GB2634769A

GB2634769A - A prepreg, a process for its manufacture and composite materials made therefrom

Info

Publication number: GB2634769A
Application number: GB2316042.7A
Authority: GB
Inventors: Russell Benjamin; Ramsdale-Capper Roderick
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2023-10-20
Filing date: 2023-10-20
Publication date: 2025-04-23
Also published as: GB202316042D0; WO2025083404A1

Abstract

A prepreg comprising a resin impregnated structural layer of electrically conductive fibres 22 having interstices therebetween parallel to an x-y plane defined by the outer faces of the structural layer. The faces are separated from each other in a z-direction, orthogonal to the x-y plane. The prepreg also includes a first layer of resin in an x-y plane and in contact with a first outer face of the structural layer. Some of the electrically conductive fibre 22 ends deviate from being parallel to the x-y planes, extend out of the structural layer and into the z-direction, the deviating fibres 30 entering and passing through essentially the entire thickness of the first layer of resin. The fibres 22, 30 are preferably metallised glass or polymeric fibres or especially carbon fibres. The method of producing the prepreg and cured composite formed from it are also claimed. They may be used in sport, leisure, industrial and aerospace applications.

Description

A Prepreg, a Process for its Manufacture and Composite Materials made therefrom

Technical Field

The present invention relates to a prepreg with improved electrical and/or mechanical 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 Al 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 Al discloses the use of glassy carbon particles in an interlayer having a maximum thickness of 50pm.

WO 2013/186389 Al and WO 2015/157486 Al teach that z-direction conductivity can be improved by the addition of potato-shaped graphite in the interleaf layer.

WO 2016/048885 Al 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 plane; 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; and a first layer of 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; wherein a number of electrically conductive fibre ends deviate from being parallel to the x-y planes, extend out of the structural layer and into the z-direction, the deviating fibres entering and passing through essentially the entire thickness of the first layer of resin.

The present inventors have found that if a number of electrically conductive structural fibres are redirected to leave the structural layer in the x-y plane and enter the first layer of thermosetting resin adopting a degree of z-directionality, such redirected fibres can form a clear and effective electrical contact across the insulating resin layer. As such fibres also remain connected to the structural layer from which they are redirected, the electrical contact across the resin layer is effectively transmitted to the electrically conductive structural layer, providing effective dissipation of any electrical charge throughout the prepreg and any eventual structure made therefrom.

Preferably the resin is a curable resin comprising thermosetting resin and the prepreg is a curable prepreg.

The first outer resin layer is a single contiguous phase and typically has a thickness of from 10 to 100pm.

Preferably the deviating electrically conductive fibres ends protrude through the thickness of the first layer of thermosetting resin. In this way, the ends may become pressed back into the thermosetting resin layer when the prepreg is laid on to another such prepreg, producing high quality electrical bridging across the resin layer.

Preferably the deviated fibre ends are not randomly located but are arranged in a regular pattern, such a rectangular array. In particular it is highly preferred that the deviated fibres create rows having a continuous electrical pathway in the x-y plane, due to the placement and contact between the deviated fibres.

Preferably the spacing between such rows is regular, and may for instance be an array of rows with a separation distance of from 1 to 10 mm in the x-y plane.

In one preferred embodiment the electrically conductive fibre ends that deviate from being parallel to the x-y plane are formed as bundles of adjacent fibres. Thus, the majority of the fibres remain in the structural layer in the x-y plane and a select proportion of bundles of cut ends of adjacent fibres deviate out of the x-y plane and have some z-directional alignment, passing through the resin layer to provide an electrically conductive pathway in the z-direction.

Such bundles may form regular rows made up of bundles positioned end-to-end provide a continuous electrical pathway in the x-y plane.

In another preferred embodiment, the deviating electrically conductive fibres ends are arranged as randomly arranged ends, that nevertheless are grouped in a regular pattern. For example, the randomly arranged cut fibre ends may form the continuous electrical pathways in the x-y plane.

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 and the rows providing a continuous electrical pathway are at an angle to the direction of the fibres in the structural layer. Preferably the rows providing a continuous electrical pathway are at an angle of from 30 to 90° to the direction of 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 HexTowTm carbon fibres, which are available from Hexcel Corporation. Suitable HexTowTm 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; 1M8-1M10 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.

The resin preferably comprises a thermosetting resin and may be selected from those conventionally known in the art, such as resins of phenol formaldehyde, urea-formaldehyde, 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 be 'single sided', in that the prepregs 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; wherein a number of electrically conductive fibre ends deviate from being parallel to the x-y planes, extend out of the structural layer and into the z-direction, the deviating fibres entering and passing through essentially the entire thickness of the first layer of resin. Such 'single sided' are so-called because only one side has manipulated electrically conductive fibre ends that deviate from being parallel to the x-y planes. Thus, 'single sided' prepregs could include an embodiment with 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 manipulated electrically conducted fibres extending out of the structural layer and into the z-direction to enter and pass through essentially the entire thickness of the second layer of thermosetting resin.

Moreover, preferred prepregs may be 'double-sided' in that the prepregs preferably 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; wherein a number of electrically conductive fibres deviate from being parallel to the x-y planes, extending out of the structural layer and into the z-direction to enter and pass through essentially the entire thickness of the second layer of thermosetting resin.

When such 'double-sided' prepregs are stacked together, as discussed below, two outer resin layers form an interleaf layer of resin in the prepreg stack.

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 alternating structural layers of electrically conductive fibres interleafed with layers of thermosetting resin. However, due to the deviating fibre ends creating electrical conductivity pathways through the interleaf layers and transmitting it to the x-y plane in the structural layers of electrically conductive fibres, a significant improvement in z-directional conductivity results.

In a particularly preferred embodiment, the prepregs are 'double-sided' and in each of the contacting layers of thermosetting resin from two adjacent prepregs, are rows providing a continuous electrical pathway that are at an angle to each other. Preferably the rows with a continuous electrical pathway in adjacent and contacting thermosetting resin layers are at an angle of from 30 to 90° to each other.

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 fibres in one prepreg are perpendicular to those in the adjacent prepreg. The to rows providing a continuous pathway that are in contact with each other may 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. Thus, the rows providing a continuous pathway that are in contact with each other provide an array of additional highly electrically conductive z-direction bridging across the interleaf thermosetting resin layer at the points of crossover of the rows. When the prepreg stacks are formed, pressure applied to these crossover points provides an additional and 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 rows providing a continuous pathway that are in contact with each other may be angled accordingly to provide effective contact between the rows.

The curable prepregs of the present invention are preferably prepared by manufacturing a precursor prepreg, which is only subsequently manipulated to have the feature of the electrically conductive fibres to deviate from being parallel to the x-y planes.

Thus, in a third aspect, the invention relates to process for the manufacture of a prepreg as described herein, the process comprising the steps of forming a precursor 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 plane; the electrically conductive fibres being parallel to the x-y planes; the precursor prepreg comprising thermosetting resin impregnated within the structural layer and present within the interstices between the fibres; and 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; followed by manipulating a number of electrically conductive fibres to deviate from being parallel to the x-y planes, so that fibre ends extend out of the structural layer and into the z-direction to enter and pass through essentially the entire thickness of the first layer of thermosetting resin, to form a curable prepreg according to the present invention.

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 precursor prepreg manufacture preferably comprising the steps of: providing a structural layer comprising fibres, having a first face and a second face, and a first impregnating layer comprising curable thermosetting resin; bringing the first face of the structural layer into contact with the first impregnating layer; compressing the structural layer and first impregnation layer together so that curable resin impregnates the structural layer so that it is present between the interstices between the fibres, thereby forming the precursor prepreg.

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.

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 150°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 the precursor prepreg has been prepared, the manipulating may involve cutting a bundle of fibres to produce bundles of parallel adjacent fibres. Preferably these bundles are of a similar length and are geometrically arranged, so that the manipulation can produce regular rows made up of bundles positioned end-to-end providing a continuous electrical pathway in the x-y plane.

Alternatively the manipulating may involve cutting a bundle of fibres to produce randomly arranged ends. For example, the randomly arranged ends are produced by scoring the first outer face of the structural layer to cut the fibres and randomly orient the ends in the z-direction.

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 perspective schematic illustration of a portion of prepreg according to the present invention.

Figure 2 is an image of the surface of a portion of a prepreg according to the present invention.

Figure 3 is an image of the surface of a portion of another prepreg according to the present invention.

Figure 4 is a schematic illustration of the interface between two prepregs according to the present invention in a prepreg stack according to the present invention, showing crossing rows providing a continuous electrical pathway.

Figure 4a is a microscopy image through a cross section of a prepreg stack according to the present invention including a row providing a continuous electrical pathway.

Figure 4b is a microscopy image through a cross section of another prepreg stack according to the present invention including the crossover point between two adjacent continuous electrical pathways.

Figure 5 is a microscopy image through a cross section of another prepreg stack according to the present invention Figure 6 is a microscopy image through a cross section of another prepreg stack 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 defining an x-y plane with the unidirectional fibres aligned in the x-direction. A first layer of thermosetting resin 14 is in an x-y plane and in contact with the first outer face of the structural layer.

A bundle of carbon fibres 16 containing n adjacent filaments and length L has been cut and manipulated so that their ends deviate from the x-y plane so that it can be repositioned at an angle 0 to the x-direction and placed onto the surface 14 of the prepreg. Thus, the fibres in the bundle extend out of the structural layer and into the z-direction (and then repositioned back into an x-y plane above the prepreg), the deviating fibres thus entering and passing through essentially the entire thickness of the first layer of thermosetting resin, passing through it and then laying on top of the prepreg in order to produce the beginnings of a row providing an continuous electrical pathway that is at an angle 0 to the x-direction.

A microscopic image of where this has been done with a value of G of 90° is shown in figure 3. In this case, rows providing a continuous electrical pathway that is in the y-direction are provided, discussed below in example 2.

Examples

Precursor Prepreg Manufacture Precursor prepregs were manufactured by bringing together a layer of unidirectional continuous carbon fibres with an upper resin sheet on backing paper and a lower resin sheet also on backing paper. The three layers were brought together by passing over and between heated rollers, in known manner, in order for the resin to impregnate the carbon fibres into the interstices between the fibres. 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 268 gsm. The nominal cured ply thickness was 250pm.

Example 1 -Scored tufted tracks In a first set of examples, curable prepregs according to the invention were made by manipulating fibres in the precursor prepregs by producing scored tufted tracks.

One or both of the external surface of thermosetting resin was scored with a series of parallel scores, at an angle of about 45° to the unidirectional fibres using an automated ply cutting machine (ZUND L-1200cv Digital Flatbed Cutter available from Zund Systemtechnik AG, Switzerland), in order to penetrate the outer layer of resin and to cut through and disturb a number of unidirectional structural fibres, so that a number of them are redirected in the z-direction.

By microscopic analysis it is estimated that one-third of the thickness of the structural layer was cut and/or disturbed by the cutting machine.

When both surfaces of resin were scored, when stacked the mating faces had tracks that ran orthogonally to each other to form a criss-cross arrangement.

An image of the surface of a prepreg according to the invention made in this manner is shown in figure 2. This shows a prepreg 10 comprising a structural layer of unidirectional carbon fibres 12 defining an x-y plane with the unidirectional fibres aligned in the x-direction. A first layer of thermosetting resin 14 is in an x-y plane and in contact with the first outer face of the structural layer. Also shown are a plurality of parallel rows 18 providing continuous electrical pathways that are at an angle 45° to the x-direction.

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 1

Table 1

Example Ply Spacing Sidedness Conductivity Absolute Relative Sequence (S) [mm] Mean (Std. Benefit Benefit (%) Dev.) [S/m] [S/m] A 3:[0/90/0] 29.4 (2.4) 1 3:[0/90/0] 5 Double 145 (2.8) 115.6 393 2 3:[0/90/0] 10 Double 62.5 (2.1) 33.1 113 3 3:[0/90/0] 15 Double 75.7 (5.8) 46.3 157 4 3:[0/90/0] 10 Single 45.0 (2.0) 15.6 53 B 6:[0/90]2s - - 9.9 (0.2) - - 6:[0190]2s 10 Double 50.3 (1.9) 40.4 408 C 12:[0/90]3s - - 5.5 (0.5) - - 6 12:[0190]3s 10 Double 24.0 (0.6) 18.5 336 Figure 4a shows a microscopic image through a cross section of a prepreg stack including a row providing a continuous electrical pathway, taken through slice A of figure 4. The image clearly shows two structural layers of unidirectional fibres 20 extending into the page, and a structural layer of unidirectional fibres 22 being orthogonal to the layers 20 and extending parallel with the page. Sandwiched between these structural layers are interleaf layers of thermosetting resin 24. Also shown is a region 26 where unidirectional fibres have been cut and have deviated from the x-y plane and entered the z-direction and their ends are now located within the resin interleaf layers 24. It can be seen that they provide an effective electrical bridge across the resin interleaf layer 24.

Figure 4b shows a microscopic image through a cross section of the same prepreg stack including a row providing a continuous electrical pathway, taken through slice B of figure 4. The image clearly shows three structural layers of unidirectional fibres 20 extending into the page, and three structural layer of unidirectional fibres 22 being orthogonal to the layers 20 and extending parallel with the page. Sandwiched between these structural layers are interleaf layers of thermosetting resin 24. Also shown is two regions 26 where unidirectional fibres have been cut and have deviated from the x-y plane in adjacent structural layers of carbon fibres and entered the z-direction and their ends are located within the resin interleaf layers 24. It can be seen that they provide an effective electrical bridge across the resin interleaf layer 24.

Further microscopic images through a cross-section of the plies of a prepreg stack according to the present invention were taken and samples are shown in figures 5 and 6. These images show a layer of unidirectional fibres 20 perpendicular to the plane of the image, a structural layer of unidirectional fibres 22 being orthogonal to the layers 20 and extending parallel with the page. Sandwiched between these structural layers is an interleaf layer of thermosetting resin 24. Also clearly shown are fibres 30 that deviate from the structural layers that are aligned in the z-direction direction.

Example 2 -Unipennate Angle Bundles In a second set of examples, curable prepregs according to the invention were made by manipulating fibres in the precursor prepregs by producing regular arrays of unipennate bundles of fibres.

Select neat cuts were made in adjacent groupings of fibres to generate a ligament (i.e. bundle) of fibres approximately 2mm in width and 20mm in length. The cut ends were lifted up with tweezers and laid down perpendicularly to the direction of the unidirectional fibres in the structural layer. The cuts were made in a rectangular repeating pattern, as shown in figure 3. Due to the length, L being just the same as the distance between the cuts, this produced continuous lines of electrical connection in the y-direction separated by spacing, S, due to the positioning of the cuts.

In the single sided configuration, the manipulated surface is mated with an unmanipulated one. When both surfaces of fibres were cut and replaced, when stacked the mating faces had tracks that ran orthogonally to each other.

An image of the surface of a prepreg according to the invention made in this manner is shown in figure 3. This shows a prepreg 10 comprising a structural layer of unidirectional carbon fibres 12 defining an x-y plane with the unidirectional fibres aligned in the x-direction. A first layer of thermosetting resin 14 is in an x-y plane and in contact with the first outer face of the structural layer. Also shown are a plurality of parallel rows 18 providing continuous electrical pathways that are at an angle 90° to the x-direction.

Four repeats were done for each test configuration, and the results are shown below in table 2.

Table 2

Example Ply Spacing Ligament Sidednes Conductivit Absolute Relative Sequence (S) [mm] length (L) s y Mean (Std. Benefit Benefit [mm] Dev.) [S/m] [S/m] (%) D 3:[0/90/0] - - - 29.4 (2.4) - - 7 3:[0/90/0] 10 20 Single 40.8 (2.6) 11.3 39 8 3:[0/90/0] 10 20 Double 50.1 (1.3) 20.7 70 Example 3 -Mechanical Testing To investigate the effects of the breaking and tufting of fibres as carried out in example 1, mechanical tests were performed on cured plies of prepregs with and without the scored tufted tracks.

The testing was carried out according to BS 7991:2001, with a geometry length of 250mm, width 25 mm and thickness 3mm, with an opening rate of 10mm/min.

The procedure was repeated twelve times and an average taken. The results were very highly clustered and the G1c value measured for the unscored prepregs was 303 J/m2 with the G1c measured for the scored tufted prepregs being 476 J/m2, showing that the prepreg according to the invention had a 47% improvement.

Claims

Claims 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 plane; 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; and 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; wherein a number of electrically conductive fibre ends deviate from being parallel to the x-y planes, extend out of the structural layer and into the z-direction, the deviating fibres entering and passing through essentially the entire thickness of the first layer of resin.
2. A prepreg according to claim 1, wherein the resin is a curable resin comprising thermosetting resin.
3. A prepreg according to claim 1 or claim 2, wherein the deviating electrically conductive fibres ends protrude through the thickness of the first layer of thermosetting resin.
4. A prepreg according to any one of the preceding claims, wherein the deviating electrically conductive fibre ends form rows providing a continuous electrical pathway in the x-y plane.
5. A prepreg according to claim 4, wherein the spacing between the rows is from 1 to 10mm.
6. A prepreg according to any one of the preceding claims, wherein the deviating electrically conductive fibres ends are arranged as bundles of parallel adjacent fibres.
7. A prepreg according to claim 4 and claim 6, wherein regular rows made up of bundles positioned end-to-end provide a continuous electrical pathway in the x-y plane.
8. A prepreg according to any one of claims 1 to 5, wherein the deviating electrically conductive fibres ends are arranged as randomly arranged ends.
9. A prepreg according to claim 4 and claim 8, wherein randomly arranged cut fibre ends form the continuous electrical pathway in the x-y plane.
10. A prepreg according to any one of the preceding claims, wherein the electrically conductive fibres in the structural layer are discrete and not interwoven.
11. A prepreg according to claim 6, wherein the electrically conductive fibres are unidirectional.
12. A prepreg according to claim 4 and claim 11, wherein the rows with a continuous electrical pathway are at an angle to the direction of the fibres in the structural layer.
13. A prepreg according to claim 12, wherein the rows with a continuous electrical pathway are at an angle of from 30 to 90° to the direction of the fibres in the structural layer.
14. A prepreg according to any one of the preceding claims, which comprises a second layer of 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; wherein a number of electrically conductive fibre ends deviate from being parallel to the x-y planes, extending out of the structural layer and into the z-direction to enter and pass through essentially the entire thickness of the second layer of resin.
15. 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.
16. A stack of a plurality of prepregs according to any one of the preceding claims.
17. A stack according to claim 16, wherein the prepregs are according to claim 4, wherein the rows providing a continuous electrical pathway in the x-y plane from two adjacent prepregs in contact in the stack are at an angle to each other.
18. A process for the manufacture of a prepreg according to any one of the claims 1 to 14, the process comprising the steps of forming a precursor 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 plane; the electrically conductive fibres being parallel to the x-y planes; the precursor prepreg comprising thermosetting resin impregnated within the structural layer and present within the interstices between the fibres; and 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; followed by manipulating a number of electrically conductive fibres so that their ends deviate from being parallel to the x-y planes, so that fibre ends extend out of the structural layer and into the z-direction to enter and pass through essentially the entire thickness of the first layer of thermosetting resin, to form a curable prepreg according to any one of claims 1 to 15.
19. A process according to claim 18, wherein the precursor prepreg is manufacture by providing a structural layer comprising fibres, having a first face and a second face, and a first impregnating layer comprising curable thermosetting resin; bringing the first face of the structural layer into contact with the first 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, thereby forming the precursor prepreg.
20. A process according to claim 18 or claim 19, wherein a second impregnating layer comprising resin is provided, wherein the second face of the fibrous layer is brought into contact with the second impregnating layer prior to the compressing.
21. A process according to any one of claims 18 to 20, wherein the manipulating involves cutting a bundle of fibres to produce bundles of parallel adjacent fibres.
22. A process according to claim 21, wherein the bundles are of a similar length and are geometrically arranged, so that the manipulation can produce regular rows made up of bundles positioned end-to-end providing a continuous electrical pathway in the x-y plane.
23. A process according to any one of claims 18 to 20, wherein the manipulating involves cutting a bundle of fibres to produce randomly arranged ends.
24. A process according to claim 23, wherein the randomly arranged ends are produced by scoring the first outer face of the structural layer to cut the fibres and randomly orient the ends in the z-direction.
25. A cured composite material, obtainable by the process of exposing a prepreg or prepreg stack according to any one of claims 1 to 17 to elevated temperature and optionally elevated pressure, to thermally set the thermosetting resin and thereby produce the cured composite material.