GB2481528A

GB2481528A - Through thickness conductive laminate

Info

Publication number: GB2481528A
Application number: GB1110579.8A
Authority: GB
Inventors: Martin Simmons; Dana Blair
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2010-06-22
Filing date: 2011-06-22
Publication date: 2011-12-28
Anticipated expiration: 2031-06-22
Also published as: GB2481528B; GB201010445D0; DE102011105377A1; FR2961514B1; DE102011105377B4; FR2961514A1; GB201110579D0

Abstract

A curable prepreg comprising a layer of structural conductive fibres and curable thermosetting resin and comprising an open-structured metal-coated polymeric sheet. There may also be a stack of the prepregs with resin interleaf layers. The coating metal may be selected from silver, copper, nickel, gold, platinum, aluminium and mixtures thereof, but is preferably silver. There is also claimed a use of such a prepreg to increase through-thickness (z- direction) electrical conductivity of a cured laminate. The laminate has particularly useful application in protecting against lightning damage to aircraft bodies.

Description

IIie: Improvements in Composite Materials

Technical Field

The present invention relates to improvements in the electromagnetic response of composite materials, particularly to providing improved resistance to damage caused by lightning strikes.

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 becoming increasingly widespread and their fields of application range from "industrial" and "sports and leisure" to high performance aerospace components.

Prepregs, comprising a fibre arrangement impregnated with resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are "laid-up" as desired and the resulting laminate is cured, typically by exposure to elevated temperatures, to produce a cured composite laminate.

A common composite material is made up from a laminate of a plurality of prepreg fibre layers, e.g. carbon fibres, interleafed with resin layers. These resin layers are believed to provide a significant toughness improvement to the eventual cured laminate.

Although the carbon fibres have some electrical conductivity, the presence of the interleaf layers means that this is only exhibited in the composite in the plane of the laminate. The electrical conductivity in the direction orthogonal to the surface of the laminate, the so-called z-direction, is low.

This lack of conductivity in the z-direction 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 lightning strike protection to such composite materials, typically involving the addition of conductive elements at the expense of increasing the weight of the composite material.

In WO 2008/056 123 significant improvements have been made in lightning strike resistance, without significantly increasing weight or affecting mechanical properties, by including metallic conductive particles in the resin interleaf layers so that they contact the adjacent fibre layers and create an electrical pathway in the z-direction.

EP 2053078 Al teaches a prepreg comprising conductive particles and thermoplastic particles.

The present invention aims to obviate or at least mitigate the above described problems and/or to provide improvements generally.

Summary of the Invention

According to the invention, there is provided a prepreg, a laminate, and a use as defined in any of the accompanying claims.

In a first aspect, the invention relates to a curable prepreg comprising a layer of structural conductive fibres and curable therniosetting resin and comprising an open-structured metal-coated polymeric sheet Such a prepreg, taken alone or when laid together with a plurality of similar prepregs and cured to form a composite laminate, has been found to provide excellent electrical conductivity in the z-direction whilst also providing -toughness. Furthermore, the metal-coated sheet is easier to handle than conductive particles. For example it is particularly difficult to ensure that they are evenly distributed in the composite material.

Additionally, adding particles to the resin tends to increase its viscosity presenting further processing constraints and difficulties.

Typically the fibres of the prepreg wifi be substantially impregnated with the resin. For example, pitpregs with aresin contentof from 30 to4S wt% arepitferred.

The resin in the prepreg is also preferably present as an essentially fibre-free layer which may be adjacent to the structural fibre layer. When a plurality of such prepiegs are laid together, the fibre-free resin layers form the interleaf layers between the structural fibre layers.

As discussed above, the prcplegs according to the invention are intended to be laid up with other prepregs, to form a curable stack of prepregs.

Thus, in a second aspect, the invention relates to a curable stack of prepregs, the stack comprising a plurality of layers of structural conductive fibres and a plurality of curable thermosetting resin interleaf layers substantially free of structural fibres, wherein at least one interleaf layer comprises an open-structured metal-coated polymeric sheet Such a stack may comprise from 4 to 20 layers of structural conductive fibres with most or all of the layers separated by a curable thermosetting resin interleaf layer. Suitable interleaf arrangements are disclosed in EP0274899.

Such interleaf layers preferably have a mean thickness of from 15 to 50 micrometres.

Typically a plurality of the interlayers comprise an open-structured metal-coated polymeric sheet. The polymeric sheet may comprise a thermoplastic polymer, a thermoset polymer and/or a combination of the aforesaid polymers. Preferably, the polymeric sheet or veil comprises a polyamide thermoplastic polymer. In a preferred embodiment at least half of the interleaf layers comprise an open-structured metal-coated polymeric sheet. It may even be desirable for at least 75% of the interleaf layers to comprise such a sheet, or even substantially all of the interleaf layers.

Typically, the fibres in the prepreg stack will be substantially impregnated with the resin.

For example, prepreg stacks with a resin content of from 30 to 45% are preferred.

It has been found to be advantageous, in the eventual cured composite laminate, that the metal-coated polymeric sheet is located at or in the interleaf layer. However, during the heating stage prior to cure, the thermosetting resin has a reduced viscosity which tends to encourage the movement of the open-structural sheet into the interleaf layer. Thus, it is only necessary in the prepreg or prepreg stack for the open-structured metal-coated polymeric sheet to be in contact with the resin layer, and not necessarily embedded therein.

It is therefore believed that the improvement in z-direction conductivity arises due to the open-structured metal-coated polymeric sheet making points of contact with adjacent layers of structural conductive fibres, by being located in the interleaf layer.

The metal coating may be selected from a variety of electrically conducting metals, for example, silver, copper, nickel, gold, platinum, aluminium and mixtures thereof. A preferred material is silver.

The open-structured sheet is preferably made from polymeric fibres, arranged in a wide variety of ways, for example, random, woven, spunlaced, knitted or laid scrim.

In this embodiment essentially all of the fibres are coated in a metal, so that the sheet has a metallic appearance.

The open-structured metal-coated polymeric sheet may be made by coating an open-structured polymeric sheet with a suitable metal coating by any suitably known method in the art such as vapour deposition, sintering, vacuum deposition, sputtering or electroless plating.

Typically the thickness of the metal coating will be less than 1.0 micrometres, typically ranging from 001 to 1.0 niicrometres, more preferably from 0.1 to 1.0 micrometres.

The polymeric sheet may comprise fibres having a thickness ranging from 5 to 40 microns, preferably from 10 to 20 micrometers.

The invention also provides the surprising effect that despite the presence of the open-structural metal-coated polymeric sheet in the interlayer, the improved toughness in the eventual cured composite laminate provided by the interleaf layer is not negatively impacted.

Utilising a polymeric material as the bulk of the material of the sheet is believed to provide a number of advantages. Firstly, such a material can be produced as a very lightweight material in view of the low material densities of most polymers. Secondly, it is believed that the polymeric material can positively contribute to the toughness of the eventual composite laminate.

Thus, rather than the open-structured sheet reducing the toughness of the eventual cured laminate, the polymeric nature of the sheet improves the toughness.

Suitable examples of polymers include polyamides, such as PA 12, PAll, PA616 and PA6, polyesters such as PET and PBT, polysulfones, polyether sulfones, polyimides, such as the Ultems, polyacrylonitrile and polyphenyl suiphide. Also, Vectran (liquid crystal PET), phenoxy (grilon MS), TPU (thermoplastic polyether polyurethane), (p-phenylenebenzobisoxazole) (PBO fibres), poly(p-phenylenebenzobisthiazole) (PBT or PBZT fibres) and also fibres made using copolymers PA6IPA 12 and mixtures thereof are

suitable.

The polyamides may comprise a melting point in the range of 170 to 280 °C, preferably from 250 to 260 °C.

A further advantage of the open-structured sheet is that it is believed to assist in maintaining an evenly spaced interleaf layer. This is because the sheet can act as a spacer between adjacent conductive fibre layers.

Thus, the open-structured sheet is typically quite thin, having a thickness in the range of from 5 to 30 micrometres, preferably from 10 to 20 micrometres. As it is important that the open-structured sheet forms contacts with adjacent structural conductive fibre layers, preferably the thickness is from 50 to 100% of the mean thickness of the interleaf layer.

It has also been found that it is beneficial that the open-structured sheet has a thickness which does not significantly vary throughout the sheet. This can, for example, be achieved by compressing the sheet prior to formation of the prepreg, for example by calendering. In this way the thickness of the sheet can be kept within tight tolerances so that the difference between the maximum and minimum thickness is less than 1.0 micrometre, The open-structured sheet may be further characterised by the degree of openness of the sheet, i.e. the percentage of an average surface area of the sheet which is made up of open holes in the sheet. The openness may also be calculated as the percentage of open areas in 1 m2 from the mesh size, fiber thickness and number of meshes in 1 m2. The open-structured sheets may have a degree of openness of from 10 to 90%, preferably from 20 to 80%, more preferably from 30 to 70%.

It is common in the art for such interleaf layers to contain thermoplastic particles, which are believed to contribute to the toughness of the eventual cured composite material. In the present invention, such particles may be employed if desired, or alternatively they may be omitted. It is believed that the polymer of the open-structured sheet can provide much, if not all, of the toughness provided by such thermoplastic particles, reducing or eliminating the toughness benefits provided by the thermoplastic particles.

Thus, the invention allows for the possibility of a further improvement, in that thermoplastic toughening particles can be omitted. This can further simplify processing difficulties involved in handling, blending and distributing such particles. Thus, preferably the fibre-free resin layer in the prepreg, and therefore also the interleaf layer in the prepreg stack, is substantially free of thermoplastic particles.

It is highly preferable if the open-structured sheet is fairly lightweight. This is all the more important in embodiments which comprise many layers of such material.

Surprisingly it has been found that the improvements in conductivity and toughness can be achieved with such a light sheet. Thus, preferably the open-structured metal-coated polymeric sheet has a weight of from 5 to 20 grams per square metre (gsm), preferably from 5 to 50 gsm. from 5 to 30 gsm, or from S to 20 gsm. The uncoated metal coated polymeric sheet has a weight ranging from 2 to 45 gsm, preferably from 2 to 40 gsm, more preferably from 2 to 35 gsm, and most preferably from 2 to 18 gsm, and/or combinations of the aforesaid ranges.

The prepregs of the present invention are themselves fairly thin, for example being from 0.2 to 5.0 mm thickness, more preferably from 0.5 to 2 mm, and even more preferably from 0.2 to 1 mm thickness, and/or combinations of the aforesaid ranges.

We have further found that the toughness of a laminate comprising the curable prepreg of the invention is improved in comparison to a laminate comprising a curably prepreg in the absence of a polymeric sheet.

The fibres in the structural fibre layers may be uni-directional, fabric form or multi-axial.

Preferably the fibres are uni-directional and their orientation will vary throughout the prepreg stack and/or laminate, for example by arranging for the fibres in neighbouring layers to be orthogonal to each other in a so-called 0/90 arrangement, signifying the angles between neighbouring fibre layers. Other arrangements such as 01+451-45190 are of course possible among many other arrangements.

The fibres may comprise cracked (i.e. stretch-broken), selectively discontinuous or continuous fibres.

The conductive fibres may be made from a wide variety of materials such as metallised glass, carbon, graphite, metallised polymers and mixtures thereof. Carbon fibres are preferred.

The thermosetting resin 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), bisrnaleimide, 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. Preferred difunctional epoxy resins include diglycidyl ether of Bisphenol F (e.g. Araldite GY 281), diglycidyl ether of Bisphenol A, diglycidyl dihydroxy naphthalene and mixtures thereof. A highly preferred epoxy resin is a trifunctional epoxy resin having at least one meta-substituted phenyl ring in its backbone, e.g. Araldite MY 0600. A preferred tetrafunctional epoxy resin is tetraglycidyl diamino diphenylmethane (e.g. Araldite MY721). A blend of di-and tn functional epoxy resins is also highly preferred.

The thermosetting resin may also comprise one or more curing agent. Suitable curing agents include anhydrides, particularly poly carboxylic anhydrides; amines, particularly aromatic amines e.g. l,3-diaminobenzene, 4,4'-diaminodiphenylmethane, and particularly the sulphones, e.g. 4,4'-diaminodiphenyl sulphone (4,4' DDS), and 3,3'-diaminodiphenyl suiphone (3,3' DDS), and the phenol-formaldehyde resins. Preferred curing agents are the amino suiphones, particularly 4,4' DDS and 3,3' DDS.

Once formed, the prepreg or prepreg stack is cured by exposure to elevated temperature and optionally elevated pressure, to produce a curable laminate. Known methods of curing may be employed such as the vacuum bag, autoclave or press cure methods.

The cured laminates produced have remarkably low electrical resistance, with a 3mm thick laminate having an electrical resistance of less than 5«=, preferably less than 2Q, less than IQ, or even less than O.5 being possible, as measured in the z-direction according to the test method described below, The cured laminates further have an improved toughness in comparison to cured laminates in which the polymeric sheet is absent, The present invention is particularly suitable for applications in the aerospace industry, particularly in the formation of aircraft body panels.

As well as lightning strike resistance, it is also desirable to reduce or prevent a phenomenon known as "edge glow" following a lightning strike. This is caused by a build up of electrical charge at the ends of a composite structure and can become a source of ignition.

It has been found that composite materials for use in aircraft body structures can suffer from such edge glow problems. This is a particularly hazardous problem if composite materials are intended to form part of a fuel tank construct.

Thus, the invention is ideally suited to provide a cured laminate composite component of an aircraft fuel tank construct.

The invention will now be illustrated by way of example and with reference to the following figures, in which Figure 1 is an image of a cross-section through a cured composite laminate according to the invention.

Figure 2 is an image of a cross-section through another cured composite laminate according to the invention.

Examples

Resistance of composite laminates test method A panel is prepared from prepreg by autoclave cure that is 300mm x 300mm x 3mm in size. The layup of the panel is 0/90. Specimens (typically three to four) for testing are cut from the panel at sizes of 36mm x 36mm. The square faces of the specimens are sanded (for example on a Linisher machine) to expose the carbon fibres. This is not necessary if peel ply is used during the cure, Excess sanding is avoided as this penetrates past the first ply. The square faces are then coated with an electrically conductive metal, typically a thick layer of gold via a sputterer. Any gold or metal on the sides of the specimens is removed by sanding prior to testing. The metal coating ensures low contact resistance.

A power source (TTi EL3O2P programmable 30V/2A power supply unit, Thurlby Thandar Instruments, Cambridge, UK) that is capable of varying both voltage and current is used to determine the resistance. The specimen is contacted with the electrodes (tinned copper braids) of the power source and held in place using a clamp whereby it is ensured that the electrodes do not touch each other or contact other metallic surfaces as this will give a false result. The clamp has a non-conductive coating or layer to prevent an electrical path from one braid to the other. A current of 1 A is applied and the voltage noted. Using Ohm's Law resistance R can then be calculated (R=V/I). The test is carried out on each of the cut specimens to give a range of values. To ensure confidence in the test each specimen is tested two times.

Example 1

Eight plies of a M21E prepreg (available from Hexcel) were laid up (0/90). Between each ply was placed polyamide veil (PA 6/6) that had been coated with silver and the calendered to produce a material that was both lightweight and very thin. The veil had a weight of 15 gsm.

The prepreg was cured in an autoclave at 180 °C and 3 bar pressure. The through thickness resistance was determined as outlined above and the result shown in the Table 1 below and compared to a Comparative Example made by the same process and having interleaf layers comprising resin but without a silver-coated veil.

Panel decriptou Through tháness re tanee {Obms) Comparative Example 7.0

Example 1 0.40

Table I

Images of cross-sections through the composite material of Example 1 are shown in Figures 1 and 2. The cross-sections show the veil acting as an interleaf with an average thickness of 25 micron. Also the veil is touching the carbon plies.

Example 2

8 plies of a M21E prepreg (available from Hexcel) were laid up to form a laminate of Sample A. This material was cured in an autoclave at 180 °C and 3 bar pressure. The through thickness resistance was determined as outlined above. 8 plies of a M2IE prepreg containing 0.5 wt% of solid carbon microspheres in the resin were laid up to form a laminate of Sample B. This material was also cured in an autoclave at 180 °C and 3 bar pressure. The through thickness resistance was again determined. Finally, 8 plies of M2 1 E prepreg with silver coated polyamide PA 12 polymeric sheets in the interleafs between the plies were laid up to form a laminate of Sample C. Again the through thickness was determined. The results are presented in the below Table 2.

Sample A 5-10 Sample B 0.4 Sample C 0.4

Table 2

Claims

Claims 1. A curable prepreg comprising a layer of structural conductive fibres and curable thermosetting resin and comprising an open-structured metal-coated polymeric sheet.
2 A prepreg according to claim 1, wherein the resin in the prepreg is also present as an essentially fibre-free layer.
3 A curable stack of prepregs, the stack comprising a plurality of layers of structural conductive fibres and a plurality of curable thermosetting resin interleaf layers substantially free of structural fibres wherein at least one interleaf layer comprises an open-structured metal-coated polymeric sheet.
4. A stack of prepregs according to claim 3, wherein at least half of the interleaf layers comprise an open-structured metal-coated polymeric sheet.
5. A prepreg or prepreg stack according to any one of the preceding claims, wherein the metal coating is selected from silver, copper, nickel, gold, platinum, aluminium and mixtures thereof, preferably silver.
6. A prepreg or prepreg stack according to any one of the preceding claims, wherein the thickness of the metal coating is less than 1.0 micrometers.
7. A prepreg or prepreg stack according to any one of the preceding claims, wherein the open-structured sheet has a thickness in the range of from 5 to 30 micrometres. preferably from 10 to 20 micrometres.
8. A prepreg or prepreg stack according to any of claims 2 to 7, wherein the open-structured sheet has a thickness of from 50 to 100% of the mean thickness of the resin or interleaf layer accordingly.
9. A prepreg or prepreg stack according to any one of the preceding claims, wherein the difference between the maximum and minimum thickness of the open-structured sheet is less than 1.0 micrometre.
10. A prepreg or prepreg stack according to any one of the preceding claims, wherein the open-structured sheet has a degree of openness of from 10 to 90%, preferably from 20 to 80%, more preferably from 30 to 70%.
11. A prepreg or prepreg stack according to any one of the claims 2 to 10, wherein the fibre-free resin layer or interleaf layer is substantially free of thermoplastic particles.
12. A prepreg or prepreg stack according to any one of the preceding claims, wherein the open-structured metal-coated polymeric sheet has a weight of from 5 to 20 grams per square metre.
13. A cured laminate obtainable by exposing a prepreg or prepreg stack according to any one of the preceding claims to elevated temperature and optionally elevated pressure.
14. A cured laminate according to claim 13, which, if 3mm thick, has an electrical resistance of less than 5), preferably less than 2), less than l«=, or even less than 0.5ft
15. A cured laminate according to claim 13 or claim 14, which is part of an aerospace structural component.
16. Use of an open structured metal-coated polymeric sheet in a curable prepreg comprising a layer of structural conductive fibres and a curable theirnosetting resin to increase the through thickness conductivity of the cured prepreg, the polymeric sheet comprising a thermoplastic polymer and the polymeric sheet being in contact with the curable resin.
17. Use according to claim 16, wherein the polymeric sheet is used in combination with multiple curable prepregs, said multiple curable prepregs forming one or more interleafs, the polymeric sheet being located in one or more of said interleafs.
18. Use according to claim 17, wherein the open-structured sheet has a thickness of from 50 to 100% of the mean thickness of the resin or interleaf.*.:r: INTELLECTUAL . ... PROPERTY OFFICE 16 Application No: GB 1110579.8 Examiner: Mr Rhys J. Williams Claims searched: 1-18 Date of search: 23 September 2011 Patents Act 1977: Search Report under Section 17 Documents considered to be relevant: Category Relevant Identity of document and passage or figure of particular relevance to claims X 1-18 US 2010/0264266 Al (TSOTSIS) See abstract particularly.Categories: X Document indicating lack of novelty or inventive A Document indicating technological background and/or state step of the art.Y Document indicating lack of inventive step if P Document published on or after the declared priority date but combined with one or more other documents of before the filing date of this invention.same category.& Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:Search of GB, EP, WO & US patent documents classified in the following areas of the UKCX: Worldwide search of patent documents classified in the following areas of the IPC B32B The following online and other databases have been used in the preparation of this search report WPI, EPODOC, RM26, RM25 International Classification: Subclass Subgroup Valid From B32B 0033/00 01/01/2006 B32B 0005/02 01/01/2006 B32B 0027/04 01/01/2006 Intellectual Property Office is an operating name of the Patent Office www.ipo.gov.uk