GB2481969A

GB2481969A - Sealing elements for use in fabrication of composite structures

Info

Publication number: GB2481969A
Application number: GB1011287.8A
Authority: GB
Inventors: Signe Bjerre Sorensen
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2010-07-05
Filing date: 2010-07-05
Publication date: 2012-01-18
Also published as: WO2012003834A3; WO2012003834A2; GB201011287D0

Abstract

An apparatus and method for fabricating a composite structure (68, figure 3) are described. A vacuum film (76, figure 3) covers the structure, and a sealing element 10 having a compressible body and an adhesive outer layer 18 of different material to compressible body is placed between the vacuum film and an opposed surface (70, figure 3) to create a substantially sealed volume (78, figure 3) containing the structure. A vacuum means is used to withdraw air from the substantially sealed volume to compress the sealing element and create a substantially airtight seal between the vacuum film and the opposed surface. The seal element may be tubular (figure 2B) or made of foam (figure 2E). It may have flat or curved surfaces (figure 2D). A different adhesive may be used in different regions of the seal. A first part (48, figure 2D) may be an acrylic adhesive and the second part (49, figure 2D) may be a silicone adhesive. This allows strong adhesion to the vacuum sheet and allows easier removal from a mould part.

Description

SEALING ELEMENTS FOR USE IN FABRICATION OF COMPOSITE STRUCTURES

Technical Field

The present invention relates to sealing elements for use in vacuum-bag fabrication processes for composite structures.

Background

Vacuum bag fabrication or vacuum bagging' is a process used to form reinforced composite structures such as modern wind turbine blades. The blades are typically moulded as two half shells, which are subsequently joined together.

The moulding process for a blade shell is illustrated schematically in Figure 1, and involves laying a stack 1 of structural fabric layers on the surface 2 of a mould 3. The structural fabric is typically glass-fibre fabric that is pre-impregnated with resin, and is known as prepreg'. The stack itself is generally referred to as a lay-up', and may additionally include other layers, for example a peel ply and/or a non-structural fabric layer on top of the structural layers. Next, a transparent plastic sheet 4, typically a sheet of polyamide (PA) e.g. nylon, or polyethylene terephthalate (PET), is placed over the lay-up 1 on the mould 3. The sheet 4 is commonly referred to as vacuum bagging film' or vacuum film', and will be referred to as vacuum film' hereafter.

As described in detail below, the vacuum film 4 is sealed against the mould 3 to eliminate air leaks and hence to create a substantially sealed volume 5 containing the laminate 1.

Air is then removed from the substantially sealed volume 5 using a vacuum pump (not shown) attached to a port 6 in communication with volume 5. The vacuum pump extracts air from the substantially sealed volume 5 and from the lay-up to create an effective vacuum, which causes the vacuum film 4 to apply pressure to the laminate 1.

The vacuum also causes the resin to flow and redistribute throughout the laminate 1.

Hot air, typically of up to about 160 Celsius, is circulated around the sealed laminate 1 to cure the resin. The vacuum and temperature conditions are maintained for a predefined duration, typically around 12-14 hours until curing is complete.

In order to create an airtight seal between the vacuum film 4 and the mould 3, a butyl rubber sealing tape 8 is used. The butyl rubber tape 8 is inherently tacky, and serves to adhere the vacuum film 4 to the mould 3 to create the seal. The butyl rubber 8 is also deformable, and compresses under the vacuum pressure to fill any small gaps between the film 4 and the mould 3 caused by any irregularities in these surfaces. This ensures that the seal is airtight, which enables a high vacuum to be created as required to remove all air from the substantially sealed volume 5 and from the lay-up.

As the vacuum film 4 and the sealing tape are disposed of once the moulding process is complete, it is preferable that the sealing tape 8 remains stuck to the film 4 rather than remaining stuck to the mould 3 when the film 4 is removed. However, once heated, the butyl rubber 8 develops a consistency similar to used chewing gum, and has a tendency to stick to the mould 3 in preference to the vacuum film 4. It can then be difficult to remove the sealing tape 8 from the mould 3 thereafter once the moulding process is complete.

To overcome this problem, the prior art solution (as shown in Figure 1) is to apply a protective adhesive tape 9 to the mould surface 2, and then to lay the butyl rubber sealing tape 8 over the protective tape 9. The protective tape 9 may be a fibreglass cloth having a PTFE coating on one side and a silicone adhesive on the other. The protective tape 9 is arranged with the silicone side in contact with the mould surface 2 and the PTFE side in contact with the butyl rubber 8. As PTFE is a non-stick material, the butyl rubber sealing tape 8 adheres more strongly to the vacuum film 4 than to the PTFE, which means that the butyl rubber tape 8 remains stuck to the vacuum film 4 when the film 4 is removed. The silicone adhesive allows the protective tape 9 to be removed cleanly from the mould 3 without leaving a residue. Flash tape', which is a polyester film with a silicone adhesive, is often used as a cheaper alternative to PTFE tape.

Aside from the fabrication of blade shells, butyl rubber tape is also used in the fabrication of blade spars, which are reinforcing structures that run longitudinally inside a modern wind turbine blade. Rather than being formed in a mould, spars may be formed by winding resin-impregnated fibres around a mandrel. A vacuum film is wrapped around this structure, and butyl rubber tape is used to seal any gaps between adjacent sections or surfaces of the vacuum film to create an enclosed volume containing the spar structure. A vacuum pump is used to withdraw air from the substantially sealed volume to create a vacuum in a similar way to that described above. Also, in common with the blade shell fabrication, hot air is circulated around the sealed lay-up and the pressure and temperature conditions are maintained until curing is complete.

Whilst butyl rubber sealing tape forms a satisfactory seal, it is expensive. Costs are further increased when protective tape is required to protect a mould. This includes the cost of the protective tape itself, and the time-related costs associated with the two-stage operation of applying the protective tape to the mould prior to applying the butyl-rubber tape. Another type of butyl rubber exists, which cures slightly when heated so that it can be removed more easily from the mould. However this type of butyl rubber is even more expensive. Accordingly, it is an aim of the present invention to provide an alternative solution that is effective but relatively inexpensive.

Summary of the invention

In accordance with the present invention, there is provided an apparatus for fabricating a composite structure, the apparatus comprising: a vacuum film for covering the structure; a sealing element between the vacuum film and an opposed surface to create a substantially sealed volume containing the structure; and vacuum means for withdrawing air from the substantially sealed volume; wherein the sealing element has a compressible body and an adhesive outer layer of different material to the compressible body.

The sealing element of the present invention is cheaper than butyl rubber tape, and equally effective at creating an airtight seal. Whereas butyl rubber acts as both an adhesive and as a filler, these functions are performed separately in the sealing element of the present invention. The adhesive layer adheres to the vacuum film and the opposed surface, whilst the body of the sealing element backs the adhesive layer and compresses when air is withdrawn from the substantially sealed volume to force the adhesive layer into any cracks, gaps or other discontinuities between the vacuum film and the opposed surface to create an airtight seal. Using different materials for the body and the adhesive allows the adhesive and filler properties of the sealing element to be optimised independently, by suitable choices of these materials, as described in more detail later.

Also in accordance with the present invention, there is provided a method of fabricating a composite structure, the method comprising: covering the structure with a vacuum film; placing a sealing element having a compressible body and an adhesive outer layer between the vacuum film and an opposed surface, the adhesive outer layer adhering to the vacuum film and the opposed surface to create a substantially sealed volume containing the structure; and withdrawing air from the substantially sealed volume to compress the sealing element and create a substantially airtight seal between the vacuum film and the opposed surface.

The inventive concept also encompasses the use of a sealing element having a compressible body and an adhesive outer layer for creating a seal between a vacuum film and an opposed surface during the fabrication of a composite structure.

The sealing elements described herein are suitable for use in the vacuum-bag fabrication of any composite structure. Preferably the composite structures referred to above are the shells or spars of a wind turbine blade.

As described by way of background to the present invention, blade shells are formed in a mould, and the sealing element is provided between the vacuum film and the mould surface. Hence, the opposed surface referred to above may be a mould surface.

Consequently, the apparatus may include a mould for defining the shape of the structure Also as described by way of background above, blade spars are not moulded, but may be formed instead by winding reinforcing fibres around a mandrel, and surrounding this structure with a vacuum film. The sealing element is used to form a seal between adjacent sections of the vacuum film. Hence, the opposed surface may be another section or surface of vacuum film. It will be appreciated that the vacuum film may be part ofavacuum bag.

The sealing element may be elongate, and is preferably in the form of a tape. The tape may be wound as a roll on a suitable reel. In cross-section, the sealing element may have a variety of shapes. Suitable shapes include circular, semi-circular, rectangular and triangular, although the skilled person will appreciate that other shapes may be suitable. For sealing elements of circular cross-section, the diameter is preferably 6-25 mm. For sealing elements of rectangular cross-section, the width of the sealing element is preferably 6-25 mm and the thickness or height is preferably 2-15 mm.

The body of the sealing element may be made of foam. The foam may be open-cell foam or closed-cell foam. Closed-cell foam is preferred because it is able to form an airtight seal.

The materials selected for the sealing element must be able to withstand the high temperatures during the curing process. In certain cases, the maximum curing temperature is approximately 80 Celsius, whilst in other cases the maximum curing temperature is around 165 Celsius. Suitable foams for use up to 80 Celsius include acrylic foam, polyurethane (PUR) foam; a copolymer comprising PE and another polymer. Preferably, polyethylene and ethylene vinyl acetate (PE-EVA) foam is used because it is easy to produce and relatively inexpensive. Whilst EVA foam would also be suitable, the inclusion of PE serves to reduce the cost of the foam. Other suitable foams for use up to 165 Celsius include styrene-butadiene rubber (SBR) foam, chloroprene rubber (CR) foam; ethylene propylene diene monomer (EPDM) foam and silicone foam.

The adhesive layer preferably includes a pressure sensitive adhesive (PSA), which bonds to the mould surface and/or the vacuum film when pressure is applied.

Advantageously, no solvent, water, or heat is needed to activate a PSA. Suitable adhesives include: acrylic adhesive, natural butyl rubber (NBR), methacrylate adhesive (MMA) and silicone adhesive. Preferably an adhesive is selected that does not leave a residue on the mould surface after use. Advantageously, therefore, an additional protective tape is not then required, thus further reducing costs.

When used to seal against a mould surface, it may be desirable for the sealing element to have differential adhesive properties. For example a first side or surface of the sealing element may include a first adhesive, whilst a second, opposed, side or surface may include a second, different, adhesive. The adhesives may be selected so that the sealing element adheres more strongly to the vacuum film than to the mould surface so that the sealing element remains stuck to the vacuum film in preference to the mould surface when the vacuum film is removed from the mould. An acrylic adhesive is particularly suitable for providing on one side of the sealing element to form a strong bond to the vacuum film. Preferably the adhesive to be in contact with the mould surface is selected so that it does not leave a residue on the mould surface. Silicone adhesive is suitable in this respect. Conveniently, and in contrast to the prior art, an additional protective tape is then not required.

In a preferred embodiment of the invention, one side of the sealing element has an acrylic adhesive, whilst an opposite side has a silicone adhesive. The sealing element is arranged in use with the silicone adhesive adjacent the mould surface and the acrylic adhesive adjacent the vacuum film. The acrylic adhesive adheres more strongly to the vacuum film than the silicone adhesive adheres to the mould surface. Consequently, the sealing element remains stuck to the vacuum film when it is removed from the mould.

The sealing element may have two or more sides of different size or shape. This allows the user to identify the correct orientation of the sealing element, especially when the sides carry different adhesives. For example, the sealing element may be of semi-circular cross-section, having a flat side and a curved side. The flat side may include a silicone adhesive, whereas the curved side may include an acrylic adhesive. As an alternative, colours or other markings could be used to identify sides or surfaces of the sealing element carrying a particular adhesive. For example, the sealing element may have opposite sides of different colour and carrying different adhesives.

The sealing element may have uniform adhesive properties about its outer periphery.

For example, the adhesive layer may comprise a single adhesive. This is desirable when the orientation of the sealing element is not important, for example when the sealing element is used to form a seal between adjacent sections of vacuum film, for example in the fabrication of blade spars. A sealing element of circular cross-section is particularly suitable for this purpose because it is easy to apply to a non-flat surface.

The sealing element may include a cavity running through the body and along the length of the sealing element. The cavity reduces the amount of material required for the body and hence reduces cost, whilst at the same time still allowing the body to compress as required to form a suitably airtight seal. The sealing element may be tubular. In one embodiment, the sealing element is annular in cross-section. In such examples, the radial thickness of the body is preferably 1-2 mm.

The sealing element may have a film layer between the foam and the adhesive. The film layer carries the adhesive and is suitably impermeable to air, which allows an airtight seal to be achieved between the mould surface and the vacuum film, or between adjacent sections of vacuum film. The adhesive layer itself may be sufficiently thick that it is impermeable to air, in which case an impermeable film layer may not be required. It has already been mentioned above that the airtight properties of the seal may be obtained through use of closed-cell foam. Consequently, suitable sealing elements may include any of the following elements, or any combination of these elements: (i) closed-cell foam; (ii) an adhesive layer that is impermeable to air; and (iii) a film that is impermeable to air between the adhesive and the body.

Brief description of the drawings

Figure 1, which is a schematic cross-sectional side view a prior art apparatus, has already been described above by way of background to the present invention.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the following figures, in which: Figures 2A to 2E are cross-sectional perspective views showing examples of sealing elements in accordance with the present invention; Figure 2A' is a schematic end view of the sealing element of Figure 2A; Figure 2B' is a schematic end view of the sealing element of Figure 2B; Figure 3 is a schematic cross-sectional side view on line Ill-Ill of Figure 4 showing an apparatus in accordance with the present invention for the vacuum-bag fabrication of a wind turbine blade shell, in which the sealing element of Figure 2D is used to form a seal between a mould surface and a vacuum film; Figure 4 is a plan view of the apparatus of Figure 3; Figure 4A is an enlarged view of part of Figure 4 near the root of the blade shell, showing an overlap between adjacent sections of the sealing element; Figure 4B is an enlarged view of part of Figure 4 near the tip of the blade shell, showing an overlap between adjacent sections of the sealing element; Figure 5A shows the sealing element between the mould surface and vacuum film before a vacuum has been created in a substantially sealed volume between the mould surface and the vacuum film; Figure SB shows the sealing element compressed once a vacuum has been created in the substantially sealed volume; and Figure 6 is a schematic cross-sectional view of a blade spar structure surrounded by vacuum film.

Detailed description

Figure 2A shows a first example of a sealing element 10 in accordance with the present invention. The sealing element 10 is elongate and, whilst only a section of the sealing element 10 is shown in Figure 2A, the sealing element 10 is in the form of a tape of indeterminate length. The sealing element 10 has a body 12 formed of open-cell polyethylene and ethylene vinyl acetate (PE-EVA) foam. The body 12 is circular in cross-section, and has a diameter (d) in this example of 20 mm. A cylindrical outer surface 14 of the body 12 supports a film layer 16 made of a thermoplastic polymer that is impermeable to air. Each surface of the film layer 16 carries a layer of acrylic adhesive. The layer of acrylic adhesive (not shown) on the inward-facing surface of the film layer 16 serves to bond the film layer 16 to the body 12, whilst the layer of acrylic adhesive 18 on the outward-facing surface of the film layer 16 defines the outermost surface of the sealing element 10. For clarity, the layers 16 and 18 are shown schematically in the end view of Figure 2A'. As described in further detail later, it is the impermeable film layer 16 that allows the sealing element 10 to create an airtight seal between a vacuum film and a mould surface, or between two surfaces of vacuum film in use.

Figures 2B and 2B' show a second example of a sealing element 20 also in the form of a tape. In common with the first example, this sealing element 20 has a body 22 of open-cell PE-EVA foam, which is of circular cross-section, again having a diameter (d') of mm. However, in contrast to the first embodiment, the body 22 of this sealing element 20 is tubular and defines a central elongate cavity 24 along the length of the sealing element 20. The elongate cavity 24 is of circular cross section, and has a diameter (d") of 16 mm. The radial thickness (r) of the foam is therefore 2 mm. The cavity 24 facilitates compression of the body 22, whilst at the same time reduces the material required for the body 22, thereby providing a cost saving.

A cylindrical outer surface 26 of the body 22 carries an adhesive layer 28, which is sufficiently thick to make it impermeable to air. Notably, there is no film layer between the foam body 22 and the adhesive layer 28. Instead, it is the impermeable adhesive layer 28 in this example that allows the sealing element 20 to create an airtight seal in use.

It will of course be appreciated that an impermeable adhesive layer could be used with the sealing element 10 of Figure 2A, which would mean that the impermeable film layer 16 may not be required. Instead, an impermeable adhesive layer could be applied to the cylindrical outer surface 14 of the foam body 12 in the same way as for the sealing element 20 of Figure 2B. It will also be appreciated that an impermeable adhesive layer may be used in combination with an impermeable film layer in either of the sealing elements 10, 20 of Figures 2A or 2B.

Figure 2C shows a third example of an elongate sealing element 30, also in the form of a tape. This sealing element 30 has a body 32 of rectangular cross section, which is formed of closed-cell PE-EVA foam. The closed-cell PE-EVA foam is impermeable to air. First and second opposed flat surfaces 34,36 of the body 32 carry an acrylic adhesive 38 that is not necessarily impermeable to air. Notably, there is no impermeable film layer between the foam body 32 and the adhesive layer 38 in this example. Instead, it is the closed-cell PE-EVA foam body 32 in this example that allows the sealing element to create an airtight seal in use.

It will of course be appreciated that suitable closed-cell foam could be used instead for the sealing elements 10, 20 of Figures 2A and 2B, in which case an impermeable film layer and/or an impermeable adhesive layer may not be required. It will also be appreciated that the closed-cell foam may be used in combination with an impermeable film layer and/or in combination with an impermeable adhesive layer in any of the sealing elements 10, 20, 30 shown in Figures 2A, 2B or 20.

Figure 2D shows a fourth example of an elongate sealing element 40, again in the form of a tape. This time, the sealing element 40 has a body 42 of semi-circular cross-section. In common with the third example above, the body 42 is formed of closed-cell PE-EVA foam, which makes the sealing element 40 impermeable to air. The semi-circular cross-section means that the sealing element 40 has a flat surface 44, or base, and a curved surface 46. The width (w) of the base 44 is approximately 12 mm, whilst the maximum thickness or height (h) of the sealing element 40 is approximately 4.5 mm.

A layer of silicone adhesive 48 is provided on the base, whilst a layer of acrylic adhesive 49 is provided on the curved surface 46. The two adhesives 48,49 provide the sealing element 40 with differential adhesive properties. As described in more detail later, the sealing element 40 may be arranged in use with its base 44 adjacent a mould surface, and its opposed curved surface 46 adjacent a vacuum film. The silicone adhesive 48 adheres less strongly to the mould surface than the acrylic adhesive 49 adheres to the vacuum film. Consequently, the sealing element 40 remains stuck to the vacuum film when the film is removed from the mould.

Figure 2E shows a fifth example of an elongate sealing element 50, again in the form of a tape. In common with the fourth example above, the sealing element 50 has a body 52 of semicircular cross-section formed of closed-cell PE-EVA foam, which has a flat base 54 carrying a layer of silicone adhesive 56 and a curved surface 58 carrying a layer of acrylic adhesive 59. However, in contrast to the fourth example, the body 52 of this sealing element 50 defines an elongate cavity 60 running along the length of the sealing element 50. In this example, the elongate cavity 60 is substantially circular in cross section, but it could be of semi-circular cross-section, or indeed of any other suitable shape. As with the tubular example 20 shown in Figure 2B, this cavity 60 facilitates compression, whilst at the same time reduces the material required for the body 52, thereby providing a cost saving.

The closed-cell foam comprises thousands of small bubbles 62 of various diameters. In the example shown in Figure 2E, the diameters of the bubbles 62 generally decrease towards the outer periphery 64 of the body 52, with the largest-diameter bubbles 62 surrounding the cavity 60.The bubbles 62 at the outer periphery 64 are sufficiently small to prevent air from penetrating the foam; this makes the foam body 52 airtight.

It will of course be appreciated that the sealing elements 40, 50 of Figures 2D and 2E could have bodies formed of open-cell foam, and/or that the or each adhesive may be carried by an impermeable film layer, and/or that the or each adhesive layer may be impermeable to air as described in the examples above.

Use of a sealing tape 40 of the variety shown in Figure 2D in the vacuum-bag fabrication of a wind turbine blade shell will now be described with reference to Figures 3 to 5.

Referring first to Figure 3, the composite lay-up process for a blade shell involves arranging a stack of prepreg glass-fibre fabric layers and one or more layers of core material (e.g. foam) in a mould 66 to create a blade shell lay-up 68. The sealing tape 40 is arranged on the mould surface 70 around the blade shell lay-up 68, with the flat base 44 of the tape 40 in contact with the mould surface 70. In this way, the silicone adhesive on the flat base 44 adheres to the mould surface 70.

Referring to Figure 4, the tape 40 is used to surround the blade shell lay-up 68. The tape can follow a gentle curvature around the lay-up 68. However, where the radius of curvature of the lay-up 68 is relatively small, or where it defines corners, it may be necessary to use several straight sections of tape 40 in overlapping relation. This can be seen in Figures 4A and 4B, which show overlapping sections of tape 40 surrounding the root 72 and tip 74 ends of the blade shell lay-up 68 respectively.

Once the sealing tape 40 has been arranged on the mould surface 70 around the blade shell lay-up 68, a vacuum film 76 is laid over the structure 68. Referring again to Figure 3, the vacuum film 76 is in contact with the curved side 46 of the sealing tape 40, and the acrylic adhesive on the curved side adheres to the vacuum film 76. The vacuum film 76 and mould surface 70 define a substantially sealed volume 78 containing the blade shell lay-up 68.

Next, a vacuum pump (not shown) is used to extract air from the substantially sealed volume 78 to create a vacuum. Withdrawing air from the substantially sealed volume 78 causes the vacuum film 76 to bear down against the mould surface 70 and compress the sealing tape 40. In this way, the sealing tape 40 is able to fill any cracks, gaps or other discontinuities between the mould surface 70 and the vacuum film 76 as described below, by way of example, with reference to Figures 5A and SB.

Referring to Figure SA, this shows the sealing tape 40 before the vacuum pump has been switched on. A small, hairline crack 80 is present in the mould surface 70 below the base 44 of the sealing element 40. Referring now to Figure SB, when the vacuum pump is switched on, the vacuum film 76 bears down against the mould surface 70 and compresses the sealing tape 40. As it compresses, the foam body 42 of the sealing tape forces the silicone adhesive layer 48 into the hairline crack 80 to form a continuous, and hence airtight, seal against the mould surface 70. The ability of the tape 40 to compress and deform to fill gaps in this way is particularly important in regions where sections of tape 40 overlap, because of the large gaps between overlapping sections.

Referring again to Figure 3, hot air is circulated around the entire assembly and the vacuum is maintained whilst the assembly is heated under the requisite conditions until the resin has cured.

Once curing is complete, the vacuum film 76 is peeled away from the mould 66 and the cured blade shell. As the vacuum film 76 is peeled away from the mould 66, the sealing tape 40 remains stuck to the film 76 in preference to the mould surface 70 because the adhesion to the film 76 via the acrylic adhesive on the curved side 46 of the tape 40 is stronger than the adhesion to the mould surface 70 via the silicone adhesive of the base 44 of the tape 40. The film 76 and sealing tape 40 may now be disposed of. The silicone adhesive does not leave a residue on the mould surface 70, so the mould 66 is ready to be used again.

Referring to Figure 6, a different process is used to manufacture the blade spars, which involves winding resin-impregnated fibres 84 around a mandrel 86. A PET vacuum film 88 is then wrapped around this structure. The sealant tape 10 of Figure 2A, which is of circular cross-section, is used to seal any gaps 90 between opposed surfaces 92, 94 of the vacuum film 88 as shown. The circular cross-section makes the tape 10 easier to apply between these flexible surfaces 92, 94, and hence provides a handling advantage in this context. Vacuum and heat are then applied in much the same way as described above until the resin is cured. Importantly, this process does not use a mould, and hence a silicone adhesive is not required.

Whilst various examples of suitable sealing elements have been specifically described, it will be appreciated that many variants exist within the scope of the present invention as defined in the claims. For example, other suitable sealing elements may be of oval or triangular cross-section, or indeed of any other suitable shape. It will also be appreciated by those skilled in the art, that other suitable materials, including other foams or adhesives, may be used to achieve the benefits described herein.

Claims

Claims 1. Apparatus for fabricating a composite structure, the apparatus comprising: a vacuum film for covering the structure; a sealing element between the vacuum film and an opposed surface to create a substantially sealed volume containing the structure; and vacuum means for withdrawing air from the substantially sealed volume; wherein the sealing element has a compressible body and an adhesive outer layer of different material to the compressible body.
2. A method of fabricating a composite structure, the method comprising: covering the structure with a vacuum film; placing a sealing element having a compressible body and an adhesive outer layer between the vacuum film and an opposed surface, the adhesive outer layer adhering to the vacuum film and the opposed surface to create a substantially sealed volume containing the structure; and withdrawing air from the substantially sealed volume to compress the sealing element and create a substantially airtight seal between the vacuum film and the opposed surface.
3. Use of a sealing element having a compressible body and an adhesive outer layer for creating a seal between a vacuum film and an opposed surface during the fabrication of a composite structure.
4. An apparatus, method or use as claimed in any preceding claim, wherein the opposed surface is a mould surface such that the sealing element forms a seal between the vacuum film and the mould surface.
5. An apparatus, method or use as claimed in any preceding claim, wherein the opposed surface is defined by a section of vacuum film such that the sealing element forms a seal between two sections of vacuum film.
6. An apparatus, method or use as claimed in any preceding claim, wherein the sealing element is elongate.
7. An apparatus, method or use as claimed in Claim 6, wherein the sealing element is in the form of a tape.
8. An apparatus, method or use as claimed in any preceding claim, wherein the sealing element has a substantially circular cross section.
9. An apparatus, method or use as claimed in any one of Claims 1 to 8, wherein the sealing element has a substantially semi-circular cross section.
10. An apparatus, method or use as claimed in any preceding claim, wherein the sealing element is tubular.
11. An apparatus, method or use as claimed in any preceding claim, wherein the sealing element includes a cavity extending through the body
12. An apparatus, method or use as claimed in any preceding claim, wherein the sealing element has opposed surfaces of different shape.
13. An apparatus, method or use as claimed in Claim 12, wherein the sealing element has a first surface that is substantially flat.
14. An apparatus, method or use as claimed in Claim 12 or Claim 13, wherein the sealing element has a second surface that is curved.
15. An apparatus, method or use as claimed in any preceding claim, wherein the adhesive is a pressure-sensitive adhesive.
16. An apparatus, method or use as claimed in any preceding claim, wherein the adhesive outer layer is impermeable to air.
17. An apparatus, method or use as claimed in any preceding claim, wherein the sealing element has differential adhesive properties around the outer surface.
18. An apparatus, method or use as claimed in Claim 17, wherein the sealing element has a first adhesive on a first side and a second adhesive on a second side, the first adhesive having stronger adhesive properties than the second adhesive.
19. An apparatus, method or use as claimed in Claim 18, wherein the first adhesive is an acrylic adhesive.
20. An apparatus, method or use as claimed in Claim 18 or Claim 19, wherein the second adhesive is a silicone adhesive.
21. An apparatus, method or use as claimed in any preceding claim, wherein the compressible body is formed from foam.
22. An apparatus, method or use as claimed in Claim 21, wherein the foam is closed-cell foam.
23. An apparatus, method or use as claimed in any preceding claim, wherein the sealing element includes a film between the compressible body and the adhesive layer that is impermeable to air.
24. An apparatus, method or use as claimed in any preceding claim, wherein the composite structure is part of a wind turbine.
25. An apparatus, method or use as claimed in Claim 24, wherein the composite structure is a wind turbine blade shell.
26. An apparatus, method or use as claimed in Claim 24, wherein the composite structure is a spar for a wind turbine blade.