A composite element with acid breakable fixation means

Abstract

A composite element (6), e.g. a wind turbine blade, comprising a stack (20) of at least two fibre mats (13) of oriented fibres (18) embedded in a matrix of cured acid breakable resin (14) is disclosed. The fibres (18) of each fibre mat (13) are fixated relative to each other by means of first fixation means (19) made from a first material, and the at least two fibre mats (13) are fixated relative to each other by means of second fixation means (16) made from a second material. The second material of the second fixation means (16) is an acid breakable material. The composite element (6) is highly recyclable by exposing the composite element (6) to an acid containing release agent (11). Furthermore, a method for manufacturing such a composite element (6) and a method for disassembling such a composite element (6) are disclosed.

Description

[0001] A COMPOSITE ELEMENT WITH ACID BREAKABLE FIXATION MEANS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a composite element, such as a wind turbine blade, comprising a stack of at least two fibre mats of oriented fibres embedded in a matrix of cured resin. The invention further relates to a method for manufacturing such a composite element and to a method for disassembling such a composite element.

[0004] BACKGROUND OF THE INVENTION

[0005] At the end of their lifetime, composite structures, such as wind turbine blades, need to be disposed of. This could, e.g., involve depositing the composite structure in a landfill, possibly after cutting the composite structure into smaller parts. In order to reduce the environmental impact, there is a desire to at least partly recycle composite structures. However, due to the composite nature of these structures, it is difficult to separate the structures into their original constituent parts, and this renders appropriate recycling of material difficult, or even impossible. For instance, the composite structures may comprise fibres arranged in a matrix of a cured resin, e.g. an epoxy based resin. Once cured, it is not possible to mechanically separate the fibres from the resin.

[0006] One previous approach for recycling composite structures, such as epoxy composite structures, is to subject the composite structure to shredding or grinding, and subsequently performing a separation process on the shredded or grinded material, in order to mechanically separate the fibres from the resin material. However, the fibres as well as the resin material retrieved in this manner are often contaminated, i.e. the fibres are contaminated by resin material and the resin material is contaminated by fibres. Furthermore, shredding shortens the fibres, and therefore reduces their recycling value. Accordingly, the retrieved fibres as well as the retrieved resin material is unsuitable for recycling for high quality products. Furthermore, in some composite structures, fibre mats and / or fibres within fibre mats, are fixated relative to each other by means of suitable fixation means, e.g. in the form of stitching yarn. Thus, it may not be possible to retrieve clean and uncontaminated fibres from composite structures being recycled, even if fibre material and cured resin material are separated.

[0007] DESCRIPTION OF THE INVENTION

[0008] It is an object of embodiments of the invention to provide a composite element which is highly recyclable.

[0009] It is a further object of embodiments of the invention to provide a method for manufacturing a highly recyclable composite element.

[0010] It is an even further object of embodiments of the invention to provide a method for disassembling a composite element which allows clean and uncontaminated fibres to be retrieved from the composite element.

[0011] According to a first aspect the invention provides a composite element comprising a stack of at least two fibre mats of oriented fibres embedded in a matrix of cured acid breakable resin, wherein the fibres of each fibre mat are fixated relative to each other by means of first fixation means made from a first material, and wherein the at least two fibre mats are fixated relative to each other by means of second fixation means made from a second material, wherein the second material of the second fixation means is an acid breakable material.

[0012] In the present context the term 'composite element' should be interpreted to mean a structure which is made from a composite material, i.e. a material which comprises suitable fibres embedded in a matrix of cured resin.

[0013] Thus, the composite element according to the first aspect of the invention comprises a stack of at least two fibre mats of oriented fibres embedded in a matrix of cured resin. In the present context the term 'fibre mat' should be interpreted to mean a substantially two-dimensional or sheet-like item made exclusively or primarily from fibres. Fibre mats may be shaped prior to curing of the matrix material by bending, twisting or cutting similar to shaping of other sheet-like materials. Accordingly, the fibres of the composite structure are provided as such substantially two-dimensional or sheet-like items, rather than in the form of loose fibres embedded in the matrix of cured resin.

[0014] The matrix of cured resin is an acid breakable resin. Accordingly, when the composite element is exposed to a suitable acid, e.g. formic acid or acetic acid, the matrix of cured acid breakable resin will break in such a manner that the matrix disintegrates, and the fibre material and the resin material are separated. This will be described in further detail below.

[0015] As used herein, an acid breakable resin is a resin that upon exposure of cured acid breakable resin to a release agent will disintegrate, e.g. by swelling breakup or by dissolving, when the release agent containing formic acid and / or acetic acid at a concentration of at least 25 wt-% and a temperature between 20°C to 90°C. Examples of acid breakable resins are amine cured epoxy resins such as for example Olin Airstone 760, Hexion R.IMR. 035C infusion epoxy, and Aditya Birla Recyclamine system. In other words, epoxy polymers based on amine cured epoxy resins form a subgroup of acid breakable epoxy polymers.

[0016] The fibres of each fibre mat are fixated relative to each other by means of first fixation means made from a first material, and the at least two fibre mats are fixated relative to each other by means of second fixation means made from a second material. Accordingly, the fibres within a given fibre mat are fixated relative to each other by means of the first fixation means, so as to keep the fibres in position relative to each other and form the structure of the respective fibre mats. In a similar manner, the fibre mats of the stack are fixated relative to each other by means of the second fixation means, so as to keep the fibre mats in position relative to each other and form the structure of the stack of fibre mats. During manufacture of the composite element this facilitates that the arrangement of the fibre mats of the final composite element are maintained, prior to as well as during and after forming of the matrix, e.g. by infusion and curing of the resin, has a desired structure, shape and composition. The second material of the second fixation means is an acid breakable material. Thus, when the composite element is exposed to an acid, in order to break the matrix of cured resin as described above, the material forming the second fixation means will also break. Accordingly, the fixation means which hold the fibre mats in position relative to each other will dissolve or disintegrate, along with the matrix of cured resin, and thereby the fibre mats are released from each other. This allows the fibre mats to be readily retrieved from the disintegrated matrix.

[0017] Thus, the composite element is highly recyclable, since the fibres of the fibre mats can easily be retrieved in a clean and essentially uncontaminated form rendering the fibres, possibly in the form of separate fibre mats, readily reusable for high quality products.

[0018] The first material of the first fixation means may be an acid breakable material. According to this embodiment, in addition to the second fixation means, the first fixation means will also break and disintegrate when the composite element is exposed to an acid. Accordingly, the fibres within the individual fibre mats are also released from each other, thereby allowing the individual loose fibres to be readily retrieved and reused as recycled fibres The fibres retrieved in this manner are very clean and pure, and essentially undamaged, and therefore they can be reused or recycled for high quality products. Recycling of for example glass fibres may involve melting of the glass fibres in order to form new virgin fibres.

[0019] As an alternative, the first material of the first fixation means may be a material which is not acid breakable. In this case the first fixation means will remain substantially intact when the composite element is exposed to an acid, and thereby the fibres will remain fixated relative to each other within the individual fibre mats. Thus, in this case the fibres may be retrieved in the form of fibre mats, rather than in the form of loose fibres. The retrieved fibre mats may be sorted in different types of fibre mats, e.g. separating fibre mats with glass fibres from fibre mats with carbon fibres, and reused as fibre mats. As an alternative, the retrieved (and optionally sorted) fibre mats may subsequently be exposed to another substance which breaks the material of the first fixation means and allows the loose fibres to be retrieved. Even in the case that the composite comprises different types of fibres arranged in different fibre mats, separate fractions of loose fibres of the same type may retrieved.

[0020] The second material of the second fixation means may be the same or a similar material as the matrix of cured acid breakable resin. According to this embodiment, the second fixation means disintegrates simultaneously with and in a similar manner as the matrix of cured resin when the composite element is exposed to an acid. As an alternative, the second material of the second fixation means may differ from the material of the matrix of cured acid breakable resin. This may, e.g., cause the matrix of cured acid breakable resin to disintegrate faster than the second fixation means, thereby allowing a controlled stepwise recycling process, and hence optionally facilitates for further separation of material fractions.

[0021] In the case that the first material of the first fixation means is also an acid breakable material, the first material may also be the same or a similar material as the matrix of cured acid breakable resin, or it may be made from a different material. The remarks set forth above with reference to the second fixation means are equally applicable here.

[0022] The matrix of cured acid breakable resin may be an epoxy-based resin, and preferably an amine cured epoxy-based resin. It has been found by the inventors of the present invention that suitable epoxy-based cured resins are acid breakable. As an alternative, the matrix of cured acid breakable resin may be of another kind which is not epoxy-based.

[0023] The first fixation means and / or the second fixation means may be in the form of stitching yarns. According to this embodiment, the fibres of the fibre mats and / or the fibre mats are fixated relative to each other by stitching the fibres / mats together by means of stitching yarns. This is a suitable and well- known manner of fixating fibres and / or mats relative to each other when manufacturing composite elements. Traditionally, stitching yarn are not acid breakable and may for example comprise glass fibres or threads of polymer material not sensitive to acid. It was found that replacing traditional stitching yarn with stitching yarn that is acid breakable is highly advantageous as this means that currently used manufacturing methods for composite elements may readily be used for manufacturing composites according to the invention. Acid breakable stitching yarn may for example be extruded or spun from acid breakable resin such as amine curable epoxy resin.

[0024] As an alternative, the first fixation means and / or the second fixation means may be of another kind, e.g. mechanical clamps or clips, such as H-clips similar to tagging gun fasteners, adhesives, etc.

[0025] The oriented fibres of the fibre mats may be glass fibres or carbon fibres. Other types of fibres are also feasible, but glass fibres and carbon fibres are preferred as they allow for relatively easy further processing for reuse or recycling of the fibres after separation from the matrix material. Furthermore, such fibres are suitable for use in composite elements, such as structural composite elements.

[0026] The composite element may be a wind turbine blade or a part of a wind turbine blade. According to this embodiment, the wind turbine blade or the part of a wind turbine blade is highly recyclable, once it has reached its end of life, thus improving the environmental impact of the wind turbine.

[0027] The composite element may comprise a third fixation means for permanently or temporarily connecting an item to the composite element. Third fixation means may for example be a clamp, a screw, a handle, a hook or a hinge. The item may for example be a sensor, an electrical cable, or data cable. The composite element may comprise a third fixation means for permanently or temporarily connecting composite element to an item. The item may then be another composite or mechanical structure. The third fixation means is made from a third material, wherein the third material of the third fixation means is an acid breakable material.

[0028] According to a second aspect, the invention provides a method for manufacturing a composite element, the method comprising the steps of: providing a plurality of oriented fibres, and grouping the oriented fibres into a plurality of fibre mats,

[0029] - fixating the oriented fibres of each fibre mat relative to each other by means of first fixation means made from a first material,

[0030] - arranging the fibre mats adjacent to each other so as to form a stack of fibre mats,

[0031] - fixating the fibre mats of the stack of fibre mats relative to each other by means of second fixation means made from a second material, where the second material of the second fixation means is an acid breakable material,

[0032] - infusing an acid breakable resin in the stack of fibre mats, and

[0033] - curing the infused resin so as to form the composite element.

[0034] The composite element being manufactured according to the second aspect of the invention may be a composite element according to the first aspect of the invention, and the remarks set forth above with reference to the first aspect of the invention are therefore equally applicable here, and vice versa.

[0035] In the method according to the second aspect of the invention, a plurality of oriented fibres, e.g. glass fibres or carbon fibres, are initially provided and grouped into a plurality of fibre mats. Fibre mats may comprise systematically oriented fibres, such as for example parallelly, bi-axially or tri-axially oriented fibres, or randomly oriented fibres.

[0036] Next, the oriented fibres of each fibre mat are fixated relative to each other by means of first fixation means made from a first material. Accordingly, the structure of the fibre mats is formed by fixating the oriented loose fibres relative to each other, by means of the first fixation means, thereby allowing each mat to be handled as a single unit. This has already been described above with reference to the first aspect of the invention. Next, the fibre mats, formed as described above, are arranged adjacent to each other so as to form a stack of fibre mats. The fibre mats of the stack of fibre mats are then fixated relative to each other by means of second fixation means made from a second material, which is acid breakable. Accordingly, the fibre mats are now kept in position relative to each other in the stack of fibre mats.

[0037] An acid breakable resin is then infused in the stack of fibre mats, and the infused resin is cured so as to form the composite element. The first fixation means and the second fixation means are hence incorporated into the composite element.

[0038] Thus, the composite element resulting from the method according to the second aspect of the invention is highly recyclable, since the matrix of cured acid breakable resin as well as the second fixation means will disintegrate when the composite element is exposed to a suitable acid, e.g. formic acid. This has already been described above with reference to the first aspect of the invention.

[0039] The first material of the first fixation means may be an acid breakable material. As described above with reference to the first aspect of the invention, this will cause the first fixation means to also disintegrate when the composite element is exposed to a suitable acid, thereby allowing the loose fibres to be retrieved. As an alternative, the first material of the first fixation means may be of a kind which is not acid breakable, in which case the fibres of the fibre mats are kept fixated relative to each other when the composite element is exposed to a suitable acid, thereby allowing the fibres to be retrieved in the form of fibre mats.

[0040] The step of fixating the oriented fibres of each fibre mat may comprise stitching the fibres together by means of stitching yarn and / or the step of fixating the fibre mats of the stack of fibre mats may comprise stitching the fibre mats together by means of stitching yarn. This has already been described above with reference to the first aspect of the invention. Using stitching yarn with acid breakable material that is disintegratable is highly advantageous as this means that currently used manufacturing methods for composite elements may be used by replacing traditional stitching yarn by stitching yarns of a suitable acid breakable material.

[0041] The invention further provides a composite element obtainable by the method according to the second aspect of the invention, and as described above. Thus, such a composite element is highly recyclable.

[0042] According to a third aspect the invention provides a method for disassembling a composite element, the composite element comprising a stack of at least two fibre mats of oriented fibres embedded in a matrix of cured acid breakable resin, wherein the fibres of each fibre mat are fixated relative to each other by means of first fixation means made from a first material, and wherein the at least two fibre mats are fixated relative to each other by means of second fixation means made from a second material, wherein the second material of the second fixation means is an acid breakable material, the method comprising the steps of:

[0043] - exposing the composite element to an acid containing release agent causing disintegration of the composite element due to disintegration of the matrix of cured acid breakable resin and of the second fixation means, and

[0044] - retrieving the oriented fibres from the disintegrated composite element.

[0045] The composite element being disassembled by means of the method according to the third aspect of the invention may be a composite element according to the first aspect of the invention and / or a composite element which has been manufactured in accordance with the method according to the second aspect of the invention. The remarks set forth above with reference to the first and / or second aspect of the invention are therefore equally applicable here, and vice versa.

[0046] In particular, the composite element comprises a stack of at least two fibre mats of oriented fibres embedded in a matrix of cured acid breakable resin. The fibres of each fibre mat are fixated relative to each other by means of first fixation means made from a first material, and the at least two fibre mats are fixated relative to each other by means of second fixation means, made from a second material, the second material being an acid breakable material.

[0047] In the method according to the third aspect, the composite element is initially exposed to an acid containing release agent. Since the matrix of cured resin as well as the second fixation means are made from acid breakable materials, this causes disintegration, e.g. by swelling break-up or by dissolving, of the matrix of cured acid breakable resin as well as disintegration of the second fixation means. Accordingly, the fibre material is separated from the resin material of the matrix of cured acid breakable resin, and the fibre mats are released from each other, causing the stack of fibre mats to disassemble.

[0048] The acid containing release agent may, e.g., be or comprise formic acid. Formic acid is readily available at low cost. Furthermore, formic acid is considered safe to humans as well as with regard to the environment. Finally, the inventors of the present invention have found that formic acid is an efficient release agent with respect to epoxy resins applied in some relevant epoxy composite structures, such as certain wind turbines blades. Accordingly, formic acid is a suitable, safe and cost effective choice for the release agent. As an alternative, the release agent may comprise other suitable kinds of acid, such as acetic acid.

[0049] Finally, the oriented fibres are retrieved from the disintegrated composite element. As described above, this can easily be done due to the disintegrated matrix of cured acid breakable resin and second fixation means. Thus, the fibres are retrieved in an easy manner, and the retrieved fibres are clean and essentially uncontaminated, and are therefore very suitable for reuse or recycling. Similarly, the disintegrated matrix resin may be collected as swelled powder resin or dissolved resin fragments and recycled in a separate process stream.

[0050] The step of retrieving the oriented fibres from the disintegrated composite element may comprise retrieving the fibre mats of oriented fibres. According to this embodiment, the oriented fibres of the fibre mats remain fixated relative to each other by means of the first fixation means, and the complete fibre mats are retrieved. This allows the fibre mats to be reused and handled as units.

[0051] As an alternative, the first material of the first fixation means may be an acid breakable material, and the step of exposing the composite structure to an acid containing release agent may further cause disintegration of the first fixation means. According to this embodiment, the exposure to the acid containing release agent will also cause the first fixation means to the disintegrate, and the fibres of the individual fibre mats are therefore no longer fixated relative to each other.

[0052] Thus, according to this embodiment, the step of retrieving the oriented fibres from the disintegrated composite element may comprise retrieving loose fibres, rather than retrieving fibre mats.

[0053] The composite element may comprise a third fixation means as described above. In this case, the method of disassembling the composite element may comprise the step of removing the item before or after exposing the composite element with the third fixation means to the acid containing release agent. Since the third fixation means is acid breakable, the third fixation means is disintegrated together with the composite element and no separate step of removing the third fixation means from the composite element is required as part of the disassembling method.

[0054] In all aspects of the invention, it is highly preferred that the first fixation means, the second fixation means, and the matrix of cured acid breakable resin are different entities. By different entities herein meant that each of the first fixation means and the second fixation means exist separately from the matrix of cured acid breakable resin prior to providing of the matrix material of acid breakable resin to form the composite element.

[0055] In some cases, the second material would have the same composition as the matrix of cured acid breakable resin. In these cases, the second fixation means should fixate the fibre mats relative to each other prior to provision of the acid breakable resin forming the matrix of cured acid breakable resin, brief description of the drawings

[0056] The invention will now be described in further detail with reference to the accompanying drawings in which

[0057] Fig. 1 is a perspective view of a wind turbine,

[0058] Fig. 2 is a perspective view of a composite element according to an embodiment of the invention, in the form of a wind turbine blade,

[0059] Figs. 3-5 illustrate steps of a method according to an embodiment of the invention for disassembling a composite element,

[0060] Figs. 6-8 illustrate various types of fibre mats for use in composite elements according to various embodiments of the invention, and

[0061] Figs. 9 and 10 illustrate various types of stacks of fibre mats for use in composite elements according to various embodiments of the invention.

[0062] DETAILED DESCRIPTION OF THE DRAWINGS

[0063] Fig. 1 is a perspective view of a wind turbine 1 comprising a tower 2 and a nacelle 3 mounted on top of the tower 2. A rotor 4 with a hub 5 carrying three wind turbine blades 6 is mounted rotatably on the nacelle 3. Accordingly, wind acting on the wind turbine blades 6 causes the rotor 4 to rotate, and the mechanical energy is transformed into electrical energy by means of a generator (not shown), in a manner which is known per se.

[0064] When the wind turbine 1, or one or more components of the wind turbine 1, has reached its end of life, it will be decommissioned. To this end, it is desirable to recycle the material of the various components to the greatest possible extent. In particular, the material of the wind turbine blades 6, which is often a composite material, may advantageously be fully or partly recycled. Thus, at least the wind turbine blades 6 may advantageously be manufactured and / or disassembled in accordance with a method according to an embodiment of the invention.

[0065] Fig. 2 is a perspective view of a composite element, in the form of a wind turbine blade 6, according to an embodiment of the invention. The wind turbine blade 6 extends in a longitudinal direction between a root end 7 and a tip end 8. A reinforcing web 9 extends along the longitudinal direction inside the wind turbine blade 6.

[0066] The wind turbine blade 6 comprises a stack of at least two fibre mats of oriented fibres, e.g. glass fibres or carbon fibres, embedded in a matrix of cured acid breakable resin. The wind turbine blade 6 may comprise other components, such as spars, metal meshes, sensors, connectors, etc. The fibres of each fibre mat are fixated relative to each other by means of first fixation means made from a first material, and the at least two fibre mats are fixated relative to each other by means of second fixation means made from a second material. At least the second material of the second fixation means is an acid breakable material.

[0067] Thus, when the wind turbine blade 6 is exposed to an acid containing release agent, the matrix of cured acid breakable resin as well as the second fixation means holding the fibre mats together will disintegrate. This allows the fibres to be easily retrieved, possibly in the form of fibre mats. Accordingly, the wind turbine blade 6 is highly recyclable.

[0068] The first material of the first fixation means holding the oriented fibres together within the respective fibre mats may further be an acid breakable material. In this case the first fixation means also disintegrate when the wind turbine blade 6 is exposed to the acid containing release agent, and the loose fibres can subsequently be easily retrieved.

[0069] Figs. 3-5 illustrate method steps of a method for disassembling a composite element 6, e.g. in the form of a wind turbine blade or a part of a wind turbine blade, in accordance with an embodiment of the invention. In Fig. 3, the composite element 6 has been submerged in an acid containing release agent 11 accommodated in a vessel 12. The composite structure 6 comprises a plurality of fibre mats 13, five of which are shown, embedded in a matrix of cured acid breakable resin 14 and forming a stack of fibre mats 13. The fibre mats 13 comprise oriented fibres, e.g. glass fibres or carbon fibres. Furthermore, the composite element 6 comprises two spar caps 15 and a reinforcing web 9.

[0070] The oriented fibres of the fibre mats 13 are fixated relative to each other by means of first fixation means (not shown), e.g. in the form of stitching yarns or adhesive, made from a first material. Furthermore, the fibre mats 13 are fixated relative to each other by means of second fixation means 16, e.g. in the form of stitching yarns, made from a second material. The second material is an acid breakable material.

[0071] The release agent 11 comprises acid, such as preferably formic acid, and it acts on the composite element 6 in such a manner that the composite element 6 disintegrates. More particularly, the release agent 11 acts on the matrix of cured acid breakable resin 14 and on the acid breakable second fixation means 16 so as to cause disintegration of these.

[0072] For instance, the release agent 11 may cause swelling of epoxy polymers of the resin 14 and of the second fixation means 16, and this may cause the matrix of cured acid breakable resin 14 and the second fixation means 16 to disintegrate into swelled epoxy particles. This, in turn, releases the fibre mats 13, as well as the spars 15 and the reinforcing web 9, from the matrix of cured acid breakable resin 14, as well as releasing the fibre mats 13 from each other. In other cases, the release agent may dissolve the resin 14.

[0073] In Fig. 4, the release agent 11 has acted on the composite element 6 for sufficiently long to have caused the disintegration of the composite element 6 described above with reference to Fig. 3. In particular, it can be seen that the fibre mats 13 are now floating freely in a mix 17 of release agent and disintegrated resin. Accordingly, the fibre mats 13 can be readily retrieved in one piece and separate from each other, and with their original fibre orientation, from the mix 17 of release agent and disintegrated resin.

[0074] In Fig. 5, the reinforcing web has been retrieved from the vessel accommodating the release agent 11. The spars 15 have moved upwards towards the surface of the release agent 11 and the top of the vessel 12, while the mix 17 of release agent and disintegrated resin has moved downwards towards the bottom of the vessel 12, along with the released fibre mats 13.

[0075] Fig. 6 illustrates a fibre mat 13 for use in a composite element according to an embodiment of the invention. The fibre mat 13 comprises a plurality of bundles of fibres 18 arranged uniaxially, i.e. in an oriented manner. The bundles of fibres 18 are fixated relative to each other by means of first fixation means 19 in the form of stitching yarns. This ensures that the bundles of fibres 18 remain in position relative to each other, and that the fibre mat 13 can therefore be handled as a single unit.

[0076] The first fixation means 19 may be made from an acid breakable material. In this case, during disassembly of a composite element comprising the fibre mat 13, the first fixation means 19 will disintegrate if exposed to a suitable acid containing release agent, thereby allowing the bundles of fibres 18 to be retrieved in an essentially pure, uncontaminated and undamaged state, as part of a recycling process.

[0077] Fig. 7 illustrates an alternative fibre mat 13 for use in a composite element according to an embodiment of the invention. The fibre mat 13 of Fig. 7 is very similar to the fibre mat 13 of Fig. 6, and it will therefore not be described in detail here. In particular, the fibre mat 13 of Fig. 7 comprises a plurality of bundles of fibres 18 arranged uniaxially, and the bundles of fibres 18 are fixated relative to each other by means of first fixation means 19. However, in the fibre mat 13 of Fig. 7, the first fixation means 19 are in the form of an adhesive adhering the bundles of fibres 18 to each other. Also in this case, the first fixation means 19 may be made from an acid breakable material, so as to cause them to disintegrate if exposed to a suitable acid containing release agent, thereby allowing the bundles of fibres 18 to be retrieved as part of a recycling process.

[0078] Fig. 8 illustrates yet an alternative fibre mat 13 for use in a composite element according to an embodiment of the invention. Similarly to the fibre mats 13 of Figs. 6 and 7, the fibre mat 13 of Fig. 8 comprises a plurality of bundles of fibres 18. However, in the fibre mat 13 of Fig. 8, the bundles of fibres 18 are arranged in a triaxial pattern, i.e. in an oriented manner. The bundles of fibres 18 are fixated relative to each other by means of first fixation means 19 following a zigzig path across the triaxial pattern of bundles of fibres 18. The first fixation means 19 may, e.g., be in the form of stitching yarns, as described above with reference to Fig. 6, or in the form of an adhesive, as described above with reference to Fig. 7. In any event, also in this case, the first fixation means 19 may be made from an acid breakable material.

[0079] It should be noted that, in the fibre mats 13 illustrated in each of Figs. 6-8, single fibres 18 could be applied instead of the illustrated bundles of fibres 18.

[0080] Fig. 9 illustrates a stack 20 of fibre mats 13, six of which are shown, for use in a composite element according to an embodiment of the invention. The fibre mats 13 are arranged adjacent to each other so as to form the stack 20. The fibre mats 13 could, e.g., be in the form of any of the fibre mats 13 illustrated in Figs. 6-8.

[0081] The fibre mats 13 are fixated relative to each other by means of second fixation means 16, in the form of stitching yarns. Accordingly, the fibre mats 13 are kept in position relative to each other, thus ensuring that the stack 20 of fibre mats 13 can be handled as a single unit.

[0082] The second fixation means 16 are made from an acid breakable material. This ensures that, during disassembly of a composite element comprising the stack 20 of fibre mats 13, the second fixation means 16 will disintegrate if exposed to a suitable acid containing release agent, thereby allowing the fibre mats 13 to be retrieved in an essentially pure, uncontaminated and undamaged state, as part of a recycling process.

[0083] Fig. 10 illustrates an alternative stack 20 of fibre mats 13, six of which are shown, for use in a composite element according to an embodiment of the invention. The fibre mats 13 could, e.g., be in the form of any of the fibre mats 13 illustrated in Figs. 6-8. Similarly to the stack 20 of fibre mats 13 of Fig. 9, in Fig. 10 the fibre mats 13 are arranged adjacent to each other, so as to form the stack 20, and are fixated relative to each other by means of second fixation means 16. However, in this case the second fixation means 16 are in the form of a plurality of clamps, each joining two or more fibre mats 13 to each other.

[0084] Also in this case, the second fixation means 16 are made from an acid breakable material, so as to cause them to disintegrate if exposed to a suitable acid containing release agent, thereby allowing the fibre mats 13 to be retrieved as part of a recycling process.

Claims

CLAIMS1. A composite element (6) comprising a stack (20) of at least two fibre mats(13) of oriented fibres (18) embedded in a matrix of cured acid breakable resin(14), wherein the fibres (18) of each fibre mat (13) are fixated relative to each other by means of first fixation means (19) made from a first material, and wherein the at least two fibre mats (13) are fixated relative to each other by means of second fixation means (16) made from a second material, wherein the second material of the second fixation means (16) is an acid breakable material.

2. The composite element (6) according to claim 1, wherein the first fixation means (19), the second fixation means (16), and the matrix of cured acid breakable resin (14) are different entities.

3. The composite element (6) according to claim 1 or 2, wherein the first material of the first fixation means (19) is an acid breakable material.

4. The composite element (6) according to any of the preceding claims, wherein the second material of the second fixation means (16) is the same or a similar material as the matrix of cured acid breakable resin (14).

5. The composite element (6) according to any of the preceding claims, wherein the matrix of cured acid breakable resin (14) is an epoxy-based resin, preferably an amine cured epoxy-based resin.

6. The composite element (6) according to any of the preceding claims, wherein the first fixation (19) means and / or the second fixation means (16) are in the form of stitching yarns.

7. The composite element (6) according to any of the preceding claims, wherein the fibres (18) of the fibre mats (13) are glass fibres or carbon fibres.

8. The composite element (6) according to any of the preceding claims, wherein the composite element (6) is a wind turbine blade or a part of a wind turbine blade.

9. The composite element (6) according to any of the preceding claims, further comprising a third fixation means for at least temporarily connecting an item to the composite element (6) or at least temporarily connecting the composite element (6) to an item, wherein the third fixation means is made from a third material, wherein the third material of the third fixation means is an acid breakable material.

10. A method for manufacturing a composite element (6), the method comprising the steps of:- providing a plurality of oriented fibres (18), and grouping the oriented fibres (18) into a plurality of fibre mats (13),- fixating the oriented fibres (18) of each fibre mat (13) relative to each other by means of first fixation means (19) made from a first material,- arranging the fibre mats (13) adjacent to each other so as to form a stack (20) of fibre mats (13),- fixating the fibre mats (13) of the stack (20) of fibre mats (13) relative to each other by means of second fixation means (16) made from a second material, where the second material of the second fixation means (16) is an acid breakable material,- infusing an acid breakable resin (14) in the stack (20) of fibre mats (13), and- curing the infused resin (14) so as to form the composite element (6).

11. The method according to claim 10, wherein the first fixation means (19), the second fixation means (16), and the matrix of cured acid breakable resin (14) are different entities.

12. The method according to claim 10 or 11, wherein the first material of the first fixation means (19) is an acid breakable material.

13. The method according to any of claims 10-12, wherein the step of fixating the oriented fibres (18) of each fibre mat (13) comprises stitching the fibres (18) together by means of stitching yarn.

14. The method according to any of claims 10-13, wherein the step of fixating the fibre mats (13) of the stack (20) of fibre mats (13) comprises stitching the fibre mats (13) together by means of stitching yarn.

15. A composite element (6) obtainable by the method of any one of the claims 10-14.

16. A method for disassembling a composite element (6), the composite element (6) comprising a stack (20) of at least two fibre mats (13) of oriented fibres (18) embedded in a matrix of cured acid breakable resin (14), wherein the fibres (18) of each fibre mat (13) are fixated relative to each other by means of first fixation means (19) made from a first material, and wherein the at least two fibre mats (13) are fixated relative to each other by means of second fixation means (16) made from a second material, wherein the second material of the second fixation means (16) is an acid breakable material, the method comprising the steps of:- exposing the composite element (6) to an acid containing release agent (11) causing disintegration of the composite element (6) due to disintegration of the matrix of cured acid breakable resin (14) and of the second fixation means (16), and- retrieving the oriented fibres (18) from the disintegrated composite element (6).

17. The method according to claim 16, wherein the first fixation means (19), the second fixation means (16), and the matrix of cured acid breakable resin (14) are different entities.step of retrieving the oriented fibres (18) from the disintegrated composite element (6) comprises retrieving the fibre mats (13) of oriented fibres (18).

18. The method according to claim 16 or 17, wherein the first material of the first fixation means (19) is an acid breakable material, and wherein the step of exposing the composite structure (6) to an acid containing release agent (11) further causes disintegration of the first fixation means (19).

19. The method according to claim 18, wherein the step of retrieving the oriented fibres (18) from the disintegrated composite element (6) comprises retrieving loose fibres (18).

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