material serves as a useful outer layer for applying pressure to achieve uniform resin impregnation. Each layer of resin applied is sometimes referred to as a film.

[0070] During this impregnation process, the resin passes through the gaps between the fibers. As already mentioned, in some cases a second impregnation layer (i.e., a second film) containing thermosetting resin is introduced, and the second surface of the fiber layer comes into contact with the second impregnation layer before compression. This can form a second layer of thermosetting resin that comes into contact with the second outer surface of the structural layer. Together with the first resin layer, this can ultimately become a resin interleaf layer during the layup of multiple prepregs.

[0071] This process can be carried out in a single step (i.e., one or two resin films are applied in a single step) with an excess resin, so that a first layer of resin (including any granular material) remains during the layup of multiple prepregs, which will ultimately become the resin interleaf layer. Such a single-step process tends to result in some degree of fracture in the structural layer, which can be advantageous, particularly with respect to conductivity.

[0072] Alternatively, resin impregnation may be carried out in a two-step process. This involves a first step in which one or two resin films are applied to a first and typically second surface of the fiber layer. The two resin films may contain only resin and no granular material such as reinforcing agents or conductive particles, and are intended to impregnate the gaps between the fibers to completely "wet out" the fiber layer. This impregnation is then followed by a second step in which the impregnation is brought into contact with another resin, often containing granular material, typically reinforcing particles, and is intended to lay down the resin layer that will ultimately become the interleaf layer when multiple such prepregs are stacked together. This second step is typically carried out after the prepreg has passed a cooling step. This two-step process is sometimes referred to in the art as "4-filming".

[0073] To facilitate resin impregnation into the fibers, the process is typically carried out at an elevated temperature, for example, 60-170°C, preferably 100-150°C, so that the resin viscosity decreases to 0.1 Pas-100 Pas, preferably 5-30 Pas, more preferably 10-20 Pas, and even more preferably 8-17 Pas. This is best achieved by heating the resin and fibers to the desired temperature before impregnation, for example, by passing them through an infrared heater.

[0074] Resin impregnation typically involves passing resin and fibers through rollers, which can be arranged in various ways. The two main arrangements are the "nip" arrangement and the "S-wrap" arrangement.

[0075] The S-wrap stage is a step in which both the resin and fibers, in sheet form, pass around two separate rotating rollers in the shape of the letter "S". In the alternative "nip" arrangement, the fibers and resin are sandwiched or pinched together as they pass between pinch points between two adjacent or opposing rotating rollers. It is generally understood that the S-wrap provides ideal conditions for reliable and reproducible resin impregnation between the fiber gaps, while also providing sufficient fracture. However, the nip stage can result in stronger impregnation, provided that care is taken to control the pressure, for example, by controlling the gap between adjacent rollers.

[0076] During this stage, several processes occur simultaneously, including resin impregnation into the gaps between the structural fiber layers and the destruction of the structural layers.

[0077] As already discussed, for example, some fracture occurs in the structural layer, but such fracture is not significant enough to cause fiber separation that could lead to a decrease in conductivity in the y-direction. Therefore, the selection of impregnation conditions must be carefully chosen to ensure that the desired amount of resin is impregnated into the structural layer. Thus, a balance must be found to give the optimal degree of structural layer fracture according to the specific conditions of the desired outcome.

[0078] Parameters such as the separation between rollers, speed, 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 breakage and resin impregnation together.

[0079] Furthermore, prior to impregnation, the structural fibers may first be passed through a fiber breaking means. This causes some of the fibers on the outer surface of the sheet to become broken filaments. Depending on how the structural fibers are configured, the breaking means may produce broken filaments in several ways, for example, by breaking the adhesion points between the structural fibers and breaking the structural fibers to shorter lengths, or by individual breaks that allow the free ends of the filaments to move into the interleaf layer of the resin. This may also be done by the process described in International Publication 2011 / 114140, in which the breaking means includes passing the fibers over an ablation surface, thereby causing the breakdown of some of the fibers on the outer surface that pass through in contact with the ablation surface, while the fibers that do not come into contact with the ablation surface remain unbroken.

[0080] As described above, unidirectional fibers are typically formed from a tow of multiple fibers spread out to integrate together before resin impregnation. A common way to achieve this is to pass the fibers over multiple sequential spreader bars or rollers. Therefore, it is convenient to incorporate the ablation surface into an existing spreader bar arrangement. Preferably, the ablation surface is the surface of the spreader bar.

[0081] Multiple sets of S-wrap or nip rollers can be used, with each set gradually increasing the pressure applied to the resin. A typical process may also involve combining sets of S-wrap and nip rollers on the same production line.

[0082] After impregnation, a cooling step is typically included to reduce the tackiness of the formed prepreg. This may be followed by further processing steps such as lamination, cutting, and separation.

[0083] Once prepared, the prepreg may be rolled up so that it can be stored for a certain period of time. Considering the adhesiveness of such materials, a backing sheet is generally provided to allow the roll to be unrolled when in use. Therefore, preferably, the prepreg according to the present invention may include a backing sheet on its outer surface to facilitate the handling and / or rolling of the material. It can then be unrolled and cut as desired.

[0084] When it is desired to manufacture composite materials, several such prepregs are typically laminated together to produce a prepreg stack or preform.

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

[0086] In one preferred configuration (arrangement), the fibers have an orientation that varies throughout the prepreg stack, for example, in a so-called 0 / 90 configuration where the angles between adjacent fiber layers are perpendicular to each other. Among many other configurations, other configurations such as 0 / +45 / -45 / 90 are also possible.

[0087] A preferred use of the prepreg of the present invention is as tape. The prepreg can be prepared as a roll of material specifically prepared for automated tape layup devices. The prepreg is supplied with a backing sheet that is removed when laid up in a mold. Typically, the prepreg supplied with the backing sheet is sufficiently flexible so that it can form a roll having a diameter of preferably less than 20 cm, preferably less than 10 cm. Known automated layup devices require that the roll meet certain dimensions. For example, the roll is wound around either a core with an inner diameter of 254 mm or 295 mm with a tolerance of ±0.5 mm. The rolls are cut into standard prepreg tape sizes, including widths of 600mm (24"), 300mm (12"), 150mm (6"), 75mm (3"), 50mm (2"), 25mm (1"), 6.34mm (1 / 4"), and 3.18mm (1 / 8"), cut within a tolerance of ±0.050mm, and then laid up as several layers of tape and cured. The tape is thus frequently used in the production of aircraft components.

[0088] As mentioned above, a further advantage of being able to produce thicker conductive prepregs is that such automated tape layup processes can produce composite materials from fewer layers of prepreg, and therefore in a shorter time.

[0089] In a third aspect, the present invention relates to a cured composite material that can be obtained by a process of thermosetting a thermosetting resin by exposing a prepreg or prepreg stack described herein to an increased temperature and optionally increased pressure, thereby producing a cured composite material.

[0090] Since the curing process described later does not substantially affect the physical composition of the resin and conductive fibers, the prepreg or prepreg stack, when cured, may preferably have any of the technical features described in relation to the prepreg alone.

[0091] Such composite materials are typically then cured by exposure to elevated temperatures, causing the thermosetting resin to harden and resulting in a cured composite material. The curing cycle used to cure prepregs and prepreg stacks is a balance of temperature and time, taking into account the reactivity of the resin as well as the amount of resin and fiber used. This may be carried out under elevated pressure in known methods such as the autoclave technique. Alternatively or additionally, curing may be carried out at near atmospheric pressure using the so-called vacuum bag technique.

[0092] As is known to those skilled in the art, such curing processes are generally exothermic, and care must be taken to prevent excessively high temperatures, which could damage the mold or cause the resin to decompose.

[0093] Typically, curable resins have a glass transition temperature of 150°C to 200°C, more preferably 160°C to 200°C.

[0094] Once cured, prepregs or prepreg stacks become composite materials suitable for structural applications, such as aerospace structures.

[0095] Here, the present invention will be explained as a simple example with reference to the following diagram. [Brief explanation of the drawing]

[0096] [Figure 1] Figure 1 is a schematic diagram of a typical current path during a lightning strike on a composite wingbox that produces the "edge glow" phenomenon. [Figure 2] Figure 2 is a schematic diagram of a process that may be used to manufacture the prepreg of the present invention. [Modes for carrying out the invention]

[0097] Figure 1 illustrates the edge glow phenomenon when lightning strikes an aerospace component composed of an outer skin and inner spar made of composite material. As explained, due to the generally low conductivity in the z-direction, current does not flow between adjacent plies and tends to form an internal potential difference. This formation can lead to electron surface emission or plasma generation at the composite edge, often manifesting as a kind of resin explosion. Uncertainty regarding the nature of this phenomenon has raised concerns about fuel vapor ignition during lightning events.

[0098] Referring to Figure 2, the prepreg manufacturing process proceeds from a creel unit 8 capable of supporting 370 spools of carbon fiber tows, moving from right to left, with each tow containing 12,000 individual carbon filaments. Each fiber bobbin on the creel is tensioned by a strap and spring configuration to ensure uniform tension in each tow fiber. The fiber tows pass from the creel to the comb. Before they enter the comb, the tension of each tow is measured at position 10 in Figure 2. The tension of each 12k carbon fiber tow is measured here using a handheld fiber tensile strength meter. The fiber breaking load from the strap and spring assembly onto the creel is controlled to achieve a fiber tension of approximately 250g / tow at this point.

[0099] Ten randomly selected tows from each process web are measured for quality control to ensure that the nominal fiber tow tension is a preferred individual tow tension of 250 g / tow. The fiber tows then pass through comb 12. The fiber comb separates the carbon fiber tows, aligns them with the fiber-opening bar section, and acts to control the overall width of the fiber web so that the basis weight of the prepreg fibers is within the required tolerance. The fiber tows then pass through load cell roller 14, which measures the overall tension applied to the carbon fibers. The fibers then pass through spreader bars 16. These bars control the tensioning and opening of the fibers to control the final fiber tension and alignment of the 10 fibers before they come into contact with the resin-coated film at pinch point 22.

[0100] The two bars forming the pinch point 22 are locked to prevent rotation, while the other bar in front of them rotates. The first spreader bar 16 is a load cell roller for monitoring the overall fiber tension entering the spreader bar system. The fiber tow is heated in this spreader bar section by an infrared heater (not shown) in preparation for impregnation with the resin composition. The infrared heater softens the fiber sizing to help promote good fiber impregnation. Fiber sizing is an epoxy solution applied to carbon fibers at the time of manufacture to aid in fiber handling, although in some cases the sizing may limit fiber opening and impregnation.

[0101] Two pre-coated resin film rolls are loaded into the prepreg web unwinder, one above the prepreg web 18 and the other below the prepreg web 20. These film rolls provide the resin supplied by the upper film unwinder 18 and the lower film unwinder 20. The resin and fibers merge at the pinch point 22. No significant impregnation occurs at this point.

[0102] The pre-coated resin film is nominally 138 gsm for this 536 gsm fiber basis product, achieving a resin content of 34% by weight in the final product. The resin (including any granular material) is coated on the right side of the supercalendered, double-sided differential value silicone release coated paper. The brake tension of the film rolls at unwinding 18 and 20 is controlled to achieve a wrinkle-free prepreg web through the hot S-wrap impregnation zones 24, 28 and to match the final fiber web tension. Although a single impregnation step involving two resin films provided by unwinding 18, 20 is shown, a resin application step following the cooled plate 30 may be included, in which the resin is not intended to enter the fiber gaps but rather to result in a resin layer that ultimately becomes a resin interleaf layer when multiple such prepregs are laid up. This is sometimes referred to as the 4-film process.

[0103] Next, the resin and fibers pass through the first S-wrap compactor 24, and then through another infrared heating stage 26 for further heating. For resin impregnation into the structural fiber layer of the 12k carbon fiber tow, the prepreg is heated to 120-130°C under the IR heater so that the resin viscosity decreases before the web enters the second, third, and fourth heated S-wrap roll sets, as shown in Figure 2. At this stage of the process, after the IR heater 26, the resin has a sufficiently low viscosity for impregnation into the fibers.

[0104] The resin and fibers then pass through three S-wrap compactors 28, where impregnation occurs, resulting in a fractured fiber layer with reliable and sufficient impregnation. These S-wrap roller sets are heated to 135°C to 160°C, have a diameter of 270 mm, and are separated to form a gap of 350 mm to 450 mm between them. One or more of the S-wrap compactors 28 can be replaced with a pair of compactors in nip formation, which are typically heated to 105°C to 125°C and have a gap of 500 to 700 microns between them, for example, 625 microns (for a prepreg with a thickness of 500 microns).

[0105] The rotation speed of these rollers is controlled to increase the wrapping force on the web so that these forces act on the prepreg web to break the structural fiber layer and cause a high resin flow to the carbon fibers for the purpose of achieving good impregnation. Breaking of the structural fiber layer by the wrapping force of the S-wrap is required for low resistance values, and impregnation is required for the success of automated prepreg tape laying operations in customer processes.

[0106] Next, the fibers and resin pass over a cooled plate 30. The prepreg web is cooled on this cooling plate to cool the prepreg to 20°C-22°C, and the process paper can be removed before the prepreg processing step, which is a further conventional process not shown herein but known to those skilled in the art, that follows the cooling plate. [Examples]

[0107] example Hextow® IM5 carbon fibers were prepared on multiple spools holding fiber tows. Each fiber tow contained multiple carbon fiber filaments (12000) with a diameter of 7 microns. These were arranged to result in a structural layer having unidirectional carbon fiber filaments of 536 gsm.

[0108] A resin system comprising a trifunctional epoxy resin, bisphenol F epoxy, and a 4,4'-DDS curing agent was prepared. The resin contained 6.75 wt% thermoplastic reinforcing particles (Orgasol 1002 DNAT1) and 6.75 wt% conductive particles (Nippon Graphite GHDR-15-4). This was provided as a two-layer resin on backing paper, provided by spools 18 and 20.

[0109] The prepreg was prepared in the 4-filming process according to the steps outlined above in relation to Figure 2. However, to change the forces involved during impregnation, some S-wrap steps were replaced with nip steps.

[0110] Measurement of electrical conductivity The conductivity in the z-direction of the composite laminate is measured by the following method.

[0111] The panel is fabricated from multiple unidirectional prepreg layers. The panel is cured by autoclave curing at 180°C for 2 hours under a pressure of 0.7 MPa to form a cured panel measuring 300 mm × 300 mm × 3 mm. A total of nine test specimens are marked in a 3 × 3 square grid configuration for testing, and then cut from the panel to a size of 40 mm × 40 mm × 3 mm. The square faces of the test specimens are polished with a linisher machine to expose the carbon fibers. Excessive polishing is avoided as it can penetrate beyond the first ply into the first interleaf layer. The square faces are then coated with metal, for example, gold to a thickness of approximately 30 nm by thermal sputtering, or tin-zinc to a thickness of at least 10 micrometers by arc spraying. The metal on the sides of the test specimens is removed by polishing before testing.

[0112] Each side of the test specimen is brought into contact with a copper braid or wire to form electrodes that extend diagonally across the metal-plated surface. A power supply capable of varying both voltage and current (TTz EL302P programmable 30V / 2A power supply unit, Thurlby Thandar Instruments, Cambridge, UK) is used to determine the resistance. Two or four electrodes can be used per sample, the latter being preferred for greater reproducibility. The power supply is brought into contact with the electrodes and held in place using clamps. The clamps have a non-conductive coating or layer to prevent electrical paths from one braid to the other. A current of 1 ampere is applied and the voltage is indicated. The resistance can be calculated using Ohm's law (R=V / I). The test is performed on each cut test specimen, giving a range of values. To ensure the reliability of the test, each test specimen is tested twice. To verify the measurement, resistivity is also measured using a flux multimeter by placing one electrode on one plated surface and the other electrode on the opposite plated surface.

[0113] The conductivity [Siemens / m] is calculated from the calculated resistance [Ohms] using the formula: conductivity (σ) = thickness of the specimen (t) / {resistance (R) × area of ​​the specimen (A)}. The thickness is measured in meters, and the area is the length (in meters) multiplied by the width (in meters).

[0114] The conductivity of the composite laminate in the xy-plane is measured by the following method.

[0115] At least three rectangular test specimens are tested from the panel, and the average value is obtained. The specimen size has a length of 100 mm and a width of 20 mm in the xy plane, and a thickness in the z direction. Thus, the specimen has two opposing rectangular ends defined by its width and thickness. The 0° reference direction in the xy plane is parallel to the length of the specimen. The panel is placed in a metal holder, and the specimen is clamped between brass plates so that the brass plates are in contact with the two opposing rectangular ends, allowing current to be passed through the specimen along its length. Electrical resistance is measured in the longitudinal direction. To ensure good and reproducible electrical contact, the ends of the specimen are cleaned, and metallization of each end of the specimen is required. The opposing rectangular ends are coated with a metal such as tin-zinc to a thickness of approximately 100 microns (similar to the z conductivity measurement) by arc spraying.

[0116] Electrical resistance is measured using a 4-probe method with a TTi EL302P power supply unit (PSU) that supplies 1A of current and a TTi1906 multimeter that measures voltage.

[0117] Resistance (in ohms) is calculated using Ohm's law (R=V / I), where I is the current in amperes (A), and conductivity is calculated using the formula: conductivity (σ) = length of the test piece (l) / {width (w) × thickness (t) × resistance (R)}.

[0118] Example 1 A prepreg stack was prepared using an 11-ply prepreg with the following layup orientations of unidirectional fibers: -45°, +45°, 90°, -45°, +45°, 0°, +45°, -45°, 90°, +45°, -45°, where the angles are the angles that the unidirectional fibers of the prepreg make with respect to any 0° direction.

[0119] The prepreg stack was heated in an autoclave to 180°C at a rate of 2°C / min, and then cured at that temperature for 2 hours under a pressure of 7 bar.

[0120] The z-conductivity value was measured using the method described above. The conductivity value in the xy-plane was also obtained using the method described above.

[0121] To measure the resin distribution between the structural layer and the interleaf layer, the resin content of the fiber layer alone was measured. This was done by analyzing images of the central specimen of nine test pieces taken under a microscope. The thickness of the interleaf layer was also measured by analyzing images of the central specimen taken under a microscope.

[0122] The resulting prepreg stacks were also tested for their ability to pass or fail an "edge glow" test, which involves passing an electric current through the panel. Two aluminum fasteners are positioned through the panel thickness, offset from each other at an angle of 22.5° with respect to the 0° direction, to prevent current from flowing directly between the fasteners along any carbon fibers. A digital camera is used to detect whether edge glow is present (i.e., the test fails) or not (i.e., the test passes).

[0123] The results are shown in Table 1 below. [Table 1]

[0124] As might be expected, the conductivity in the 90° direction is consistently greater than that in the 0° direction. This is due to the fact that two prepregs are aligned in the 90° direction and only one is aligned in the 0° direction. However, they are comparable in value because there are many prepregs aligned at 45° for each direction measured, which tends to make the anisotropy in the xy plane fairly uniform.

[0125] It can also be observed that the conductivity in the xy plane is variable depending on the type of impregnation process 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 interfiber gaps. The conductivity in the x-direction is expected to remain unaffected. As can be seen, when the resin content reaches or exceeds 28 volume%, the conductivity in the xy plane drops significantly, resulting in failure of the edge glow test.

[0126] Example 2 To investigate this further, four of the prepared sets of prepregs (Examples 1, 6, 9, and 10) were formed into 8-ply stacks, all aligned in the 0° direction. The prepreg stacks were cured in an autoclave at 2°C / min up to 180°C for 2 hours under a pressure of 7 bar.

[0127] The results are shown in Table 2 below. [Table 2]

[0128] As can be seen, the conductivity in the x-direction (i.e., the 0° direction) is much greater than the conductivity in the y-direction (i.e., the 90° direction). It can also be seen that if the ratio of conductivity in the x-direction parallel to the conductive fiber to conductivity in the y-direction perpendicular to the conductive fiber is less than 1000, for example, 50-500, then even with a relatively low conductivity in the z-direction, it may be possible to pass the edge glow test.

Claims

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

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

3. The prepreg according to claim 1 or claim 2, 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 claims 1 to 3, wherein the ratio of the thickness of the structural layer to the total thickness of the first outer resin layer and, if present, the second outer resin layer is 3:1 to 6:

1.

5. The fiber basis weight of the fibers in the structural layer is 200 g / m². ２ Larger, preferably 500-1200 g / m ２ The prepreg according to any one of claims 1 to 4.

6. The prepreg according to any one of claims 1 to 5, wherein the structural layer has a thickness greater than 300 μm, preferably 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 claims 1 to 6, wherein the total thickness of the first outer resin layer and, if present, the second outer resin layer is greater than 50 μm, preferably 70 to 150 μm.

8. The prepreg according to any one of claims 1 to 7, wherein the resin in the first outer resin layer and, if present, the second outer resin layer, contains thermoplastic particles.

9. The prepreg according to claim 8, wherein the thermoplastic particles are present in an amount of 5 to 15% by weight based on the total resin in the prepreg.

10. The prepreg according to any one of claims 1 to 9, wherein the conductive fiber is selected from the list consisting of carbon fiber, metallized glass fiber, graphite fiber, metallized polymer and mixtures thereof, and is preferably carbon fiber.

11. The prepreg according to any one of claims 1 to 10, wherein the first outer resin layer and, if present, the second outer resin layer, contain conductive particles.

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

13. A prepreg according to claim 11 or claim 12, wherein the conductive particles are present in an amount of 5 to 15% by weight 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, if present, the second outer resin layer is more than 10 μm thicker than the size of the conductive particles.

16. The prepreg according to any one of claims 1 to 15, wherein the resin content in the structural layer is less than 30.0% by weight, preferably less than 28.0% by weight, and more preferably less than 26% by weight.

17. A prepreg stack comprising a plurality of prepregs according to any one of claims 1 to 16, wherein the prepreg stack comprises a plurality of conductive fiber structural layers and a plurality of resin interleaf layers formed by the first outer resin layer and, if present, a second outer resin layer.

18. A cured composite material that can be obtained by a process of thermosetting a thermosetting resin by exposing a prepreg or prepreg stack according to any one of claims 1 to 17 to an increased temperature and optionally increased pressure, thereby producing a cured composite material.

19. A cured composite material according to claim 18 for forming aircraft components.

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