GB2531045A

GB2531045A - Improved Surfacing Material

Info

Publication number: GB2531045A
Application number: GB1417818.0A
Authority: GB
Inventors: Arcidiacono Marco
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2014-10-08
Filing date: 2014-10-08
Publication date: 2016-04-13
Anticipated expiration: 2034-10-08
Also published as: GB2531045B; WO2016055421A1; GB201417818D0

Abstract

The present invention relates to a surfacing material for use in an automatic tape laying device, comprising a layer of reinforcement material 412 and a conducting surface layer 414 in contact with the reinforcement material, wherein the surface layer 414 extends beyond the surface of the reinforcement material 412. Also claimed is a composite article comprising multiple adjacent lengths of the surfacing material. Preferably the reinforcement material of a first surfacing material 602 is spaced from the reinforcement material of a second surface material 604, but the conductive surface layer of one length of surfacing material overlaps the conductive surface layer of a second length of surface material 608. The surfacing material provides improved surface conductivity perpendicular to the tape direction whilst also presenting fewer defects, and may protect aircraft from lightning strikes.

Description

IMPROVED SURFACING MATERIAL

INTRODUCTION

The present invention relates to a surfacing material for use in an automatic tape laying device and a composite material comprising the surfacing material.

BACKGROUND

Composite materials are being increasingly used on aircraft structures in place of aluminium because they possess excellent strength and weight properties. The skin which covers the airframe on an aircraft is one part of the aircraft structure that is frequently being made from composite materials.

The drawback of composite materials is that they have a lower electrical conductivity than metals.

This is particularly important for skin panels which need to conduct electrical energy around the external surface of the aircraft when struck by lightning. The use of composites on skin panels makes the aircraft more vulnerable to damage from lightning strikes unless their inherent lack of conductivity is addressed.

To mitigate against the effect of a lightning strike plane builders often incorporate a conductive surface into the outer layer of composite material used on the skin panels. Typically the conductive surfacing layer is an expanded copper or aluminium foil combined with a curable resin. This layer is usually placed into a mould and successive layers preimpregnated composite material (prepreg) are placed on top to form a layup. The surfacing film is typically slit into a tape and deposited by automated tape layup (ATL) apparatus onto the surface of a mould, the surfacing film may also include an integrated layer of prepreg to give it sufficient structural strength to resist forces used in ATL. Once the surface film and subsequent prepreg layers have been laid in the mould the layup is then cured to form a completed part. Surfacing films comprising expanded metal foils do not flow to occupy the spaces between tape lengths and so do not form a conductive network. They also add weight to the structure and their fragility presents handling problems, especially when placed by ATL.

Conductive films comprising metal particles in a curable resin have recently been developed to replace expanded metal foils (for example patent number US 8178606 B1).The metal particles in these films form an interconnected conductive network within the resin during the cure stage. In order to retain the metal particles in position in the centre of the resin it is necessary to use a high viscosity resin. This may be one that is specifically formulated to have a high viscosity, or it can be a lower viscosity resin that has been B-staged. The use of a high viscosity resin, whilst necessary) presents problems when it is applied to a mould by ATL apparatus. When tapes are deposited by ATL apparatus it is necessary to maintain a separation of approximately (0.1 -2 mm) between adjacent strips of tape. This is to prevent overlaps occurring between adjacent tapes which would result in defects in the layup. Gaps between adjacent tape lengths, whilst not ideal are accepted because they are preferable to overlaps. The separation between adjacent lengths of the surfacing film tape prevents conductive networks from forming between the conductive elements within the surfacing film; this lowers the overall conductivity of the surfacing layer, particularly directions perpendicular to the tape direction. The same reduction of conductivity occurs in surfacing layers that comprise expanded metal foils also exhibit reduced conductivity for the same reason.

Because these conductive films comprise high viscosity resins, they exhibit minimal flow during the cure phase. As a result they do not flow to completely bridge the gaps between adjacent tapes, and as such they also exhibit surface defects on the final cured laminate and limit the overall conductivity.

Accordingly, it is desirable to develop a surfacing material which overcomes the above described problems and provides improvements generally.

SUMMARY

According to the present invention, there is provided a surfacing material and a composite article as defined in any of the accompanying claims.

According to a first aspect of the present invention there is a surfacing material for use in an automatic tape laying device comprising a layer of reinforcement material and a conducting surface layer in contact with the reinforcing material, wherein the conducting surface layer extends beyond the reinforcement material.

According to a second aspect of the present invention there is provided a composite article comprising multiple adjacent lengths of surfacing material wherein the conducting surface layer from a first length of surfacing material contacts the conducting surface layer or reinforcement material from a second length of surfacing material.

The larger width of the conducting surface layer in respect of the reinforcement material width bridges the gap between adjacent lengths of surfacing material when deposited onto a mould surface. This maintains a conductive network and prevents visual defects from occurring on the surface of the finished part.

The conducting surface layer is thin and therefore flexible in comparison to the reinforcement material. The length of tape is deposited with the surfacing layer overlapping the neighbouring tape.

The part of the surfacing layer that is wider than the reinforcement material folds upwards against the neighbouring tape length as the tape is pushed against the mould. It will then occupy the space between both lengths of tape making contact with the conductive surface layer or reinforcement material of the neighbouring length of tape. If no overlap occurs, the surfacing layer bridges or reduces the gap improving conductivity between adjacent surfacing material tapes. If a length of reinforcement material accidentally overlaps a portion of conductive surface layer alone, the thinner conductive surface layer means an acceptable amount of disruption to the layup occurs. Whereas when reinforcement material overlaps reinforcement material, the disruption renders the lay-up un-useable.

In an embodiment of the present invention the conductive surface layer extends beyond the reinforcement material on one edge only. This embodiment provides a neat finish on the surface layer.

In an embodiment of the present invention the conductive surface layer extends beyond the reinforcement material on both sides. Preferably the distance the surface layer extends is equal on both sides. This embodiment has the advantage that the length that the conductive surface layer projects beyond a single edge of the reinforcement material is minimised. This reduces the likelihood of the conductive film being damaged during storage and handling. Additionally lengths of tape of this embodiment can be deposited onto a mould surface in any direction.

In an alternative embodiment of the present invention the conductive surface layer extends beyond the reinforcement material and is conformed around one or both edges of the reinforcement material. This has the advantage that all of the conductive surface film is in contact with the reinforcement material and therefore it is less likely to be damaged during storage or handling, but still places conductive material in the gap between lengths of surfacing material.

In an alternative embodiment of the present invention the conductive surface film is applied to the reinforcement material and the conductive surface film has an uneven thickness across its width.

Preferably the conductive surface film has an increased thickness at one or both edges, so that when it is pressed into the mould, the thicker regions of conductive surface film are squeezed against the mould surface, causing the conductive surface film to flow out from under the reinforcement material so that the conductive surface film then extends beyond the reinforcement material into the gap between adjacent lengths of surfacing material. This embodiment has the advantage that it damage during storage or handling is reduced because the surfacing material does not extend beyond the reinforcement material until it is placed into the mould.

SPECIFIC DESCRIPTION

In an embodiment, the conductive surface layer is wider than the reinforcement material. The conductive surface layer can be wider than the reinforcement material by from 0.1 mm to 5 mm, or more preferably from 0.5 mm to 2mm.

In another embodiment, the conductive surface layer is wider than the reinforcement material by from 0.01% to 50%) or more preferably from 0.2% to 10%.

In an embodiment, the conductive surface layer and the reinforcement material are arranged so that the conductive surface layer extends beyond the reinforcement material both sides or on one side only.

In another embodiment) the conductive surface layer comprises a curable resin and a conductive filler as disclosed in US 8178606 Al incorporated herein by reference.

In an alternative embodiment of the present invention the conductive filler comprises an expanded metal foil, for example a copper or aluminium foil. The present invention is also compatible with any other conductive element for a surfacing film that is known in the art. This may include metal or carbon structures such as conductive veils, or arrangements of metallised or metal coated fibres.

In a further embodiment, the conductive surface layer comprises a curable resin and an electrically conductive filler that during the cure of the curable resin is capable of self-assembling into a heterogeneous structure comprised of a continuous, three-dimensional network of metal situated among (continuous or semi-continuous) polymer rich domains whose electrical conductivity is within several orders of magnitude of that of bulk metals.

In an embodiment of the present invention, the electrically conductive filler is preferably a coated silver filler, and the filler and the curable resin exhibit an interaction during the cure of the resin, said interaction causing the filler to self-assemble into conductive pathways.

In yet another embodiment of the present invention, the composition is cured thereby forming conductive pathways within, and the conductivity of the cured self-assembled composition is greater than 100 times the conductivity of a cured non-self-assembled composition having an equivalent amount of the conductive filler.

In further embodiments of the present invention, the curable resin of the surfacing layer comprises diglycidyl ether of bisphenol F, and further comprises a cure agent, preferably comprising a polyamine anhydride adduct based on reaction between phthalic anhydride and diethylenetriamine.

In an additional embodiment of the present invention, the conductive surface layer composition further provides shielding of electromagnetic radiation having a frequency of between 1 MHz and 20 GHz, wherein said shielding reduces the electromagnetic radiation by at least 20 decibels.

Because of the heterogeneous structure formed, the conductive surface layer composition is able to induce a percolated network of conductive particles at particle concentrations considerably below that of traditional compositions that possess homogenous structures comprised of particles uniformly situated throughout the polymer matrix. Moreover, the heterogeneous structure formed during curing permits the sintering of particles thereby eliminating contact resistance between particles and in turn leading to dramatic improvements in thermal and electrical conductivity.

Moreover, the continuous pathway of sintered metal permits carrying of substantial amounts of heat and electrical current encountered during a lightning strike event. The combination of lower filler loading and the related self-assembling of continuous pathways permits conductive surface layer materials that are lighter weight and easier to manufacture and repair which are desirable for fuel savings, payload capacity reasons, and construction and repair reasons.

Due to its isotropic nature, the composition is conductive in all orthogonal directions; thereby lending to significantly improved electrical and thermal conductivity in the z-direction of composite structures. In turn, this improvement allows for considerable reduction in capacitive effects and heat buildup associated with non-conductive resins layers present in composite laminate as well as existing EMF LSP systems and the like.

The mechanism of self-assembly and structure formation is achieved through the proper selection of component materials and adherence to particular processing conditions. In one embodiment of the present invention, the conductive surfacing layer comprises a conductive filler (thermal, electrical or both) and a curable resin comprising a monomer and optionally a curative agent. The formation of filler rich domains during reaction of the resin allows for direct filler-to-filler particle contacts to be made. In the presence of heat the particles may further sinter together. Sintering eliminates the contact resistance between the previously non-sintered filler particles thereby substantially improving the thermal and/or electrically conductivity of the composite.

While not fully understood and not wishing to be bound by this theory, it is believed that the self-assembly and domain formation and sintering are sensitive to the resin's cure temperature, the cure time, and the level of pressure applied during the cure. In other words, domain formation and sintering are kinetically driven processes. In a still a further embodiment, the rate at which the resin is heated will affect the extent of domain formation and sintering. In total, the processing conditions

S

can be tailored to achieve a conductive adhesive having the best combination of properties at minimal filler loading, which often translates to lower cost and opportunity to take advantage other properties that are adversely affected by high filler loadings. In some cases, when the adhesive is employed in an application that is not able to withstand high sintering temperatures, higher pressures or non-traditional sintering techniques may be used to achieve exceptionally high co nd u ctivities.

The filler component and curable resin of the conductive surfacing layer are chosen so as to create a homogeneous mixture when mixed. However) during the cure) it is believed that the resulting polymer formed from the cured resin then has a repulsive interaction with the filler so as to allow the composition to self-assemble into a heterogeneous compound having filler-rich domains wherein the filler composition is significantly higher than the bulk filler concentration. Thus, while the overall (bulk) filler concentration of the compound does not change) the filler particles and the resin self-assemble in situ into respective regions of high concentration. This phenomenon can lead to a self-assembled network of interconnected filler particles formed in situ from a mixture having very few, if any) initial filler-filler contacts.

There are several approaches which may be employed to create the repulsive interaction between the filler component and the curable resin. However, in a preferred embodiment of the present invention, this is achieved by coating a filler particle with a non-polar coating and mixing the coated filler in a reactive organic compound comprising a relatively non-polar resin and a polar curing agent.

In an uncured state, the resin, curative, and filler form a relatively homogeneous mixture in which the coated filler and the resin are compatible with one another and form a relatively homogeneous mixture. However, with the application of heat the curing agent reacts with the resin forming a polymer having polar moieties thereon, resulting in a repulsive interaction between the non-polar coating on the filler and the polar moieties on the polymer. This repulsive interaction leads to the self-assembling of polymer-rich and filler-rich domains whose respective concentrations are significantly higher than the bulk concentrations of polymer and filler, respectively. Moreover, extensive domain formation is capable of creating continuous filler-rich domains with substantial particle to particle contact between most of the filler particles.

Other types of interactions capable of creating repulsive effects upon curing of the curable resin in the presence of the filler, could consist of) but are not limited to, electrostatic interactions, hydrogen bonding interactions, dipole-dipole interactions, induced dipole interaction, hydrophobic-hydrophilic interactions, van der Waals interactions) and metallic interactions (as with an organometalic compound and metallic filler). Other forms of repulsive interactions could arise from entropic related effects such as molecular weight differences in the polymers formed from the curable resin(s). Additionally, repulsive interactions could arise as a result of an external stimulus such as

electrical field.

The domains in the conductive surfacing layer that are formed upon curing of the curable resin in the presence of the filler results in filler-rich domains having a higher than bulk (average) filler concentrations and in organic rich domains having lower than bulk (average) filler concentrations.

The areas of higher than average filler concentration can form semi-continuous or continuous pathways of conductive filler material extending throughout the body of the cured composition.

These pathways provide a low resistance route through which electrons and/or thermal phonons can travel. In other words) the pathways or channels allow for greatly enhanced thermal or electrical conductivity. This conductive pathway may be further enhanced by sintering the filler particles together. Such highly conductive pathways are particularly beneficial for LSP given the large amount of electrical current and heat that must be dissipated during a strike event.

Sintering, as it is understood in the art, is a surface melting phenomenon in which particles are fused together at temperatures below the material's bulk melting temperature. This behavior is brought about by a tendency of the material to relax into a lower energy state. As such, selection of filler type, size, and shape can greatly affect the sinterability of the filler particles. Certain particles, such as thin, wide, flat, plates are often formed by shearing large particles via various milling processes.

This process imparts a large amount of internal stress in addition to creating a large amount of surface area. When a certain amount of heat is added to the particles, they will have the tendency melt and fuse together thereby relieving the internal strain and decreasing the overall surface energy of the particles. For this reason, the preferred filler particles for use in the present invention are those that comprise some degree of thermal or electrical conductivity and sinter easily. In a still further embodiment of the present invention, the preferred filler comprises a metallic particle that has been subjected to cold working which has imparted strain into the structure of the filler which further enables sintering.

The sintering temperature will vary according to the material chosen as the filler, as well as the geometry of the filler particle. However, in a preferred embodiment of the present invention, it is advantageous to balance the cure of the curable resin and the sintering of the filler such that they occur simultaneously. In this embodiment, the cure temperature and profile is selected to coincide with the sintering temperature of the filler, so as the resin becomes repulsive to the filler and the filler particles are forced together, the individual filler particles can sinter once particle to particle contact is made. This is believed to be responsible for the continuous filler structure seen throughout the fully cured composition. In a preferred embodiment of the present invention, the sintering temperature is at least about 100 °C, more preferably about 150 °C, and even more preferably above 150°C for a silver flake filler.

In another embodiment of the present invention, a low-temperature cure may be desirable. For example when coating/applying the curable composition to a heat sensitive substrate, the cure agent and cure mechanism may be tailored to achieve a cured, self-assembled material at temperatures below 50°C, and alternately below room temperature (20 -25 °C). In embodiments of the present invention where sintering does not take place during a cure step, for example in a low-temperature cure environment, the particles may initially form self-assembled pathways that are not sintered. A sintering step may then be later added. This later-added sintering step may comprise heating of the self-assembled material, either through ambient heating, or electrically induced heating such as through a lightning strike.

In embodiments of the present invention, the self-assembling composition may be cured without the addition of heat. However, in a preferred embodiment of the present invention, the composition is cured via application of heat to the composition. Heat curing is commonly accomplished in a cure oven such as a convection oven or an autoclave, whereby hot air or radiated heat is used to increase the temperature of the composition. In alternate embodiments of the present invention, other methods of cure may be employed such as induction curing in an electromagnetic field, microwave curing, infrared curing, electron beam curing, ultraviolet curing, and curing by visible light.

Additionally, the cure reaction may be self-accelerated through the use of an exothermic cure reaction. A non-thermal cure may be desirable, for example, when the composition is coated on a temperature sensitive substrate such as a plastic.

In one embodiment of the present invention the filler comprises inorganic fillers. Available fillers include pure metals such as aluminum, iron, cobalt, nickel, copper, zinc, palladium, silver, cadmium, indium) tin, antimony, platinum) gold, titanium, lead) and tungsten, metal oxides and ceramics such as aluminum oxide, aluminum nitride) silicon nitride, boron nitride, silicon carbide, zinc oxide.

Carbon containing fillers could consist of graphite, carbon black, carbon nanotubes, and carbon fibers. Suitable fillers additionally comprise alloys and combinations of the aforementioned fillers.

Additional fillers include inorganic oxide powders such as fused silica powder, alumina and titanium oxides, and nitrates of aluminum, titanium, silicon, and tungsten. The particulate materials include versions having particle dimensions in the range of a few nanometers to tens of microns.

In an embodiment of the present invention, the filler is present at about 40 volume percent or less, based on the total volume of the conductive surface layer. In a more preferred embodiment of the present invention, the filler is present at about 30 volume percent or less, based on the total volume of the conductive surface layer. In a most preferred embodiment of the present invention, the filler is present at about 15 volume percent or less, based on the total volume of the conductive surface layer.

In a preferred embodiment of the present invention, the filler comprises a material that is either electrically conductive, thermally conductive, or both. Although metals and metal alloys are preferred for use in several embodiments of the present invention, the filler may comprise a conductive sinterable non-metallic material. In an alternate embodiment of the present invention the filler may comprise a hybrid particle wherein one type of filer, for example a non-conductive filler, is coated with a conductive, sinterable material, such as silver. In this manner) the overall amount of silver used may be reduced while maintaining the sinterability of the filler particles and conductivity of the sintered material.

In an embodiment of the present invention, the filler component must be able to interact with the curable resin to impart a heterogeneous structure in the cured conductive surfacing layer. In a preferred embodiment of the present invention as discussed above, this is accomplished through the interaction of a polar organic compound with a non-polar filler. For preferred filler materials, such as metals, the filler is coated with a material comprising the desired degree of polarity. In one preferred embodiment of the present invention, the filler coating comprises a non-polar fatty acid coating, such as stearic, oleic, linoleic, and palmitic acids. In a still further embodiment of the present invention, the filler coating comprises at least one of several non-polar materials, such as an alkane, paraffin, saturated or unsaturated fatty acid, alkene, fatty esters, waxy coatings, or oligomers and copolymers. In additional embodiments of the present invention, non-polar coatings comprise ogranotitanates with hydrophobic tails or silicon based coatings such as silanes containing hydrophobic tails or functional silicones.

In a further embodiment of the present invention, the coating (or surfactant, coupling agent, surface modifier, etc.) is applied to the filler particle prior to the particles' incorporation into the conductive surfacing layer. Examples of coating methods are, but not limited to, are deposition of the coating from an aqueous alcohol, deposition from an aqueous solution, bulk deposition onto raw filler (e.g. using a spray solution and cone mixer, mixing the coating and filler in a mill or Attritor), and vapour deposition. In yet a further embodiment, the coating is added to the composition as to treat the filler prior to the reaction between the organic components (namely the resin and curative).

In a preferred embodiment of the present invention curable resin of the surfacing layer comprises an epoxy resin and a cure agent. In this embodiment, the curable resin comprises from about 60 to about 100 volume percent of the total composition. In this embodiment, the curable resin of the surfacing layer comprises approximately from 70 to 85 percent by weight of a diglycidyl ether of a bisphenol compound) such as bisphenol F, and 15 to 30 percent by weight of a cure agent, such as a polyamine anhydride adduct based on reaction between phthalic anhydride and diethylenetriamine.

In additional embodiments of the present invention, suitable curable resins of the surfacing layer comprise monomers, reactive oligomers, or reactive polymers of the following type siloxanes, phenolics, novolac, acrylates (or acrylics)) urethanes, ureas, imides, vinyl esters, polyesters, maleimide resins) cyanate esters, polyimides, polyureas, cyanoacrylates, benzoxazines, unsaturated diene polymers, and combinations thereof. The cure chemistry would be dependent on the polymer or resin utilized in the curable resin. For example, a siloxane matrix can comprise an addition reaction curable matrix, a condensation reaction curable matrix, a peroxide reaction curable matrix, or a combination thereof. Selection of the cure agent is dependent upon the selection of filler component and processing conditions as outlined herein to provide the desired self-assembly of filler particles into conductive pathways.

In an embodiment of the present invention the surfacing material comprises a conductive surface layer having a conductivity of from 0.2 to 0.4 mO/mm2, measured using a Lucas Signatone SP4, 4-point probe head connected to a Keithley 2750 Digital Multimeter.

In an embodiment of the present invention the reinforcement material of the surfacing material is a prepreg, comprising structural fibres and a curable resin. Preferably the reinforcement material comprises structural fibres with a unidirectional arrangement. Unidirectional structural fibres have the advantage that few or no fibres are cut if the material is cut into tapes for ATL use.

In an alternative embodiment of the present invention the reinforcement material may comprise dry reinforcement fibres, which are subsequently infused with a curable resin.

The structural fibres may comprise cracked (i.e. stretch-broken), selectively discontinuous or continuous fibres. The structural fibres may be made from a wide variety of materials such as glass, carbon) graphite) metallised polymers aramid and mixtures thereof. The prepreg typically comprises from 30 to 70 wt % structural fibres.

Preferably the structural fibre comprises a fabric or a unidirectional arrangement of fibres having an areal weight of 4 to 100 gsm, or more preferably 10 to 60 gsm, or more preferably 5 to 30 gsm, or more preferably 12 to 24 gsm or more preferably still, of 17 to 19 gsm.

As discussed above, the prepreg of the surfacing material comprises a curable resin. The curable resin may be present as a discrete layer or may be fully or partially impregnated into a layer of structural fibres. The prepreg typically comprises from 15 to 50 w.t % curable resin or more preferably 32 to 45 wt. % curable resin.

The curable resin of the prepreg may be selected from those conventionally known in the art, such as resins of phenol-formaldehyde, urea-formaldehyde, 1,3,5-triazine-2,46-triamine (melamine), bismaleimide, epoxy resins, vinyl ester resins, benzoxazine resins, polyesters) unsaturated polyesters, cyanate ester resins, or mixtures thereof.

Particularly preferred are epoxy resins, for example monofunctional, difunctional or trifunctional or tetrafunctional epoxy resins.

Suitable difunctional epoxy resins, by way of example, include those based on; diglycidyl ether of Bisphenol F, Bisphenol A (optionally brominated), phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldelyde adducts, glycidyl ethers of aliphatic diols, diglycidyl ether, diethylene glycol diglycidyl ether, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidised olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof. Difunctional epoxy resins may be preferably selected from diglycidyl ether of Bisphenol F, diglycidyl ether of Bisphenol A, diglycidyl dihydroxy naphthalene, or any combination thereof.

Suitable trifunctional epoxy resins, by way of example, may include those based upon phenol and cresol epoxy novolacs, glycidyl ethers of phenol-aldehyde adducts, aromatic epoxy resins, aliphatic triglycidyl ethers, dialiphatic triglycidyl ethers, aliphatic polyglycidyl ethers, epoxidised olefins, brominated resins, triglycidyl aminophenyls, aromatic glycidyl amines, heterocyclic glycidyl imidines and amides, glycidyl ethers, fluorinated epoxy resins, or any combination thereof.

Suitable tetrafunctional epoxy resins include N,N,N',N'4etraglycidyl-m-xylenediamine (available commercially from Mitsubishi Gas Chemical Company under the name Tetrad-X, and as Erisys GA- 240 from CVC Chemicals), and N,N,N',N'-tetraglycidylmethylenedianiline (e.g. MY721 from Huntsman Advanced Materials).

The therniosetting resin may also comprise one or more curing agent(s). Suitable curing agents include anhydrides, particularly poly carboxylic anhydrides; amines, particularly aromatic amines e.g. 1,3-diaminobenzene, 4,4'-diaminodiphenylrnethane, and particularly the sulphones, e.g. 4,4J diaminodiphenyl sulphone (4,4' DDS), and 3,3'-diaminodiphenyl sulphone (3,3' DDS), and the phenol-formaldehyde resins. Preferred curing agents are the amino sulphones, particularly 4,4' DDS and 3,3' DDS. Further examples of the type and design of the resin and fibres can be found in In embodiments where the reinforcement material is a prepreg, the prepreg may include additional materials as desired, such as performance enhancers or modifying agents. Such materials may be selected from flexibilisers, toughening agents/particles, additional accelerators, core shell rubbers, flame retardants, melting agents, pigments/dyes, plasticisers, UV absorbers, anti-fungal components, fillers, viscosity modifiers/flow control agents, stabilisers and inhibitors.

In an embodiment of the present invention a composite article comprises a length of surfacing material wherein the conducting surface layer projects beyond the reinforcement material of the same surface layer and overlaps the conductive surface layer of a second length of surface material, or contacts the conductive surface layer or reinforcement material of a second length of surface material.

In an embodiment of the present invention a composite article comprises multiple lengths of surfacing material arranged parallel lengths.

In an embodiment of the present invention a composite article has been laid up using ATL deposition of a surfacing material.

Preferably the surfacing material has a thickness of from 0.5 mm to 5.0 mm, more preferably 0.5 mm to 4.0 mm, most preferably from 1.0 mm to 3.0 mm, so that it can be conveniently handled by the ATL apparatus.

As the surfacing material is intended to be provided in the form of a roll, preferably it is sufficiently flexible so as to be able to form a roll with a minimum diameter of less than 20 cm, preferably less than 10 cm, or more preferably less than 250 mm when the backing sheet is applied to the composite material or less than 70mm when it is applied to a resin film.

The surfacing material according to the invention can be prepared by any known method in the art.

Such methods typically involve bringing together the structural fibres and curable resin and impregnating the structural fibre layer with the resin to form a prepreg. The prepreg can be slit to the desired width and then combined with the conductive surfacing layer which had been slit to a wider width. Once prepared the surfacing material is typically rolled up ready for deposition by an ATL apparatus.

The surfacing material of the present invention can also be made by alternate methods, for example, two sheets of conductive surfacing layer can be lightly applied to both sides of a sheet of prepreg.

This resulting product can then be slit by cutting through one layer of conductive surfacing material and through the prepreg, leaving one layer of conductive surfacing material intact. The cuts are made into alternate faces across the width of the material. The adjacent tape lengths can then be separated in opposite directions and wound into rolls. Each tape length comprises a width of prepreg and a length of conductive surfacing material twice the width of the prepreg projecting beyond one edge of the prepreg.

The invention relates to a process of laying down onto a mould surface, a surfacing material material as described herein, which is fed automatically from a roll to the mould surface such that the backing paper on the prepreg is uppermost, and the conductive surfacing layer material adhering to the mould surface by application of pressure onto the backing paper by a tool) followed by removal of the backing paper) leaving the composite material in place on the mould surface without its backing paper. The conductive surface layer may comprise a tackifier layer to improve adhesion to the mould surface, or alternatively, it may be heated prior to deposition.

This process is typically followed by the subsequent placement of additional composite material, such as prepregs or dry fibrous reinforcement, to generate a stack of composite material that forms the composite part.

Once the surfacing material and any subsequent composite material has been laid down, the arrangement is cured by exposure to elevated temperature, and optionally elevated pressure) to produce a cured composite part.

In a further embodiment there is provided a reinforcement material comprising a conductive surfacing layer extending beyond the reinforcement material layer such that upon placement of spaced adjacent reinforcement material tapes that are in contact with a mould surface, a continuous surfacing layer is formed.

In an embodiment of the present invention multiple lengths of tape of the surfacing material of the present invention are incorporated into a cured composite part) the part having a conductivity measured across the surface of the part, perpendicular to the tape direction, from 0.2 to 0.4 mO/mm2, measured using a Lucas Signatone SP4, 4-point probe head connected to a Keithley 2750 Digital Multimeter.

The invention will now be clarified by way of example only and with reference to the accompanying drawing in which:

Figure 1 is a representation of the prior art.

Figure 2 is a representation of an embodiment of the surfacing material of the present invention.

Figure 3 is a representation of another embodiment of the surfacing material of the present invention.

Figure 4 is a representation of a further embodiment of a composite material comprising the surfacing material of the present invention.

Figure 5 is a representation of an embodiment of a composite material comprising the surfacing material of the present invention.

Figure 6 is a representation of a further embodiment of a composite material comprising the surfacing material of the present invention.

Figure 1 represents the prior art, where two adjacent lengths of surfacing material tape, 102, 104 comprising a conductive surface layer 106 and prepreg 108 have been deposited on a mould surface (not shown). To minimise the risk on an overlap occurring during deposition, the two lengths of tape have been deposited with a gap between them 100. The gap reduces the conductivity between the conductive surface layers. Because the conductive surface layers comprise viscous resin the gaps can persist in the final cured parts, presenting as surface defects.

In Figure 2 the surfacing material comprises a conductive surface layer 204, and a prepreg 20. The conductive surfacing layer 202 is wider than the prepreg such that it extends beyond the prepreg on both sides of the tape. In use the surfacing material 202 is deposited by an ATL machine so that the conductive surface layer 204 contacts the mould surface. A subsequent length of surfacing material 202 is then placed adjacent to this so that portion of the projecting conducive surface layer of the second tape length is placed over the projecting conducive surface layer of the first length. Thus the conductive surfacing layers are overlapping to provide a conductive bridge between adjacent lengths of surfacing material and the prepregs 206 of the surfacing material are not overlapping.

In Figure 3 the surfacing material 302 comprises a conductive surface layer 304, and a prepreg 306.

The conductive surfacing layer is wider than the prepreg such that it extends beyond the prepreg on one side of the tape. In use a length of surfacing material 302 is deposited by ATL so that the conductive surface layer is deposited onto a mould surface. A second length of surfacing material is then deposited parallel to the first length. The second length is deposited so that the portion of conductive surface layer 304 that extends beyond the prepreg 306 contacts the side of the first length of surfacing material where no conductive surface layer projects beyond the prepreg. The conductive surface layer of the second length then bridges the gap between the adjacent lengths of surfacing material maintaining a conductive network.

In Figure 4 multiple lengths of the surfacing material of the present invention 402, 404 similar to the material 302 of Fig 3, have been deposited onto a mould 406 in tape form. The surfacing material comprises prepreg tape 412 and a conductive surface layer 414. The conductive surface layer from one tape contacts the conductive surface layer of the adjacent tape 408.This maintains the conductive network between adjacent tape lengths. Even if tape lengths are deposited so that no contact occurs, the projecting portion of conductive surface layer still partially spans the gap and still functions to improve conductivity.

In Figure 5 multiple lengths of the surfacing material of the present invention 502, 504 have been deposited onto a mould 506 in tape form. The surfacing material comprises prepreg tape 512 and a conductive surfacing layer 514. The conductive surface layer from one tape has been displaced away from the mould during deposition, because of an overlap. The conductive surfacing layer now occupies the gap between the adjacent tapes 508, making contact with the conductive surface layer and the prepreg of the adjacent tape. In this embodiment contact between conductive surfacing layers is maintained but crucially the prepreg layers do not overlap each other.

In Figure 6 multiple lengths of the surfacing material of the present invention 602, 604, which are similar to that which is illustrated in figure 1, have been deposited onto a mould 606 in tape form.

The surfacing material comprises prepreg tape 612 and a conductive surfacing layer 614. The conductive surface layer from one tape has been displaced during deposition onto the conductive surface layer of an adjacent tape. The conductive surfacing layer now occupies the gap between the adjacent tapes 608, making contact with the conductive surface layer. Crucially contact is made whilst the prepreg layers do not overlap each other.

Claims

CLAIMSl.A surfacing material for use in an automatic tape laying device, comprising a layer of reinforcement material and a conducting surface layer in contact with the reinforcement material, wherein the surface layer extends beyond the surface of the reinforcement material.
2. A surfacing material according to any of the preceding claims wherein the surface layer comprises and electrically conductive filler capable of self-assembling into a heterogeneous structure during cure.
3. A surfacing material according to any of the preceding claims wherein the surface material is in the form of a tape.
4. A surfacing material according to any of the preceding claims wherein the surface layer is wider than the reinforcement material by no less than 0.01%.
5. A surfacing material according to any of the preceding claims wherein the surface layer extends beyond the reinforcement material on one edge only.
6. A surfacing material according to claims ito 4 wherein the surface layer extends beyond the reinforcement material evenly on both sides.
7. A surfacing material according to any of the preceding claims wherein the reinforcement comprises a structural fibre and a curable resin.
8. A surfacing material according to any of the preceding claims wherein the reinforcement material comprises unidirectional structural fibres.
9. A surfacing material according to any of the preceding claims wherein the surface layer has a surface resistance of from to 0.2 to 0.4 mO/mm2, measured using a Lucas Signatone SP4, 4-point probe head connected to a Keithley 2750 Digital Multimeter.
10. A composite article comprising multiple adjacent lengths of surfacing material according to any of the preceding claims wherein the conducting surface layer from a first length of surfacing material contacts the conducting surface layer or reinforcement material from a second length of surfacing material.
11. A composite article according to claim 10 wherein the conducting surface layer of one length of surfacing material overlaps the conductive surface layer of a second length of surface material.
12. A composite article according to any of claims 10 and 11 wherein the reinforcement material of a first length of surfacing material is spaced from the reinforcement material of a second length of surface material.
13. A composite article according to any of claims 10 to 11 wherein the multiple lengths of surfacing material comprises parallel lengths of a surfacing material as defined in any of claims ito 10.
14. A composite article according to any of claim 13 wherein the surfacing material is deposited by ATL apparatus.