A prepreg, a process for its manufacture and composite materials made therefrom

Abstract

A prepreg 10 comprises a structural layer of electrically conductive fibres 12 parallel to an x-y plane and resin impregnated within the structural layer and present within interstices between the fi

Description

Technical Field The present invention relates to a prepreg with improved electrical properties, a stack of such prepregs, a method of manufacturing such a prepreg and a cured composite material resulting therefrom. Background Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is widely used and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components. Prepregs, comprising a fibre or fabric arrangement impregnated with thermosetting resin such as epoxy resin, are widely used in the generation of such composite materials. The resin may be combined with the fibres or fabric in various ways. The resin may be tacked to the surface of the fibrous material, however more usually it partially or completely impregnates the interstices between the fibres. In a common arrangement, a discrete layer of resin remains unimpregnated on the external surface of the prepreg. Once manufactured, typically a number of plies of such prepregs are “laid-up” as desired and the resulting prepreg stack, i.e. a laminate or preform, is cured, typically by exposure to elevated temperatures, to produce a cured composite structure. Curing may be performed in a vacuum bag which may be placed in a mould for curing. Alternatively the stack may be formed and cured directly in a mould. When such a laminate is made from a plurality of prepregs that comprise a discrete resin layer, this results in fibre layers interleafed with discrete resin layers. Such an arrangement is known to provide desirable mechanical properties in any resulting cured composite material. However, lightning strikes on aircraft skins consisting of such composites will likely sustain damage due to energy concentrations. Among the physical phenomena observed from lightning strikes is a phenomenon known as "edge glow," which describes the condition in which a glow of light, combined with particle or plasma ejections appears at the tips or ends of the carbon fibers in the exposed fiber surfaces of composite components in a composite structure. Edge glow is caused by voltage differences between conductive, composite layers, and typically occurs in high current density areas resulting from a lightning strike, where the voltage potential is at its maximum, such as the exposed fiber surfaces. Edge glow is a potential fuel ignition source when it occurs in areas containing fuel or fuel vapor such as in fuel tanks or near fuel lines (collectively referred to herein as "a fuel environment"). The phenomenon occurring at the edge is called “Edge Glow” and the one occurring on the surface is called “Surface Discharge”. Both can be considered being an ignition hazard. Furthermore, the presence of the interleaf layers, being electrically insulating, results in the electrical conductivity in the direction orthogonal to the surface of the laminate, the so-called z-direction, being low, which can exacerbate phenomena such as edge glow, and is generally accepted to contribute to the vulnerability of composite laminates to electromagnetic hazards such as lightning strikes. A lightning strike can cause damage to the composite material which can be quite extensive, and could be catastrophic if occurring on an aircraft structure in flight. This is therefore a particular problem for aerospace structures made from such composite materials. Additionally, composites for use in aerospace applications must meet exacting standards on mechanical properties. Thus, any improvements in conductivity must not impact negatively on mechanical properties. A wide range of techniques and methods have been suggested in the prior art to provide electrical conductivity to the z-direction of such composite materials. WO 2008 / 056123 discloses how improvements have been made in conductivity by adding hollow conductive particles in the resin interleaf layers, so that they contact the adjacent fibre layers and create an electrical pathway in the z-direction. This relies on bridging across the electrically insulating interleaf layer to the relatively electrically conducting fibre layers (if they are made from e.g. carbon fibre). WO 2010 / 150022 A1 teaches the disruption of the interface between the structural layer and the interleaf layer, so as to induce points of contact between adjacent structural layers. WO 2011 / 027160 A1 discloses the use of glassy carbon particles in an interlayer having a maximum thickness of 50pm. WO 2013 / 186389 A1 and WO 2015 / 157486 A1 teach that z-direction conductivity can be improved by the addition of potato-shaped graphite in the interleaf layer. WO 2016 / 048885 A1 discloses that z-direction conductivity can be increased by use of conductive nano-sized particles and a lightweight carbon veil comprised of randomly arranged carbon fibers located in the interleaf layer. However, such methods can only increase the z-direction conductivity to a modest degree and largely rely on random electrical connections being formed between distributed entities within the resin layer. Methods of providing a direct electrical pathway through the resin layer are known such as the use of through-thickness pins of metal alloys or carbon composites, however these methods suffer from side effects such as damage of the laminate structure, and additional process stages and complications in fabrication. Therefore there remains the need for prepregs that have an electrically insulating resin layer and yet have improved overall electromagnetic properties. Summary of Invention In a first aspect, the invention relates to a prepreg comprising a structural layer of electrically conductive fibres having interstices therebetween, the structural layer having a first outer face and an essentially parallel second outer face, the faces each defining an x-y plane separated from each other by a distance equal to the thickness of the structural layer in a z-direction, orthogonal to the x-y planes; the electrically conductive fibres being parallel to the x-y planes; the prepreg comprising resin impregnated within the structural layer and present within the interstices between the fibres; wherein the first outer face comprises a plurality of raised regions, each raised region comprising a plurality of electrically conductive fibres that are oriented with the x-y planes, and have a raised thickness extending out from the first outer face in the z-direction. The present inventors have found that if a number of raised regions of electrically conductive fibres are located on the structural layer, such raised regions can form a clear and effective electrical contact across the insulating resin layer. Additionally, the raised regions experience an increased pressure during compaction during curing, which can help to increase fibre contact in the underlying structural layer, providing additional conductivity in the x-y direction. Preferably the prepreg comprises a first layer of thermosetting resin in an x-y plane and in contact with the first and / or second outer face of the structural layer having a thickness in the z-direction. The first outer resin layer is a single contiguous phase and typically has a thickness of from 10 to 100pm. Preferably the resin is a curable resin comprising thermosetting resin and the prepreg is a curable prepreg. Raised Regions Preferably the raised regions are parallel longitudinal raised strips having a defined width in the x-y plane, a defined length in the x-y plane, and comprising a bundle of electrically conductive fibres. Preferably the fibres are unidirectional whose direction is aligned with the length of the raised strips. Preferably the parallel longitudinal strips are evenly spaced apart, although differing separation distances are also possible. Preferably the strips cover from 10% to 90%, preferably from 20 to 80%, of the surface of the first outer face. It is believed that improvements in electrical conductivity arise from two interrelated factors: a pressure effect, and the area of the raised region. As the area fraction reduces, the pressure experienced by the strips increases, and so this acts to induce a more intimate contact between the electrically conductive fibres in the region of the strips, increasing the electrical conductivity. Conversely, as the area fraction increases, the pressure experienced by the strips decreases, however the pressure experienced by the strips transfers to a greater proportion of the underlying structural layer. Therefore, preferably the strips cover from 10% to 90%, preferably from 20 to 80%, of the surface of the first outer face. Preferably the average raised thickness of the raised regions is from 10 to 100% of the thickness of the structural layer. This provides a meaningful thickness without unduly extending too far from the surface of the structural layer, and allows for compression and spreading of the raised regions during curing, to provide a meaningful increased pressure and therefore electrical contact with and in the underlying structural layer. The prepreg Conveniently, the prepreg according to the invention is essentially rectangular in the x-y planes with a length in an x-y plane much greater than a width in an x-y plane, and wherein the raised strips run parallel with the length of the prepreg. Such a prepreg may be produced in a continuous manner, and formed into a roll. The fibres may be in the form of a fabric or be formed from tows of discrete fibres. Preferably the fibres are discrete and not interwoven, and in particular they are unidirectional, in that they are arranged parallel to each other. The fibres may comprise cracked (i.e. stretch-broken), selectively discontinuous or continuous fibres. The roving nature of fibres that are woven are not considered to have any aspect of z-directional orientation. In a particularly preferred embodiment, the electrically conductive fibres in the structural layer are unidirectional. It is then highly preferred that the raised regions are provided as rows which are also unidirectional fibres, aligned with the fibres in the structural layer. The fibres may be made from a wide variety of materials, such as carbon fibres, metalised glass fibres, graphite fibres, metal-coated fibres, metallised polymers and mixtures thereof. Carbon and glass fibres are preferred. Typically the fibres in the structural layer will generally have a circular or almost circular cross-section with a diameter in the range of from 3 to 20 pm, preferably from 5 to 12 pm. Exemplary layers of unidirectional fibres are made from HexTow™ carbon fibres, which are available from Hexcel Corporation. Suitable HexTow™ carbon fibres for use in making many unidirectional fibre layers include: IM7 carbon fibres, which are available as fibres that contain 6,000 or 12,000 filaments and weigh 0.223 g / m and 0.446 g / m respectively; IM8-IM 10 carbon fibres, which are available as fibres that contain 12,000 filaments and weigh from 0.446 g / m to 0.324 g / m; and AS7 carbon fibres, which are available in fibres that contain 12,000 filaments and weigh 0.800 g / m. The prepreg of the present invention is typically predominantly composed of thermosetting resin and structural fibres, although other materials are often present such as curing agents or other additives. Typically the prepregs comprise from 25 to 50 wt % of curable resin. Additionally the prepregs typically comprise from 45 to 75 wt % of structural fibres. Resins The resin preferably comprises a thermosetting resin and may be selected from those conventionally known in the art, such as resins of phenol formaldehyde, ureaformaldehyde, 1, 3, 5-triazine-2, 4, 6-triamine (Melamine), Bismalemide, epoxy resins, vinyl ester resins, Benzoxazine resins, polyesters, unsaturated polyesters, Cyanate ester resins, or mixtures thereof. Epoxy resins are particularly preferred. Curing agents and optionally accelerators may be included as desired. The thermosetting resins are preferably epoxy resins, and may comprises one or more monofunctional, difunctional, trifunctional and / or tetrafunctional epoxy resins. Such resins may become brittle upon curing, and therefore toughening materials may be included in the resin to impart durability, although this may also increase the viscosity of the resin. The toughening material may be provided as a separate layer such as a veil. Where the toughening material is a thermoplastic polymer, it should be insoluble in the resin at room temperature and at the elevated temperatures at which the resin cures. Depending on the melting point of the thermoplastic polymer, it may melt or soften to varying degrees during curing of the resin at elevated temperatures and re-solidify as the cured laminate is cooled. Suitable thermoplastics include thermoplastics such as polyamides (PAS), polyethersulphone (PES) and polyetherimide (PEI). Polyamides such as nylon 6 (PA6), nylon 11 (PA11) or nylon 12 (PA12) and / or mixtures thereof are preferred. As discussed, the prepregs of the present invention may comprise a first layer of thermosetting resin in an x-y plane and in contact with the first outer face of the structural layer having a thickness in the z-direction. The prepregs may also comprise a second layer of thermosetting resin in an x-y plane and in contact with the second outer face of the structural layer having a thickness in the z-direction, but wherein there are no raised regions extending out of the structural layer and into the z-direction. Thus, in a second aspect, the invention relates to a stack of a plurality of curable prepregs as described herein. Such stacks will form a laminated structure of structural layers of electrically conductive fibres and raised regions of electrically conductive fibres. In one particularly preferred embodiment, the structural layers of electrically conductive fibres are unidirectional and adjacent prepregs are in a 0 / 90 arrangement, i.e. the structural fibres in one prepreg are perpendicular to those in the adjacent prepreg. The raised regions, which are preferably rows, may comprise unidirectional fibres aligned desirably be at angles of +45 / -45, i.e. they are perpendicular to each other and also at an angle of 45° to the unidirectional fibres in their respective prepregs. When the prepreg stacks are formed, pressure applied concentrates at the raised regions and provides a significant electrical connection in the z-direction of the prepreg stack. Other arrangements of unidirectional fibres are also commonly known, such as 0 / 90 / +45 / -45 or similar arrangements. The raised regions, e.g. rows, may be angled accordingly to provide effective contact. Manufacturing The precursor prepregs may be manufactured in known manner, typically in a continuous process involving the passage of many thousands of fibres, forming a structural layer of fibres, through a series of impregnation stages, typically guided by rollers, which act to impregnate resin into the structural layer. The point where the fibres meet the resin, usually in sheet form, is the start of the impregnation stage. The process of prepreg manufacture preferably comprising the steps of feeding a continuous stream of unidirectional electrically conductive fibres over a sequence of rollers, to produce the structural layer, and wherein an additional stream of unidirectional electrically conductive fibres is passed over a sequence of rollers to produce the raised strips, and wherein thermosetting resin is impregnated within the structural layer and present within the interstices between the structural fibres. Before the fibres are contacted with the resin and reach the impregnation zone they are typically arranged in a plurality of tows of unidirectional fibres, each tow comprising many thousands of filaments, e.g. 12,000. These tows are mounted on bobbins and are fed initially to a combing unit to ensure even separation of the fibres. Conveniently, a single tow may provide a single raised region. In order to improve handling of the resin it is conventional that it is supported onto a backing material, such as paper. The resin is then fed, typically from a roll, such that it comes into contact with the fibres, the backing material remaining in place on the exterior of the resin and fibre contact region. During the subsequent impregnation process the backing material provides a useful exterior material to apply pressure to, in order to achieve even impregnation of resin. Optionally, a second impregnating layer comprising thermosetting resin is provided, wherein the second face of the fibrous layer is brought into contact with the second impregnating layer prior to the compressing, wherein the second impregnating layer comprises a region where no impregnation occurs, and arranged to be aligned with the region where no impregnation occurs in the first impregnating layer, thereby producing the precursor prepreg. To facilitate impregnation of the resin into the fibres it is conventional for this to be carried out at an elevated temperature, e.g. from 60 to 190°C preferably from 100 to 130°C, so that the resin viscosity reduces. This is most conveniently achieved by heating the resin and fibres, before impregnation, to the desired temperature, e.g. by passing them through an infra-red heater. Following impregnation there is typically a cooling step, to reduce the tackiness of the formed prepreg. This cooling step can be used to identify the end of the impregnation stage. This may be followed by further treatment stages such as laminating, slitting and separating. Once prepared the prepreg may be rolled-up so that it can be stored for a period of time. It can then be unrolled and cut as desired. Once the prepregs are produced by the process of the present invention, a plurality of them are typically stacked together, producing a prepreg stack or preform. The prepreg stack or perform may then be cured by exposure to elevated temperature, wherein the thermosetting resin cures. This is typically carried out under elevated pressure in known manner such as the autoclave or vacuum bag techniques. Thus, in a fourth aspect, the invention relates to cured composite material, obtainable by the process of exposing a prepreg or prepreg stack as described herein, to elevated temperature and optionally elevated pressure, to thermally set the thermosetting resin and thereby produce the cured composite material. The invention will now be illustrated, with reference to the following figures, in which: Figure 1 is a side sectional view through a portion of prepreg according to the present invention. Figure 2 is a side sectional view through a portion of a prepreg stack according to the present invention. Figure 3 is side sectional view through a portion of a cured prepreg stack according to the present invention. Figure 4 is a side schematic view of a process for forming a prepreg according to the present invention. Figure 5 is an image of an apparatus carrying out the process illustrated in figure 4. Figure 6 is a microscopy image side sectional view through a cured composite laminate according to the present invention. Figure 7 is a microscopy image side sectional view through another cured composite laminate according to the present invention. Figure 8 is a microscopy image side sectional view through another cured composite laminate according to the present invention. Figure 9 is a microscopy image side sectional view through another cured composite laminate according to the present invention. Turning to the figures, figure 1 shows a prepreg 10 according to the present invention comprising a structural layer of unidirectional carbon fibres 12 having a first outer face 20 defining an x-y plane, and a second outer face 22, also defining a parallel x-y plane, with the unidirectional fibres aligned in the x-direction. The structural layer 12 comprises resin impregnated and present within the interstices between the fibres. Placed on top of the structural layer 12 are three strips 14, providing raised regions, each strip 14 comprising a plurality of electrically conductive fibres that are oriented in the y-direction (i.e. into the page). Each strip has a raised thickness, h, extending out from the first outer face in the z-direction and a width, t, in the x-direction, are parallel to each other and are spaced apart by a distance s. In the examples below, t = 2 mm and h = 120 to 140 pm. The spacings, s, were varied in the range: 3.3 <s <20 mm. Figure 2 shows a prepreg stack 30 according to the present invention, comprising two structural layers of unidirectional carbon fibres 12 with the unidirectional fibres aligned in the x-direction. The structural layers 12 comprise resin impregnated and present within the interstices between the fibres. Placed on top of each structural layer 12 are three strips 14, providing raised regions, each strip 14 comprising a plurality of electrically conductive fibres that are oriented in the y-direction (i.e. into the page). Placed on top of the stack is a further structural layer 13, which is identical to a structural layer 12, to provide a flat surface to the stack 30. It will be noted that the stack also comprises open regions 17, where no material is present. The prepreg stack 30 is then exposed to elevated pressure on its outer surfaces, and elevated temperature, together causing a reduction in the viscosity of the thermosetting resin and a rearrangement of the structural fibres. The thermosetting resin cures and hardens, fixing the cured structure in place. This results in the cured composite material 50 shown in figure 3. It is noticeable that there has been a reduction in the overall thickness of the prepreg stack in the z-direction. The lowermost structural layer 12 has become a cured structural layer 52, with very little change in its physical dimensions. Likewise, the further structural layer 13, has become a further cured structural layer 53, also with very little change in its physical dimensions. However, the central structural layer 12 has radically changed its dimensions to an undulating cured structural layer 55. Additionally, the raised strips 14 have become significantly flattened cured strips 54. As the pressure is applied to the outer surfaces, the raised strips 14 experience an increased pressure, as it is concentrated through their reduced surface area. This acts to compress the strips, resulting in their spreading to the sides, as shown in figure 3. The pressure acting through the strips is transmitted to the central structural layer 12, causing its undulating profile in its final cured structural layer 55 form. The regions of no material 17 allow for the overall compression and spreading to take place, and become far smaller regions of no material 57 following curing, and may even vanish completely. Figure 4 is a side schematic view of a process for forming a prepreg according to the present invention. A continuous stream of unidirectional electrically conductive fibres 102 is passed over a sequence of rollers comprising spreader bars 110 providing a spreader zone and compaction rollers 120 in a compaction zone. The impregnated fibres then pass to a cooling zone 130 so the resin hardens in place, and then passes to a traction zone 140. Additionally, a number of strips of unidirectional electrically conductive fibres are introduced onto the top surface of the carbon fibres, either at location 1 or location 2. The prepreg produced 104 is according to the present invention can then be rolled up and stored for later use. Examples Lab scale examples A prepreg stack having the arrangement shown in figure 2 was prepared. The structural layers were prepared by taking commercially available prepregs with a structural layer of unidirectional IMA-12k (available from Hexcel) carbon fibre with an areal weight of 268 gsm. The resin comprised trifunctional epoxy resin, bisphenol-F epoxy and 4,4’ DDS curing agent (selected as a particularly electrically insulating resin system) in an amount of 34% (M21EV / 34% / 268 / IMA, available from Hexcel). This impregnates the structural layer and leaves a resin layer on top including particles. These were selected because they are known to have low through-thickness conductivity when formed into a stack and cured. Strips of prepreg with width, t, and spacing, s; were laid onto the regular prepreg plies in the arrangement shown in figure 1. The resin material used for the strips did not include any particles. Commercially available prepreg 8552 / 34% / 134 / IMA (available from Hexcel) was selected. The layup used, as shown in figure 2, was 5:[0a / 90b / 0a / 90b / 0a] where (a) and (b) refer to M21EV and 8552 materials respectively. The area fraction (Af) is defined as: In these examples, the width, t, was kept constant at 2mm, whilst the separation, s, was varied. The prepreg stacks were cured by autoclave. The laminates were vacuum bagged under 1 bar of vacuum pressure. The peak positive air pressure in the autoclave was 7 bar and applied when the temperature reached 60 °C and the hold temperature was 180 °C for 120 minutes. The heating and cooling ramp rates were 2 °C.min’1. To test conductivity, cured samples with a dimension of 40 by 40mm are produced. The outer surfaces are sanded to remove resin and expose the structural fibres. This is metalised to provide a highly conductive contact to the electrodes. A constant current (1 Amp) is maintained and the voltage measured and this allows calculation of the resistance. The results are shown below in Table 1. Table 1 Sample Area Fraction Conductivity (S / m) Normalised Conductivity (S / m) 0 17.9 0 0.1 20.5 33.7 0.2 24.7 47.3 0.4 46.5 87.8 0.6 33.5 43.1 1 22.8 22.8 Notably, the z-direction conductivity rises and reaches a maximum for intermediate area fractions between 0 and 1.0. This suggests an interaction effect between two factors, with a synergistic maximum around 0.3-0.5 area fraction. The strips concentrate the pressure experienced by the laminate during curing in an autoclave. The elevated pressure reduces the contact resistance as the fibres in the strip are more intimately connected with fibres in the main ply, forming a conductive bridge that spans the interlayer. It is believed that this peak arises from two interrelated factors: a pressure effect, and the area of the conductive strip. As the area fraction reduces, the pressure experienced by the strips increases, and so this acts to induce a more intimate contact between the electrically conductive fibres in the region of the strips, increasing the electrical conductivity. Conversely, as the area fraction increases, the pressure experienced by the strips decreases, however the pressure experienced by the strips and thus the conductivity imparted to the strips transfers to a greater proportion of the underlying structural layer. At very low levels of area fraction, the high local conductivity experienced by the strips does not translate into high global conductivity in the structural layer, being limited by the available strip area. At very high levels of area fraction the local conductivity in the strips that are translated to the structural layer are no higher than that would be experienced with no strips at all being present. It therefore seems that an optimal condition that maximises overall conductivity, exists when the strips cover around half of the surface, so that a meaningful increase in pressure is developed, which is translated over a substantial portion of the structural layer. The normalised conductivity is an estimate the local strip conductivity. We make this estimate by subtracting the baseline (weighted by the area fraction) and then dividing by the area fraction. _ _ ^total &base(^ ^strip 1 There appears to be a further factor involved, the response of the strips to pressure results in a morphological change acting to broaden the strip and diminish its height, such that the strip geometry between the uncured (Fig.2) and cured prepreg stacks (Fig.3) evolves during the cure process. The initial strip geometry is drives the conditions to give the resultant properties. Pilot Plant Examples Prepregs according to the present invention were manufactured at pilot plant scale, in an apparatus 100 as shown in figure 4. An image of the actual process in use is shown in figure 5 with the strips of fibres being added at insertion point 2. This involves passing a continuous supply of unidirectional carbon fibres over spreader bars 110, before passing them over a series of heated compaction rollers 120. At this stage the layer of unidirectional continuous carbon fibres is impregnated with an upper resin on a sheet on backing paper and a lower resin on a sheet also on backing paper. The three layers were brought together by passing over and between heated compression rollers 120, in order for the resin to impregnate the carbon fibres into the interstices between the fibres. The prepreg then passes to a cold table 130 to cool the material and finally to a laminator 140. The resin comprised trifunctional epoxy resin, bisphenol-F epoxy and 4,4’ DDS curing agent (selected as a particularly electrically insulating resin system) and the fibre was unidirectional IMA-12k (available from Hexcel) carbon fibre with an areal weight of 134 gsm. The nominal cured ply thickness was 125pm. In addition, strips of carbon fibre are introduced to the upper surface of the prepreg, either at position 1 (at the end of the spreader bars 110) or at position 2 (after leaving the cold table 140). Also, two types of strips were added, either a ‘normal’ unidirectional type (designed as ‘N’), as shown in figure 1, or a twisted type, where the fibres are still aligned in the y-direction but are twisted around the y-axis (designated as T). As a comparative example, regular plies of a material M21EV / 34% / 134 / IMA (available from Hexcel) with particles in the interleaf was selected, with no additional strips. In these examples, the separation, s, was kept constant at 20mm, whilst the width, t, was varied. Prepreg stacks were formed with the laminate sequence 24:[0 / 90]6s and were cured by autoclave with the same conditions as for the lab examples. To test conductivity, cured samples with a dimension of 40 by 40mm with a 0 / 90 layup are produced. The outer surfaces are sanded to remove resin and expose the structural fibres. This is metalised to provide a highly conductive contact to the electrodes. A constant current (1 Amp) is maintained and the voltage measured and this allows calculation of the resistance. The results are shown below in table 2. Table 2 Sample Area Fraction Twisted? Position Conductivity (S / m) Normalised Conductivity (S / m) 0 - - 37.0 - 0.14 T 2 91.9 390.4 0.17 T 1 68.5 186.3 0.36 N 2 95.0 162.5 0.43 N 1 154.3 271.8 Although the examples only cover area fractions up to 0.43, it is striking that the results obtained at the lab scale are repeated in the pilot plant scale. As the examples had fairly low area fractions, the area effect seems to be the limiting factor rather than the pressure effect. Microscopy images through cross-sections of the cured samples are provided in Figure 6 (N1), figure 7 (N2), figure 8 (T1) and figure 9 (T2). In the top half of each figures is the original microscopy image post curing. Consecutive layers of 0° (A) and 90° (B) plies can be seen. The bottom half is the same image but with everything but the fibres coming out of the plane of the page masked out. A false colour was then applied to differentiate the IM5 fibres (C) from the IMA fibres (B). carried out by image analysis based on the different diameters of each fibre type. It will be seen that N1 and N2 result in a wider final tow width in the prepreg, assumed to be due to the lack of constraint imposed by twisting. N1 is wider than N2 as this has also passed through the compactor stage 120 which has aided its spreading. This is further evidenced by the shape of the surface tows with N1 and N2 being noticeably flatter in cross-section. A further difference between N1 and N2 is that N1 was laid on top of the fibre bed prior to the (insulative) resin film being applied, perhaps allowing for better electrical contact. It will be noted that the additional strips that are twisted are significantly more pronounced and cause deformation of the structural layer to a significant extent. Typical z-conductivities for the fibre bed with no interlayers is in the range of 200-500 S / m. N1, N2 and T1 achieve relatively similar values at the bottom of this range. T2 is significantly higher. The primary contribution is thought to be from the large difference in heights (for example compare Figures 6 to 8 with Figure 9). It is anticipated that this large height would act to increase the pressure effect. The additional conductivity seen in N1 vs N2 may be attributed to the aforementioned effect of the strips having an increased thickness and thus increased width, t. Mechanical Testing Mechanical testing of the N1 prepreg formed in the pilot plant trials was carried out, in comparison to known prepregs without additional strips applied. Unidirectional laminates were cured, and specimens manufactured according to BS EN2561. Ultimate tensile strength (UTS) reference data from panels cured of standard prepreg manufactured using IMA carbon fibre and IM5 carbon fibre were measured. Data from ribbed prepreg variant N1 has also been presented. N1 has base IMA fibres with additional IM5 fibres placed onto the prepreg in a manufacturing method as described above in the pilot plant manufacturing. The results are shown below in table 3. Table 3 UTS Data Prepreg IMA Fibre Prepreg IM5 Fibre Ribbed Prepreg N1 Mean UTS [MPa] 3173 3199 3321 2929 2924 2997 3038 Standard Deviation [MPa] 77 121 115 - - - 155 IMA is a carbon fibre with a filament diameter of 5 pm and IM5 a carbon fibre with a filament diameter of 7 pm. Additional fibres of different filament diameters were placed onto the base fibre during the manufacturing process to aid the identification of the structure created during the manufacture of the conductive panels. However, fibres of other filament diameters could be effective as additional fibres. The UTS data for the N1 prepreg lies within the range between known comparative IMA and IM5 prepreg results. This provides evidence that there is no measurable change in UTS of the product, N1 according to the present invention, when compared to reference prepregs. Thus, the addition of the strips to improve through-thickness 5 laminate conductivity, does not come at a detriment to mechanical performance.

Claims

1. A prepreg comprising a structural layer of electrically conductive fibres having interstices therebetween, the structural layer having a first outer face and an essentially parallel second outer face, the faces each defining an x-y plane separated from each other by a distance equal to the thickness of the structural layer in a z-direction, orthogonal to the x-y planes; the electrically conductive fibres being parallel to the x-y planes;the prepreg comprising resin impregnated within the structural layer and present within the interstices between the fibres;wherein the first outer face comprises a plurality of raised regions, each raised region comprising a plurality of electrically conductive fibres that are oriented with the x-y planes, and have a raised thickness extending out from the first outer face in the z-direction.

2. A prepreg according to claim 1, which comprises a first layer of thermosetting resin in an x-y plane and in contact with the first and / or second outer face of the structural layer having a thickness in the z-direction.

3. A prepreg according to claim 1 or claim 2, wherein the resin is a curable resin comprising thermosetting resin.

4. A prepreg according to any one of the preceding claims, wherein the raised regions are parallel longitudinal raised strips having a defined width in the x-y plane, a defined length in the x-y plane, and comprising a bundle of electrically conductive fibres, aligned with the length of the raised strips.

5. A prepreg according to claim 4, wherein the bundle of electrically conductive fibres are unidirectional whose direction is aligned with the length of the raised strips.

6. A prepreg according to claim 4 or claim 5, wherein the parallel longitudinal strips are evenly spaced apart.

7. A prepreg according to claim 4 or claim 5, wherein the strips cover from 10% to 90%, preferably from 20 to 80%, of the surface of the first outer face.

8. A prepreg according to any one of the preceding claims, wherein the average raised thickness of the raised regions is from 10 to 100% of the thickness of the structural layer.

9. A prepreg according to any one of claims 4 to 7, which is essentially rectangular in the x-y planes with a length in an x-y plane much greater than a width in an x-y plane, and wherein the raised strips run parallel with the length of the prepreg.

10. A prepreg according to any one of the preceding claims, wherein the electrically conductive fibres in the structural layer are unidirectional.

11. A prepreg according to claims 5 and claim 10, wherein the fibres in the raised regions are aligned with the fibres in the structural layer.

12. A prepreg according to any one of the preceding claims, wherein the electrically conductive fibres are selected from the list consisting of carbon fibres, metalised glass fibres, graphite fibres, metal-coated fibres, metallised polymers and mixtures thereof, preferably carbon fibres.

13. A stack of a plurality of prepregs according to any one of the preceding claims.

14. A stack according to claim 13, wherein the prepregs are according to claim 4, whereinthe strips from one of two adjacent prepregs in contact in the stack are positioned in between the raised strips of the second of the two adjacent prepregs in contact in the stack.

15. A stack according to claim 13 or claim 14, wherein the structural layers of electrically conductive fibres are unidirectional and adjacent prepregs are in a 0 / 90 arrangement.

16. A process for the manufacture of a prepreg according to any one of claims 10 to 12, the process comprising the steps of feeding a continuous stream of unidirectional electrically conductive fibres over a sequence of rollers, to produce the structural layer, and wherein an additional stream of unidirectional electrically conductive fibres is passed over a sequence of rollers to produce the raised strips, and wherein thermosetting resin is impregnated within the structural layer and present within the interstices between the structural fibres.

17. A process according to claim 16, wherein the first face of the structural layer is brought into contact with a first resin impregnating layer; compressing the structural layer and first impregnation layer together so that curable resin impregnates the fibrous layer so that it is present between the interstices between the fibres.

18. A cured composite material, obtainable by the process of exposing a prepreg or prepreg stack according to any one of claims 1 to 15 to elevated temperature and optionally elevated pressure, to thermally set the thermosetting resin and thereby produce the cured composite material.s

Images