Technical Field of the Invention

The present invention relates to a pre-impregnated composite, to a method of producing the pre-impregnated composite, to a composite article comprising the pre-impregnated composite and to a method of producing the composite article.

Background to the Invention

A composite is formed when two or more different materials interact. This interaction can be physical or chemical in nature. Nowadays, composite materials are used regularly in the aerospace, automotive, rail, marine, sports and construction industries since such materials typically exhibit a high strength to weight ratio. However, there is still a desire within these industries to reduce the weight of composites further in order to reduce operating costs associated with related fuel and handling requirements.

Research has shown that pre-impregnating carbon fibre materials with polymeric resins that comprise carbon nanotubes and/or carbon black is an effective approach for improving a composite’s strength to weight ratio and its conductivity. However, in recent years much research has focussed on producing carbon fibre materials that incorporate graphene since further improvements in the strength to weight ratio, conductivity, ductility and density can be obtained.

To date, the incorporation of graphene into polymeric systems has been achieved by (i) dispersing graphene flakes in a polymeric resin or (ii) chemical interaction between functional groups in graphene oxide or reduced graphene oxide and a corresponding hardener. In the first case where graphene flakes are dispersed in a resin, it has been found that individual graphene flakes are distal from each other, unfavourably aligned or agglomerate due to a lack of compatibility with the resin, whereas in the second case, it has been found that the residual oxide defects and smaller flake size suppressed the properties of the resulting composite. In light of the above, it is an object of embodiments of the present invention to provide a composite comprising graphene which, relative to existing composites that comprise graphene, exhibits improved mechanical and physical properties. It is another object of embodiments of the present invention to provide a composite that exhibits improved conductivity relative to existing composites that comprise graphene.

It is also an object of embodiments of the present invention to provide an improved method for incorporating graphene into a fibrous reinforcing material. Summary of the Invention

According to a first aspect of the invention there is provided a composite comprising a fibrous reinforcing material impregnated with a matrix material that comprises a resin, a hardener and graphene, wherein the resin or the hardener is an active hydrogen-containing component and graphene is functionalised with a dispersing agent and the active hydrogen-containing component. The dispersing agent may comprise any of the following functional groups: -NH2, -OH, -

0=C-NH, -(NH2)2 and -(NH2)3. It has been found that dispersing agents comprising amino, hydroxyl, carboamide, diamine and triamine functional groups are very suitable for reacting with the edge electrons of graphene and that improvements in the conductivity and mechanical properties of the corresponding composite articles can be obtained. The composite articles also exhibit reduced water permeability and hence improved durability.

The matrix material may comprise 0.1 to 10 wt% of the dispersing agent. In some embodiments the matrix material may comprise 0.1 to 5 wt% of the dispersing agent, while in other embodiments the content of the dispersing agent in the matrix material may be 0.1 to 2 wt%.

The matrix material may comprise a wetting agent. In particular, the matrix material may comprise 0.1 to 10 wt% of the wetting agent. In some embodiments the matrix material may comprise 0.1 to 5 wt% of the wetting agent. In other embodiments the matrix material may comprise 0.1 to 2 wt% of the wetting agent.

The dispersing agent and the wetting agent (if present) lowers the interfacial tension between the graphene and the liquid (solvent borne or water borne system) thereby making the graphene easier to disperse. The use of high molecular weight wetting and dispersing agents increases the stability of the dispersion whereas faster wetting is obtained through the use of lower molecular weight wetting and dispersing agents. The use of dispersing agents and wetting agents has also been found to positively influence the optical properties (colour development, hiding power and gloss) of the resulting composites and also improves the rheological performance of the matrix material before curing.

The dispersing agent and the wetting agent may be a copolymer. In particular, the dispersing and wetting agents may comprise a polyacrylate copolymer, a maleic anhydride copolymer, a styrene copolymer or a polyether copolymer.

The copolymer may comprise multiple pigment affinic groups which adsorb to the surface of graphene. In water borne systems, the dispersing and wetting agents are able to transfer an electrical charge via their pigment affinic groups to the graphene surface. This serves to reinforce the surface charge, but also results in the graphene flakes becoming equivalently charged which stabilises the system through electrostatic repulsion.

The copolymer may comprise resin affinic side chains. In water borne systems, the resin affinic side chains comprise long hydrophilic groups, e.g. polyether, which interact with water and the resin system as well as each other. The intermolecular steric hindrance of these side chains further mediates the dispersion stabilisation, as the repulsion prevents the flocculation of the graphene flakes.

The matrix material may comprise graphene that is free from oxides. In particular, the graphene may be pristine graphene, i.e. graphene which has not been reduced from graphene oxide since it is known that some graphene oxide remains after the reduction step which cannot be removed. When oxide-free graphene is used, the dispersing agent and the active hydrogen- containing component are able to react with sp ² electrons located at the graphene edges to form functionalised graphene. Moreover, the use of oxide-free graphene rather than graphene oxide (GO) or reduced graphene oxide (RGO), enables composite articles to be obtained that exhibit improved electrical conductivity, thermal conductivity and mechanical properties. In some embodiments the graphene may be substantially free from surface defects such as oxides. The outer surface of graphene may also be substantially free from functional groups. This promotes functionalisation at the edges rather than on the outer surface, which is preferred since it preserves the inherent properties of graphene, e.g. good mechanical strength and conductivity, when incorporated within the matrix material. Moreover, edge functionalisation rather than functionalisation of the graphene outer surface means that the graphene surface is free or substantially free of stress risers which result in faster crack propagation. The matrix material may comprise graphene flakes. The graphene flakes may be in the form of graphene nano platelets (GNP) and/or few layer graphene (FLG). The graphene flakes may have a lateral size of 0.5 to 10 microns and in some embodiments the graphene flakes may have a lateral size of 2 to 7 microns. The graphene flakes may have a thickness of 1 to 25 nm and in some embodiments the flakes may have a thickness of 1 to 10 nm. The matrix material may comprise at least 0.1 wt% graphene. In one embodiment the matrix material may comprise 0.1 to 20 wt% graphene. In some embodiments the matrix material may comprise 1 to 20 wt% graphene, while in other embodiments the matrix material may comprise 1 to 10 wt% graphene or 1 to 5 wt% graphene. In some embodiments the matrix material may comprise 5 to 16 wt% graphene. Very good electrical percolation is obtained when the matrix material comprises more than 5 wt% graphene, e.g. 7 to 9 wt% graphene. This has been attributed to the pre-functionalisation of graphene with the dispersing agent and optionally the wetting agent which increases the spacing between graphene layers to maximise the availability of edges (reactive sites) for subsequent functionalisation with the active hydrogen-containing component.

The active hydrogen-containing component may be a hardener comprising any of the following functional groups: aromatic amides, cycloaliphatic amides, aliphatic amides, aromatic amines, cycloaliphatic amines, aliphatic amines, phenols, anhydrides, aromatic diamine, dimethylthiotoluenediamine (DMTDA) and thiols. These functional groups are very suitable for reacting with epoxy resins.

The epoxy resin may comprise multi-functional epoxy monomers. In particular, the epoxy resin may comprise bifunctional epoxy monomers, tri-functional epoxy monomers, tetra- functional epoxy monomers or a combination thereof. Examples of commercial epoxy resins that may be used in accordance with the present invention include, but are not limited to epoxy resins such as: Araldite®, Airstone®, Baxxodur®, Litestone®, Loctite®, Epikote®, Epanol®, Epon®, Epicron®, Epicron®, jER®, Epocast®, Epikotetm®, Epotek®, Epotec®, Cetepox®.

When the matrix material comprises an epoxy resin, the epoxy resin: hardener ratio is between 1 :1 and 5:1. In some embodiments, the resin: hardener ratio may be 1 :1 , 1 .5:1 , 2:1 , 2.5:1 , 3: 1 , 3.5:1 , 4: 1 , 4.5: 1 or 5: 1 , depending on the number of functional groups in the epoxy resin and hardener.

The active hydrogen-containing component may be a resin comprising hydroxyl functional groups, e.g., the active hydrogen-containing component may comprise a polyol resin, a phenol, formaldehyde resin or a glycidyl ether resin. In particular, the polyol resin may comprise any of the following, either alone or in combination: acrylic polyols, polyester based polyols and polyether based polyols such as polyethylene glycol, polypropylene glycol and poly (tetramethylene ether) glycol.

If a polyurethane composite is desired then the polyol resin may be reacted with an isocyanate hardener. The hardener may for instance comprise aliphatic isocyanates, aromatic isocyanates or a combination of aliphatic and aromatic isocyanates as desired or as appropriate. Examples of aromatic isocyanates that may be used in accordance with the present invention include: diisocyanates such as Toluene Diisocyanate (TDI) and Methylene diphenyl diisocyanate (MDI), whereas suitable aliphatic isocyanates include 1 ,6-hexamethylene diisocyanate (HDI), 1- isocyanato-3-isocyanatomethyl-3, 5, 5-trimethyl-cyclohexane (isophorone diisocyanate, IPDI), 4,4'- diisocyanato dicyclohexylmethane, (H12MDI and hydrogenated MDI).

If a polyester composite is desired, then the active hydrogen-containing component may be a polyol resin and the hardener may comprise a carboxylic acid. For instance, the hardener may comprise a dicarboxylic acid and the dicarboxylic acid may comprise any of the following, either alone or in combination: Oxalic Acid, Malonic acid, Succinic acid, Glutaric acid, Adipic acid, Pimelic acid, Suberic acid, Azelaic acid, Sebacic acid, Brassylic acid and Thapsic acid.

When the matrix material comprises a polyol resin and a hardener, the polyol resin: hardener ratio may be between 1 :1 and 5:1. In some embodiments, the polyol resin: hardener ratio may be 1 :1 , 1.5:1 , 2:1 , 2.5: 1 , 3:1 , 3.5: 1 , 4:1 , 4.5:1 or 5:1 , depending on the number of functional groups in the polyol resin and hardener.

The pre-impregnated composite may comprise a fibrous reinforcement material formed from carbon fibres, glass fibres, aramid fibres, textile fibres or any combination thereof. In some embodiments the fibrous reinforcement material may be formed from natural and/or synthetic fibres. The pre-impregnated composite may comprise a plurality of reinforcement material layers. Each reinforcement layer may be impregnated with the matrix material.

According to a second aspect of the invention there is provided a method of producing a pre-impregnated composite which comprises the step of impregnating a fibrous reinforcement material with a matrix material that comprises a resin, a hardener, a dispersing agent and graphene, wherein the resin or the hardener is an active hydrogen-containing component and the graphene is functionalised with the dispersing agent and the active hydrogen-containing component.

The method according to the second aspect of the invention can be used to produce the pre-impregnated composite of the first aspect of the invention. Accordingly, the method according to the second aspect of the invention may incorporate any or all features described in relation to the first aspect of the invention.

The method may comprise the step of pre-functionalising graphene with the dispersing agent, and optionally the wetting agent, and then reacting pre-functionalised graphene with the active hydrogen-containing component. It is understood that the step of pre-functionalising graphene with the dispersing agent helps prevent against the agglomeration of GNP and FLG and facilitates the subsequent functionalisation of graphene with the active hydrogen-containing component. Moreover, by functionalising graphene with the dispersing agent first, this ensures that good dispersion of graphene in the active hydrogen-containing component is obtained. The pre-functionalisation of graphene with the dispersing agent may be carried out in the presence of a wetting agent. This helps to better disperse the graphene flakes which maximises the available area for edge functionalisation. The pre-functionalisation of graphene with the dispersing agent may be carried out in the presence of an organic solvent such as xylene, toluene, ethyl acetate, ethanol, hexane, isopropanol, Oimethyiformamide (DMF), N-Methyi-2-pyrrolidone (NMP), cyrene or propyl acetate, in which case, the dispersing agent may be a solvent based dispersing agent. The solvent based dispersing agent may comprise any of the following functional groups, either alone or in combination: amino, hydroxyl, carboamide, diamine, and triamine. The solvent based dispersing agent may comprise a high molecular weight copolymer. In particular, the dispersing agent may be an alkylammonium salt of a high molecular-weight copolymer such as BYK9076. The solvent based dispersing agent could also be a high molecular-weight copolymer having pigment affinity groups such as BYK9077.

The pre-functionalisation of graphene may also be carried out in the presence of water, in which case, the dispersing agent may comprise a water based dispersing agent. The water based dispersing agent may comprise any of the following functional groups, either alone or in combination: amino, hydroxyl, carboamide, diamine and triamine. The water based dispersing agent may comprise a high molecular-weight copolymer having pigment affinic groups. Examples of water based dispersing agents that may be used in accordance with the present invention include: DisperBYK2010, DisperBYK2012, Anti terra 250, Anti Terra U, Anti terra U100, DisperBYK 190, DisperBYK 192, BYK093, BYK022 and BYK1640.

In some embodiments, oxide-free graphene may be functionalised with the dispersing agent and the active hydrogen-containing component. Oxide-free graphene may be obtained by mining graphite ore from a graphite ore body, subjecting the graphite ore to an electrolytic treatment to obtain an oxide-free expanded graphitic material and subjecting the oxide-free expanded graphitic material to an exfoliation treatment to obtain single-layer graphene, few-layer graphene and graphene nano platelets The electrolytic treatment may be carried out in the presence of a non-oxidising electrolyte. For instance, the non-oxidising electrolyte may comprise ammonium sulphate and/or sulphuric acid.

The pre-impregnated composite may be produced via a pre-preg process or a wet lay-up process. However, it will be appreciated that the reinforcement material can be impregnated with the matrix material by alternative processes such as Spray Lay-up, Liquid Resin Infusion, Resin Transfer Moulding (RTM), Filament Winding, Pultrusion, Hot Melt Processing, Sheet Moulding Compound (SMC), Bulk Moulding Compound (BMC), Dough Moulding Compound (DMC).

In some embodiments the pre-impregnated composite may be produced by: a) providing a layer of fibrous reinforcement material; b) applying the matrix material on the reinforcement material; c) providing another layer of reinforcing material on the matrix material; and d) repeating steps (b) and (c) until a pre-impregnated composite of the desired thickness is obtained. The fibrous reinforcement material may be provided in a mould prior to the step of applying the matrix material on the layer of fibrous reinforcement material. The matrix material may be spread across the surface of the reinforcement material using a brush, a spatula or a roller. Alternatively, the matrix material may be spread across the surface of the reinforcement material by vacuum infusion. The pre-impregnated composite may be partially cured. In some embodiments the preimpregnated composite may be partially cured under vacuum.

A protective film may be provided on both sides of the partially cured pre-impregnated composite.

According to a third aspect of the invention there is provided a composite article which comprises the pre-impregnated composite according to the first aspect of the invention or the preimpregnated composite produced according to the method of the second aspect of the invention.

The composite article according to the third aspect of the invention may incorporate any or all features described in relation to the first aspect of the invention or the second aspect of the invention. The composite article may comprise a plurality of pre-impregnated composites. The plurality of pre-impregnated composites may be provided in a stacked arrangement.

According to a fourth aspect of the invention there is provided a method of producing the composite article according to the third aspect of the invention, which comprises the step of curing the pre-impregnated composite.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a schematic of a matrix material comprising functionalised graphene.

Figure 2 shows a schematic of a matrix material comprising un-functionalised graphene.

As will be elucidated in more detail below, the incorporation of functionalised graphene (Figure 1) into the matrix material enables improvements in mechanical strength to be obtained relative to matrix materials that comprise unfunctionalised graphene (Figure 2). These improvements have been attributed to functionalised graphene being homogeneously distributed and substantially aligned within the matrix material (Figure 1). In contrast, Figure 2 shows that unfunctionalised graphene is more randomly distributed and orientated within the matrix material which results in the matrix material exhibiting inferior strength. Matrix materials of the type shown in Figure 1 also exhibit good lightning strike protection properties since the aligned graphene flakes are able to effectively dissipate the electricity from the strike through the matrix material.

Graphene production

A suitable graphite material“Vittangi graphite”, being a strong, conductive graphite bearing ore, was identified and is available to the Applicant in the Nunasvaara deposit in Sweden, being a predominantly microcrystalline flake Joint Ore Reserves Committee (JORC 2012) mineral resource of 9.8 Mt at 25.3% - 46.7% graphite (Cg). Grades for this deposit have been drill tested at an average of 35% Cg, with grades attaining up to 46.7% Cg. The rock strength has been measured at approximately 120 MPa and the resistivity at less than 10 Ohm-meter, for example 0.0567 Ohm-meter. A graphite deposit of the nature of the Nunasvaara deposit in Sweden would not be, and has not been to date, considered an appropriate source of graphitic material feedstock for the production of graphene. Graphite bearing ore obtained from the Nybrannan deposit as part of the Jalkunen Project is also a suitable material that is available to the Applicant for the production of graphene.

The graphite ore is extracted by known quarry mining methods with abrasive disks, saws or wires and other known non-explosive methods of rock extraction in an ore extraction step. The blocks of ore obtained have sizes which are suitable for transport, transfer movement, and handling. The blocks may be further cut into smaller shapes or forms of electrodes which are considered more suitable for presentation to an electrolytic process. The blocks may be cubic, cylindrical, trapezoidal, conical, or rectangular in shape and have a preferred minimum dimension of 50 mm and maximum dimension of 500 mm. More particularly, the blocks have a minimum dimension of 100 mm and maximum dimension of 300 mm, or still more particularly a minimum dimension of 150 mm and maximum dimension of 200 mm. The ore blocks from the graphitic deposit are employed directly as electrodes in electrolysis for the production of nano-micro platelet graphite. In this embodiment the extracted graphite ore is used as the anode, copper metal is used as the cathode and the electrolytic treatment is carried out in the presence of a 1 M ammonium sulphate solution having a pH of 6.5-7.5. The voltage applied to exfoliate the extracted graphite into nano-micro platelet graphite was 10V and the ammonium sulphate solution was concurrently stirred at 1000rpm.

The nano-micro platelet graphite obtained after the electrolytic treatment has substantially unaltered properties relative to the graphite ore from which it is produced. Moreover, the obtained nano-micro platelet graphite exhibited increased interlayer spacing between adjacent graphitic sheets relative to the observed interlayer spacing of nano-micro platelet graphite obtained from synthetic graphite or highly ordered pyrolytic graphite (HOPG).

Following the electrolytic treatment and before further exfoliation of the micro-nano platelet graphite into graphene, sulphate anions were separated from the solution containing the micro- nano platelet graphite. This was achieved by subjecting the solution containing the micro-nano platelet graphite to a liquid-liquid separation treatment in which the solution was added to kerosene. Since sulphate anions are more soluble in kerosene than in water they readily migrate and are solubilised into the organic solvent, which facilitates their removal from the solution containing the micro-nano platelet graphite. The micro-nano platelet graphite obtained following this beneficiation treatment comprises 90-99.9 % by weight of carbon.

The micro-nano platelet graphite obtained from the beneficiation treatment was then subjected to a combined chemical and high-pressure exfoliation treatment. The chemical treatment involves mixing the micro-nano platelet graphite (100 g) with an aqueous intercalant such as tetrabutyl ammonium sulphate solution and/or Sodium dodecyl sulphate (SDS), and/or Sodium dodecyl cellulose (SDC) and/or Carboxy methyl cellulose (CMC) (0.5 wt%) to intercalate ammonium ions between the graphitic layers of the micro-nano platelet graphite. The aqueous dispersing agents used are Anti terra U, Antiterra U100 and/or DisperBYK 192, and/or Antiterra 250 (1 wt %) and/or DISPERBYK 2012 (2 wt %) both of which are manufactured by BYK. However, equivalent products from other wetting agent and dispersing agent suppliers could also be used. This solution can be kept at room temperature and pressure for a period of 7 days to increase the content of intercalated ammonium ions between the graphitic layers.

The solution containing the intercalated micro-nano platelet graphite and surfactants is then subjected to a high-pressure treatment in an M-1 10Y high pressure pneumatic homogenizer or equivalent high pressure wet milling process machine with min 50 - 100 cycles with 100 micron nozzles which involves the use of a high pressure jet channel in an interaction mixing chamber. The solution containing intercalated micro-nano platelet graphite and surfactants is pumped from opposite sides of the homogeniser into the mixing chamber. This causes two highly accelerated liquid dispersion streams to collide with pressurised gas (1200 bar), resulting in de-agglomeration of the graphitic layers and the exfoliation of single-layer and few-layer graphene in high yield. The combination of high pressure and reduced bond strength between adjacent graphitic layers of the micro-nano platelet graphite increases the amount of single-layer graphene and few- layer graphene that is formed relative to graphene that is exfoliated from graphite using a high sheer exfoliation route. Advantageously, it has been found that by following the method of the present invention the graphene yield could be increased by 60-80%, and in certain instances up to 90%, relative to the graphene yields obtained when using conventional high shear treatments to exfoliate graphene from graphite.

Following the combined chemical and high pressure exfoliation treatment the solution obtained is ultra-centrifuged at 8,000-12,000 rpm for 30 minutes using a Fisher scientific Lynx 4000 or Beckmann Coulter (ProteomeLab® XL-A) centrifuge in order to substantially separate the exfoliated pristine graphene from any residual nano-micro platelet graphite.

Pre-functionalisation of graphene

Pristine, oxide-free and moisture free graphene nano platelets were then pre-functionalised with a dispersing agent (BYK9076) by mixing pristine graphene with the dispersing agent (e.g 0.25 or higher wt%) in the presence of xylene or any other appropriate solvent.

Matrix material preparation

Exemplary matrix materials (E1-E5) were prepared by mixing pre-functionalised graphene with a hardener (Aradur 295) for 5 minutes at 2000 RPM. This was to ensure that the pre- functionalised graphene was homogeneously dispersed throughout the hardener and that the graphene was further functionalised with the hardener to obtain“functionalised graphene”, i.e. graphene that is functionalised with both the dispersing agent and the hardener. An epoxy resin (Ardalite LY1564) was then added to the functionalised graphene solution and this mixture was stirred for a further 15 minutes at 5000 RPM. Comparative matrix material formulations (C1-C4) were prepared by mixing pristine graphene (which had not been pre-functionalised with a dispersing agent) with the epoxy resin (Ardalite LY1564) for 5 minutes at 2000 RPM and then adding this epoxy/graphene mixture to a hardener (Aradur 295). This mixture was then stirred for a further 15 minutes at 5000 RPM. A “control” matrix material formulation was prepared without graphene by mixing a hardener (Aradur 295) with an epoxy resin (Ardalite LY1564) for 15 minutes at 5000 RPM. The control (C1), comparative (C1-C4) and exemplary matrix material formulations (E1-E5) are shown in Table 1 below:

Table 1

Preparation of the pre-impreqnated composite and composite article

To prepare the pre-impregnated composite a layer of carbon fibre (Carbon Fibre 2/2 Twill 3k 21 Og) reinforcement material is laid on the surface of a mould. The matrix material is then applied on the layer of carbon fibre reinforcement material and spread across its surface using a spatula which causes the matrix material to become partially or fully embedded in the carbon fibres of the reinforcement material. This process is repeated until a pre-impregnated composite of the desired thickness is obtained. In this embodiment, a pre-impregnated composite comprises three layers of carbon reinforcement material. The pre-impregnated composite is then placed in a vacuum bag and transferred into an oven. To partially cure the matrix material the pre-impregnated composite is heated at 120°C for a period of 90 to 120 minutes. The pre-impregnated composite is then fully cured in an autoclave at 180°C for four hours.

The cured pre-impregnated composites were then subjected to a tensile strength test and a flexure strength test to evaluate their mechanical properties. The results of each test are shown in Table 2 below.

Tensile Strength test The cured pre-impregnated composites were cut into dumbbell-shaped test samples and were tested in accordance with ASTM D638. It can be seen that the tensile strength of the cured pre-impregnated composites increases with the increasing graphene content in all cases. However, it is also evident that improvements in tensile strength are obtained when the cured pre- impregnated composites comprise functionalised graphene (E1-E4) rather than un-functionalised graphene (C1 -C4).

Flexure Strength Test

The cured pre-impregnated composites were cut to the required shape and were tested in accordance with ASTM 790 to evaluate their flexural properties. It can be seen that cured pre- impregnated composites with improved resistance to flexing can be obtained by increasing the content of graphene in the composite. Moreover, the results also demonstrate that irrespective of the graphene content, improved resistance to bending can be obtained by pre-impregnating the carbon reinforcement material with a matrix that comprises functionalised graphene rather than un-functionalised graphene.

Tests were also conducted to determine the lightning protective properties and anti-icing properties of pre-impregnated composites that comprise matrix materials in accordance with the present invention. Lightning strike test

Laminate panels were manufactured by stacking four plies of woven carbon fibre epoxy prepreg (TenCate E750 resin system) and curing them for 60 mins at 180°C in a Fontijne LabPro600 heated press. Three panels were tested. Two of the panels were coated with matrix materials E4 and E5 respectively while a third panel comprised an expanded copper mesh which is conventionally used in aircraft to mitigate lightning strikes. The panels were not post-cured, although the two panels coated with matrix materials E4 and E5 respectively were subjected to a secondary heating cycle in order to cure the epoxy resin. Testing was carried out in accordance with EUROCA ED-105A (2013) - ‘Aircraft Lightning Test Methods’ with a Zone 2A strike specification. In each test, the position of the strike was set to approximately the centre of the test panel. The results of the lighting strike test are shown in Table 3 below.

From Table 3 below it can be seen that the panels coated with matrix materials E4 and E5 exhibit comparable performance to that of the panel provided with the copper mesh and that no punctures were observed in any of the panels tested. Accordingly, it will be appreciated that comparable lightning strike protection can be obtained at significantly reduced weights, i.e. by an order or magnitude, by replacing the expanded copper mesh with a matrix material coating according to the present invention.

Table 3 Anti-icing test

Anti-icing tests were conducted using a MacGregor resistive implant welding power supply SN M025C on a single ply of carbon reinforcement fabric (Carbon Fibre 2/2 Twill 3k 210g) that was pre-impregnated with matrix material E5. Anti-icing tests were also conducted on a panel formed from four plies of woven carbon fibre epoxy prepreg (TenCate E750 resin system) and coated with matrix material E5.

The results of the anti-icing tests indicated that significant increases in temperature could be obtained when a current (Power supply 50V/600A DC) was applied to the two test samples. In particular, a temperature increase of 170°C was observed in a 60 second period for the single ply of carbon reinforcement fabric impregnated with the E5 matrix material, whereas an increase of 95°C was observed for the E5 coated carbon fibre epoxy pre-preg within the same period. Accordingly, the results demonstrate that carbon reinforcement fabrics coated or impregnated with a matrix material according to the present invention are suitable for and can be used to de-ice critical areas of aircraft, e.g. the wings of an aeroplane, before taking flight.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention.