GB2408012A

GB2408012A - A composite panel

Info

Publication number: GB2408012A
Application number: GB0326484A
Authority: GB
Inventors: Robert Charles Backhouse
Original assignee: Trysome Ltd
Current assignee: Trysome Ltd
Priority date: 2003-11-13
Filing date: 2003-11-13
Publication date: 2005-05-18
Anticipated expiration: 2023-11-13
Also published as: GB2408012B; GB0326484D0

Abstract

A composite panel 100 for example an automotive body panel comprises a first layer of two materials 2 and 4 with different thermal expansion characteristics such that a surface relief of the first layer is changeable in response to varying temperature. A sheet 20 is bonded over the surface of the first layer, wherein the sheet is deformable to accommodate changes in surface relief of the first layer. The two materials of the first layer may be an absorptive matrix such as carbon fibres and a two part epoxy resin such as cycloaliphatic amine. The fibres 3 may be a warp knitted polymer fabric with a polyester flow media incorporating thermoplastic binder threads. The sheet may be a thermally stable polyacrylonitrile fleece which is blended with a polymer and bonded to the first layer by a thermally activated system. Other embodiments relate to a method of manufacturing said panel and a process for obtaining said panel.

Description

COMPOSITE ARTICLE

The invention relates to a composite panel and a method of manufacturing a composite panel, in particular but not exclusively for composites comprising carbon fibre for use in automotive exterior body components.

There are various known materials for and methods of forming composite structural panels, particularly panels intended for use in the highperformance automotive field.

For such panels one significant advantage is their low mass compared with conventional steel or aluminium panels manufactured by pressing or superplastic forming (SPF).

Such composite materials comprise two or more materials integrated together.

Typically the composites comprise high-strength fibres (such as carbon fibre, glass fibre or aramid fibre) in a resin matrix. Other composites have a laminate structure.

The composite panels are generally painted to improve their appearance and to achieve a desired finish to the panel.

A composite material has an "equilibrium temperature" at which the two components are in equilibrium such that the differential stresses due to thermal expansion mismatch are zero. Such a composite is typically cured at an elevated temperature at which the composite will transform from liquid to a solid, thereby setting the structure at its equilibrium state. In this way, the two materials together can form a smooth outer surface when appropriately moulded. if subsequently heated to a temperature above or below this equilibrium temperature, one of the materials will expand or contract more than the other, thus creating unevenness in the outer surface. For a fibre-resin composite, the equilibrium temperature is typically over 1 00 C. Thus since the composite is set at the equilibrium temperature, at temperatures above or below this temperature, the composite will not be in equilibrium and will deform due to one of the components expanding or contracting more than the other.

Such articles tend to be subjected to a wide variation in temperature in use. For example, the in-use temperature of the exterior of a car can vary from around -50 C to 105 C depending on the climate in which it is used. Most paints begin to soften at around 65 C, which is typically lower than the equilibrium temperature of the composite material. When the paint softens, the underlying composite structure becomes visible to the eye and since the surface of the composite material has already become uneven due to its being below its setting temperature, this unevenness will be visible through the softened paint. This particular problem does not occur with a metal panel for example, since a metal panel effectively only contains one material. If plastic deformation occurs in the composite material, this "fabric print-through" will be permanent and result in a visually deficient product.

It would be desirable to provide a thermally stable composite article that may be considered to yield what is termed a class-A surface finish component, most preferably over a wide temperature range. This high quality finish would ideally include properties such as an ability to be painted, impact resistance and durability.

According to a first aspect of the present invention, there is provided a composite panel comprising: a first layer comprising two materials having different thermal expansion characteristics such that a surface relief of the first layer is changeable in response to varying temperature; and a sheet bonded over the surface of the first layer, which sheet is deformable to thereby accommodate changes in surface relief of the first layer.

According to a second aspect of the present invention, there is provided a method of manufacturing a composite panel, the method comprising the steps of: providing a first layer comprising two materials having different thermal expansion characteristics such that a surface relief of the hrst layer is changeable in response to varying temperature; and bonding a sheet over a surface of the first layer, which sheet is deformable to thereby accommodate changes in surface relief of the first layer.

According to a third aspect of the present invention, there is provided a composite panel obtainable by a process comprising the following steps: providing a first layer comprising two materials having different thermal expansion characteristics such that a surface relief of the first layer is changeable in response to varying temperature; and bonding a sheet over a surface of the first layer, which sheet is deformable to thereby accommodate changes in surface relief of the first layer.

The sheet suitably accommodates changes in relief of the first layer such that the surface of the sheet opposite the surface by which it is bonded to the first layer exhibits less change in surface relief with varying temperature than does that surface of the first layer. In this way the presence of the sheet can reduce visibility of the said changes at the surface of the sheet opposite to a surface adjacent the first layer.

The sheet is applied in sheet form to the first layer. Thus the sheet is formed as one or more integral sheet structures which are subsequently bonded to the first layer.

The sheet is thus distinguished from paint and the like, which is applied in liquid form.

The sheet is preferably a fibrous sheet, most preferably a fleece sheet. The sheet is preferably elastically deformable. The sheet is typically an un-carbonised or partially carbonised Polyacrylonitrile (PAN) needle punched random fleece, typically in the range of 75-100g/m2 areal weight. Such material is typically employed in sound and thermal insulation in automotive engine compartments.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows an example of a laminate construction to which the method of the invention can be applied; Figure 2 shows a schematic representation of the preforming and resin infusion process of an automotive body panel to which the invention can be applied; and Figure 3 shows an example of a laminate construction in accordance with the invention.

In the figures, like reference numerals indicate like parts.

Figure 1 shows an example of a laminate structure suitable for carrying out embodiments of the invention, labelled generally with reference numeral 1. The structure has three layers: layer 2 is a i45 warp Sniffed carbon reinforcement fabric, layer 3 is a layer of warp knitted polymer fabric which in this example is a polyester flow media incorporating thermoplastic binder threads, and layer 4 is another +45 warp knitted carbon reinforcement fabric. Layers 2 and 4 are either woven or preferably stitched multi-axial fabric. It can be seen that in this case layer 2 is arranged with the stitching vertically in the figure whereas layer 4 is arranged with the stitching horizontally, for reasons of panel balance and symmetry. The multidirectional reinforcement improves the mechanical properties of the resulting composite. The structure 1 is a resin permeable construction and the layer 3 is preferentially permeable over layers 2 and 4.

As mentioned above, the layer 3 of warp knitted polymer fabric contains a proportion (typically 0.5 to 5%) of low-melting point binder threads or powder. In this embodiment there are provided binder threads of low-melting point thermoplastic polyester or polyamide which serve to bind the surrounding reinforcement fabric in layers 2 and 4 under the application of applied heat and pressure.

The structure of figure 1 is manufactured by laminating the pre-woven or pre-knitted sheets together.

A general process for turning the structure of figure 1 into an automotive body panel will now be described with reference to figure 2. The various steps are shown in the seven sub-figures as follows: (a) A portion of the structure 1 of figure 1 is taken and cut using Computer Numerical Control (CNC) to roughly the correct size and shape for the body panel to be made.

The structure is substantially flat at this stage.

(b) The cut portion of the structure 1 is located over a male preform tooling 6. This mould tool 6 represents the impression of the desired body panel surface.

(c) The structure 1 is formed to shape over the tooling 6 using a hot vacuum technique. The structure is covered by an air-tight membrane 12 and forced against the tooling 10 by drawing a vacuum between the tooling and the membrane.

(d) The shaped structure 1 is then placed in a female preform tooling 10, which has the equivalent concave shape to the convex shape of the male tooling 6.

(e) The structure 1 is encapsulated by a flexible membrane 12 laminated onto the female mould tool 10 and infiltrated by liquid resin using a Resin Infusion technique that involves applying a vacuum across the encapsulated assembly such that resin is infused into the structure 1. Thus a resin-infused panel 14 is manufactured.

The encapsulated structure 14 is cured in an oven whilst still under a vacuum.

(9) The flexible membrane 12 is removed and the cured structure 14 is cut to its final shape using, for example, a CNC technique. The panel is then removed from the female mould 10.

The liquid resin used in this embodiment is a two-part epoxy resin, high purity Bisphenol-F Epoxy resin cured with Isophoronediamine, a cycloaliphatic amine. The resin is preferably mixed in the correct stoichiometric ratio. Any suitable thermosetting resin could be used instead, epoxy resins being especially suitable.

The resulting panel is suitable for subsequent application of a protective coating.

The most commonly-used protective coating is paint or a coating comprising paint.

However, if such a coating is applied directly to the structure 14, problems can occur in use due to differential thermal expansion of the structure 1 and the liquid resin e.g. surface bumps caused by expanded areas of resin. Any disruption of the surface of the structure 14 can become permanently visible through the protective coating.

In order to address the above-discussed problem, in accordance with the invention, a further layer is applied to the structure 1, resulting in a structure 100 as shown in figure 3.

The structure 100 includes in addition to the layers of the structure 1 a sheet 20 of partially carbonised polyacrylonitrile fleece. Suitable fleece can be obtained from the suppliers: Eswegee or Freudenburg, as PAN or pre-oxidised PAN fleece. The sheet is applied to the outer surface of the layer 2 i.e. the surface which is not adjacent to the layer 3. The density of the sheet 20 is 20-12Og/m2 but preferably 80-10Og/m2.

This compares to a density for layers 2 and 4 of around 44Og/m2 areal weight, and for layer 3 10Og/m2 areal weight. The purpose of the sheet 20 is to create a relatively homogeneous fibre reinforced surfacing layer. The sheet 20 is thermally stable and provides an advantageous surface for the application of protective coatings such as paint systems.

Furthermore the said fleece may be a blend of partially carbonised polyacryonitrile with another polymer so as to enhance the impact resistance. The sheet 20 is adhered to the layer 2 by application of a thermally activated binding system and subsequent heating. A suitable polymer is thermoplastic Polyester and a suitable binding system is Polyamide binder. Other polymers and binding systems could be used.

The sheet 20 is chosen to be mechanically compatible with the underlying surface and is therefore formed of a similar material to that of the layers 2, 3, 4. In other words the sheet 20 is elastic and is thereby deformable so as to accommodate changes in the underlying structure of layer 2. The sheet 20 is approximately 0.5mm, which is substantially the same thickness as each of the other layers 2, 3 and 4).

Once formed, the structure 100 can be used in the process of figure 2 to form a panel of the desired shape. Thus during step (e) of figure 2, the fleece is infiltrated with resin together with the rest of the structure 100.

The above-described embodiment can be varied within the scope of the invention.

For example, instead of using an Rl system to infiltrate the structure with resin, other prior-art techniques can be used such as Resin Transfer Moulding (RTM), VARTM (Vacuum Assisted RTM) or VARI (Vacuum Assisted Resin Injection). Instead of the flexible membrane used in figure 2 (e) another rigid or semi-rigid tool could be used to encapsulate the structure 1.

The sheet 20 could be applied to both surfaces should this be desired.

An alternative to the above-discussed fleece would be another fabric type such as a fine woven cloth layer. The fleece has the advantage of being conformable to complex surfaces i.e. it yields easily.

The fleece is furthermore compatible with the other carbon fibre layers since PAN is the precursor to carbon fibre manufacture. It is also low cost and in the needle punched fleece form is very conformable to complex shapes.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A composite panel comprising: a first layer comprising two materials having different thermal expansion characteristics such that a surface relief of the first layer is changeable in response to varying temperature; and a sheet bonded over the surface of the first layer, which sheet is deformable to thereby accommodate changes in surface relief of the first layer.

2. A composite panel according to claim 1, wherein the first layer is formed of a material which is mechanically compatible with a material of which the sheet is formed.

3. A composite panel according to claim 1 or claim 2, wherein the two materials of the first layer are an absorptive matrix and a resin.

4. A composite panel according to claim 3, wherein the absorptive matrix comprises carbon fibres.

5. A composite panel according to claim 4, wherein the fibres comprise warp knitted carbon reinforced fabric.

6. A composite panel according to claim 5, wherein the fibres comprise a polyester flow media incorporating thermoplastic binder threads.

7. A composite panel according to any of claims 3 to 6, wherein the resin is an epoxy resin.

8. A composite panel according to claim 7, wherein the resin is a twopart epoxy resin.

9. A composite panel according to claim 7 or claim 8, wherein the resin comprises a cycloaliphatic amine.

10. A composite panel according to any preceding claim, wherein the sheet is at least partially carbonised.

11. A composite panel according to any preceding claim, wherein the sheet is substantially thermally stable.

12. A composite panel according to claim 10 or claim 11, wherein the sheet comprises a polyacrylonitrile fleece.

13. A composite panel according to claim 12, wherein the polyacrylonitrile fleece is needle punched random fleece.

14. A composite panel according to claim 12 or claim 13, wherein the sheet is blended with a polymer.

15. A composite panel according to any preceding claim, wherein the first layer and the sheet are adhered together by means of a binding system which has been thermally activated.

16. A composite panel according to any preceding claim, further comprising a coating applied to the surface of the sheet opposite to a surface adjacent the first layer, which softens at elevated temperature and re-hardens on cooling.

17. A composite panel according to claim 16, wherein the coating comprises paint.

18. A composite panel according to any preceding claim, for use as a vehicle body component.

19. A composite panel substantially as herein described with reference to the accompanying drawings.

20. A method of manufacturing a composite panel, the method comprising the steps of: providing a first layer comprising two materials having different thermal expansion characteristics such that a surface relief of the first layer is changeable in response to varying temperature; and bonding a sheet over a surface of the first layer, which sheet is deformable to thereby accommodate changes in surface relief of the first layer.

21. A method according to claim 20, comprising the further step of coating the surface of the sheet opposite to a surface adjacent the first layer with a coating that softens at elevated temperature and re-hardens on cooling.

22. A method according to claim 21, wherein the coating comprises paint.

23. A method substantially as herein described with reference to the accompanying drawings.

24. A composite panel obtainable by a process comprising the following steps: providing a first layer comprising two materials having different thermal expansion characteristics such that a surface relief of the first layer is changeable in response to varying temperature; and bonding a sheet over a surface of the first layer, which sheet is deformable to thereby accommodate changes in surface relief of the first layer. \