Search

US-20260125524-A1 - IMPROVED UNIDIRECTIONAL PREPREGS

US20260125524A1US 20260125524 A1US20260125524 A1US 20260125524A1US-20260125524-A1

Abstract

A curable prepreg comprising a structural layer of unidirectional electrically conductive fibres having interstices therebetween, having a first outer face and an essentially parallel second outer face, and comprising thermosetting resin impregnated within the structural layer and present within the interstices, and a first layer of thermosetting resin in contact with the first outer face of the structural layer, wherein the ratio of the electrical conductivity in the x-direction, parallel to the conductive fibres, to the electrical conductivity in the y-direction perpendicular to the conductive fibres, is less than 1000.

Inventors

  • Leela Sequeira
  • Dana Blair
  • Benjamin Mitchell
  • Michael Snape
  • Emily ROE

Assignees

  • HEXCEL COMPOSITES LIMITED

Dates

Publication Date
20260507
Application Date
20231025
Priority Date
20221026

Claims (20)

  1. 1 . A curable prepreg comprising a structural layer of unidirectional electrically conductive fibers having interstices therebetween, having a first outer face and an essentially parallel second outer face, and comprising thermosetting resin impregnated within the structural layer and present within the interstices, and a first layer of thermosetting resin in contact with the first outer face of the structural layer, wherein the ratio of the electrical conductivity in the x-direction, parallel to the conductive fibers, to the electrical conductivity in the y-direction perpendicular to the conductive fibers is less than 1000.
  2. 2 . A curable prepreg according to claim 1 , wherein the ratio of the electrical conductivity in the x-direction to the electrical conductivity in the y-direction is less than 500.
  3. 3 . A curable prepreg according to claim 1 , which comprises a second layer of thermosetting resin in contact with the second outer face of the structural layer.
  4. 4 . A curable prepreg according to claim 1 , wherein the ratio of the thickness of the structural layer to the total thickness of the first, and if present second outer resin layers, is 3:1 to 6:1.
  5. 5 . A curable prepreg according to any one of the preceding claims , wherein the fiber areal weight of the fibers in the structural layer is greater than 200 g/m 2 .
  6. 6 . A curable prepreg according to any one of the preceding claims , wherein the structural layer has a thickness, in the direction perpendicular to the first outer face and the second outer face, of greater than 300 μm.
  7. 7 . A prepreg according to claim 1 , wherein the total thickness of the first, and if present second outer resin layers, is greater than 50 μm.
  8. 8 . A curable prepreg according to claim 1 , wherein the resin in the first, and if present, second, outer resin layers comprise thermoplastic particles.
  9. 9 . A curable prepreg according to claim 8 , wherein the thermoplastic particles are present at a level of from 5 to 15 wt % based on the total resin in the prepreg.
  10. 10 . A curable prepreg according to any one of the preceding claims , wherein the electrically conductive fibers are selected from the list consisting of carbon fibers, metalized glass fibers, graphite fibers, metalized polymers and mixtures thereof.
  11. 11 . A curable prepreg according to claim 1 , wherein the first, and if present, second, outer resin layers comprise electrically conductive particles.
  12. 12 . (canceled)
  13. 13 . A curable prepreg according to claim 11 , wherein the electrically conductive particles are present at a level of from 5 to 15 wt % based on the total resin in the prepreg.
  14. 14 . A curable prepreg according to claim 11 , wherein the electrically conductive particles have a particle size of from 10 to 80 μm.
  15. 15 . A curable prepreg according to claim 1 , wherein the total thickness of the first, and if present second outer resin layers, is greater than 10 μm thicker than the size of the electrically conductive particles.
  16. 16 . A curable prepreg according to claim 1 , wherein the resin content in the structural layer is less than 30.0 wt %.
  17. 17 . A curable prepreg stack, comprising a plurality of curable prepregs according to claim 1 , thereby comprising a plurality of structural layers of conductive fibers and a plurality of resin interleaf layers formed by the first, and if present second, outer resin layer.
  18. 18 . A cured composite material, obtainable by the process of exposing a curable prepreg or curable prepreg stack according to claim 1 to elevated temperature and optionally elevated pressure, to thermally set the thermosetting resin and thereby produce the cured composite material.
  19. 19 . A cured composite material according to claim 18 , which forms an aircraft component.
  20. 20 . A method of preparing a curable prepreg, said method comprising: i) providing a plurality of substantially parallel unidirectional electrically conductive fibers having interstices therebetween; ii) impregnating the fibers and the interstices with a thermosetting resin, thereby forming a structural layer having a first outer face and a substantially parallel second outer face; and iii) applying a first layer of thermosetting resin to the first outer face of the structural layer, thereby forming the curable prepreg; wherein the ratio of the electrical conductivity in the x-direction, parallel to the conductive fibers, to the electrical conductivity in the y-direction perpendicular to the conductive fibers is less than 1000.

Description

TECHNICAL FIELD The present invention relates to curable prepreg comprising a structural layer of unidirectional electrically conductive fibres and resin, that provides improved electrical conductivity properties. BACKGROUND Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is widely used and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components. Prepregs, comprising a fibre or fabric arrangement impregnated with thermosetting resin such as epoxy resin, are widely used in the generation of such composite materials. The resin may be combined with the fibres or fabric in various ways. The resin may be tacked to the surface of the fibrous material, however more usually it partially or completely impregnates the interstices between the fibres. In a common arrangement, a discrete layer of resin remains unimpregnated on the external surface of the prepreg. Once manufactured, typically a number of plies of such prepregs are “laid-up” as desired and the resulting prepreg stack, i.e. a laminate or preform, is cured, typically by exposure to elevated temperatures, to produce a cured composite structure. Curing may be performed in a vacuum bag which may be placed in a mould for curing. Alternatively the stack may be formed and cured directly in a mould. When such a laminate is made from a plurality of prepregs that comprise a discrete resin layer, this results in fibre layers interleafed with discrete resin layers. Such an arrangement is known to provide desirable mechanical properties in any resulting cured composite material. However, lightning strikes on aircraft skins consisting of such composites will likely sustain damage due to energy concentrations. Among the physical phenomena observed from lightning strikes is a phenomenon known as “edge glow,” which describes the condition in which a glow of light, combined with particle or plasma ejections appears at the tips or ends of the carbon fibers in the exposed fiber surfaces of composite components in a composite structure. Edge glow is caused by voltage differences between conductive, composite layers, and typically occurs in high current density areas resulting from a lightning strike, where the voltage potential is at its maximum, such as the exposed fiber surfaces. Edge glow is a potential fuel ignition source when it occurs in areas containing fuel or fuel vapor such as in fuel tanks or near fuel lines (collectively referred to herein as “a fuel environment”). The phenomenon occurring at the edge is called “Edge Glow” and the one occurring on the surface is called “Surface Discharge”. Both can be considered being an ignition hazard. Furthermore, the presence of the interleaf layers, being electrically insulating, results in the electrical conductivity in the direction orthogonal to the surface of the laminate, the so-called z-direction, being low, which can exacerbate phenomena such as edge glow, and is generally accepted to contribute to the vulnerability of composite laminates to electromagnetic hazards such as lightning strikes. A lightning strike can cause damage to the composite material which can be quite extensive, and could be catastrophic if occurring on an aircraft structure in flight. This is therefore a particular problem for aerospace structures made from such composite materials. This edge glow phenomenon may therefore appear during a lightning strike event, especially on composite laminates having low z-direction electrical conductivity. During a lightning strike event, a transient charge with high intensity current travels through the skin and then enters the wing substructure (e.g. structural spar or ribs) because of the fasteners connecting the two composite parts. So typically, in a composite skin/spar assembly, current travels partially on the skin and partially through the spar which represents one of the walls of the fuel tank. The current passes laterally from the fasteners through adjacent composite plies of the spar and tends to travel along the fibers because of the higher electrical conductivity as compared to the resin matrix. This path may generate the typical bright glow or sparks at the spar/rib cap edge, providing the “edge glow” phenomenon. Additionally, composites for use in aerospace applications must meet exacting standards on mechanical properties. Thus, any improvements in conductivity must not impact negatively on mechanical properties. A wide range of techniques and methods have been suggested in the prior art to provide electrical conductivity to the z-direction of such composite materials. WO 2008/056123 discloses how improvements have been made in conductivity by adding hollow conductive particles in the resin interleaf layers, so that they contact the adjacent fibre layers and create