WO2010000782A1 - Method of curing a composite structure - Google Patents

Method of curing a composite structure Download PDF

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Publication number
WO2010000782A1
WO2010000782A1 PCT/EP2009/058274 EP2009058274W WO2010000782A1 WO 2010000782 A1 WO2010000782 A1 WO 2010000782A1 EP 2009058274 W EP2009058274 W EP 2009058274W WO 2010000782 A1 WO2010000782 A1 WO 2010000782A1
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WO
WIPO (PCT)
Prior art keywords
composite structure
resin
nanoparticles
wind turbine
millimetres
Prior art date
Application number
PCT/EP2009/058274
Other languages
French (fr)
Inventor
Edwin Bodros
Original Assignee
Vestas Wind Systems A/S
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Vestas Wind Systems A/S filed Critical Vestas Wind Systems A/S
Publication of WO2010000782A1 publication Critical patent/WO2010000782A1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03D—WIND MOTORS
    • F03D1/00—Wind motors with rotation axis substantially parallel to the air flow entering the rotor 
    • F03D1/06—Rotors
    • F03D1/065—Rotors characterised by their construction elements
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
    • C08J3/247—Heating methods
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/005—Reinforced macromolecular compounds with nanosized materials, e.g. nanoparticles, nanofibres, nanotubes, nanowires, nanorods or nanolayered materials
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/04—Reinforcing macromolecular compounds with loose or coherent fibrous material
    • C08J5/10—Reinforcing macromolecular compounds with loose or coherent fibrous material characterised by the additives used in the polymer mixture
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2363/00—Characterised by the use of epoxy resins; Derivatives of epoxy resins
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2230/00—Manufacture
    • F05B2230/40—Heat treatment
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2280/00—Materials; Properties thereof
    • F05B2280/60—Properties or characteristics given to material by treatment or manufacturing
    • F05B2280/6003—Composites; e.g. fibre-reinforced
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2280/00—Materials; Properties thereof
    • F05B2280/60—Properties or characteristics given to material by treatment or manufacturing
    • F05B2280/6013—Fibres
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2280/00—Materials; Properties thereof
    • F05B2280/60—Properties or characteristics given to material by treatment or manufacturing
    • F05B2280/6015—Resin
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05C—INDEXING SCHEME RELATING TO MATERIALS, MATERIAL PROPERTIES OR MATERIAL CHARACTERISTICS FOR MACHINES, ENGINES OR PUMPS OTHER THAN NON-POSITIVE-DISPLACEMENT MACHINES OR ENGINES
    • F05C2253/00—Other material characteristics; Treatment of material
    • F05C2253/04—Composite, e.g. fibre-reinforced
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05C—INDEXING SCHEME RELATING TO MATERIALS, MATERIAL PROPERTIES OR MATERIAL CHARACTERISTICS FOR MACHINES, ENGINES OR PUMPS OTHER THAN NON-POSITIVE-DISPLACEMENT MACHINES OR ENGINES
    • F05C2253/00—Other material characteristics; Treatment of material
    • F05C2253/16—Fibres
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05C—INDEXING SCHEME RELATING TO MATERIALS, MATERIAL PROPERTIES OR MATERIAL CHARACTERISTICS FOR MACHINES, ENGINES OR PUMPS OTHER THAN NON-POSITIVE-DISPLACEMENT MACHINES OR ENGINES
    • F05C2253/00—Other material characteristics; Treatment of material
    • F05C2253/20—Resin
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/72—Wind turbines with rotation axis in wind direction
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to an improved method of heat curing a fibre composite structure.
  • the present invention also relates to a composite structure, and to a wind turbine comprising a composite structure.
  • the structure When curing a composite structure, comprising fibres and a thermosetting resin, the structure is heated to a temperature sufficient to cure the thermosetting resin and then cooled down. During heating, a temperature gradient is formed over the structure material, since the heat is applied to the surface of the structure, e.g. by means of an oven or a heated mould, and the heating of the interior of the structure is dependent on heat travelling from the surface to the interior. In a corresponding way, a heat gradient is also formed during cooling of the composite structure.
  • the present invention relates to a method of curing a fibre composite structure, comprising: providing a composite structure, said structure comprising fibres and a thermosetting resin; and heating said composite structure; wherein said resin comprises thermally conductive nanoparticles providing a thermally percolating effect within the composite structure during heating; and wherein the fibre composite structure has a material thickness of more than 10 millimetres.
  • a fibre/resin composite is cured, fully or partly, by heating the composite, since the composite comprises a thermosetting resin.
  • the resin comprises thermally conductive nanoparticles, which allows for thermal percolation within the composite structure.
  • the thermally conductive nanoparticles thus improve the heat conduction within the structure, e.g. by facilitating for heat to travel from a surface of the structure towards an interior of the structure.
  • the heat gradient over the composite structure is smaller, resulting in a more even temperature throughout the structure during heating, as well as during cooling. This is a great advantage since the more even the temperature is, the more even the curing is throughout the structure, whereby a more uniform structural strength throughout the structure is obtained.
  • the curing process, and thus the structural strength of the final composite structure may thus be better controlled.
  • thermal percolation or “thermally percolating effect” describes the effect that is achieved, in accordance with the present invention, when the thermally conductive nanoparticles interact with each other to form a thermally conductive path within the resin.
  • the structure temperature should exceed a certain temperature, depending predominantly on the resin used, for a certain time.
  • heat conductivity of the composite it is possible to reach that temperature also in the interior of the composite structure, or reach it faster than would otherwise be possible, even if heat is only applied to the surface of the structure.
  • the advantage is more pronounced, the larger the structure is, or the further away from an outer surface any point within the structure is.
  • the improved heat conductivity also facilitates the cooling down of the structure after heating, allowing it to reach ambient temperature faster, thus speeding up the curing process even more. It might, due to the risk of introducing detrimental thermal stress in the material, not be desirable to allow the outer parts of the structure to exceed the temperature which is needed for curing by much, or to allow these parts to be heated to said temperature for much longer than needed for the curing of said parts. However, in order to also achieve sufficient curing of the interior of the structure, both the temperature, and the time for which that temperature is held, may have to exceed the levels needed for curing the outer parts. By improving the heat conductivity of the composite structure in accordance with the present invention, also this problem may be alleviated.
  • the time for curing a composite structure may thus be reduced with the help of the present invention, which significantly reduces the production costs of composite structures by increasing the throughput of a production line and by allowing production equipment for curing the composite structures to be more efficiently utilized.
  • the nanoparticles may be any type of nanoparticles able to conduct heat, such as carbon black or carbon nanotubes or a mixture thereof.
  • An advantage with using carbon nanotubes is that, by virtue of their needle shape, formation of paths within the composite structure able to conduct heat may be facilitated, even if the nanotubes are randomly intermixed with the resin, because there is a higher probability that a nanoparticle will be positioned in contact with another nanoparticle in the resin. Thus a lower concentration of nanoparticles may be sufficient to achieve the percolation effect if carbon nanotubes are used.
  • An advantage of carbon black nanoparticles over carbon nanotubes is the significantly lower cost of the carbon black nanoparticles.
  • a mixture of carbon black and carbon nanotubes may have the advantage of combining a relatively low cost with a relatively low particle concentration requirement.
  • the thermal conductivity of the composite structure is improved even if there are not enough nanoparticles to form continuous conductive paths formed by particles in direct contact with each other throughout the structure, but the thermal conductivity is more markedly improved if such paths are formed.
  • the concentration of nanoparticles in the resin should not be too low. The concentration needed depends e.g. on the properties of the particles, whereby a higher concentration might be needed e.g. if carbon black is used as compared with if carbon nanotubes are used, and on the composition of the composite, whereby e.g. the fibre content might influence the needed concentration of particles in the resin.
  • the particle concentration in the resin is at least 0.005% by weight, but preferably it is above 0.05% by weight, more preferably above 0.5% by weight, most preferably above 1 % by weight, such as above 2, 3 or 4 percent by weight of the resin.
  • the concentration of nanoparticles in the resin may thus be below 10% by weight, but preferably it is below 5 percent by weight, such as below 4, 3, 2, 1.9, 1.8, 1.7, 1.6 or 1.5 percent by weight of the resin. In a particular embodiment, the concentration of nanoparticles in the resin may be below 1.9 percent by weight of the resin, especially below 1.5 percent by weight of the resin.
  • the nanoparticle concentration in the resin may thus typically be between 0.005 and 10, such as between 0.05 and 5, such as between 0.5 and 3, or such as between 1 and 2, percent by weight of the thermosetting resin, particularly between 1 and 1.9, such as between 1 and 1.5, percent by weight of the thermosetting resin.
  • the nanoparticle concentration in the resin should preferably be between 1 and 1.5, percent by weight of the thermosetting resin.
  • the nanoparticles may be randomly intermixed in the resin.
  • the inclusion of the particles in the resin is made simple, and no specific measures need to be taken to ensure a specific distribution or orientation of the particles in the resin.
  • the thermosetting resin may be any type of resin that is cured, or partly cured, by applying heat, possibly in combination with additional curing measures, such as the addition of a hardener.
  • the resin may be a thermosetting epoxy resin.
  • Such a resin has a latency, allowing curing whenever desired by heating the resin to above a certain temperature, but without the resin being cured below said certain temperature.
  • An epoxy resin also provides a very strong bond when cured and is free of solvents.
  • the fibres may be any type of fibres, or filaments, possibly in the form of yarns, or tows, for extra strength.
  • the fibres may have the function of reinforcing the resin.
  • carbon or glass fibres may be used.
  • the fibres may have a length of at least 1 millimetre, such as at least 10 millimetres or at least 100 millimetres.
  • the fibres may have a diameter of at least 1 micrometre, such as at least 5 micrometres or at least 10 micrometres.
  • the fibres, yarns or tows may be arranged in any way within the structure to reinforce said structure, such as monodirectionally in parallel with each other or bidirectionally at e.g. 90 degrees to each other.
  • the temperature which the structure is heated to is dependent on the curing properties of the resin. As discussed above, there might be a temperature gradient over the composite structure during heating, especially pronounced for thicker structures, resulting in a higher temperature at the surface of the composite structure than in the centre of the structure.
  • the surface of the structure may be heated to at least 6O 0 C, preferably to at least 8O 0 C, more preferably to at least 100 0 C, such as to about 12O 0 C.
  • it may be convenient to heat the surface of the structure to less than 300 0 C, preferably to less than 200 0 C, especially to less than 16O 0 C, during curing of the thermosetting resin of the fibre composite structure.
  • the surface of the composite structure is preferably heated to a temperature between 100 0 C and 160 0 C
  • the heating may be made e.g. by means of an oven or by means of a heated mould, depending e.g. on the size and shape of the composite structure.
  • the composite structure may have been formed by means of an infusion process prior to heating.
  • the composite structure may have a material thickness of more than 10 millimetres, such as more than 20, 30, 40, 50, 60, 70, 80, 90, 100 or 110 millimetres.
  • the problems with heat gradients, as discussed above, are more pronounced with increasing material thickness, why the present invention may be particularly beneficial for composite structures having a large material thickness.
  • the composite structure may have a material thickness of between 10 and 200 millimetres, such as between 50 and 170 millimetres, especially between 100 and 150 millimetres, particularly between 110 and 130 millimetres, such as about 120 millimetres.
  • the composite structure may be a wind turbine blade, or a part of a wind turbine blade. Since, as discussed above, the inventive method may be specifically advantageous for larger composite structures, it may be desired to use it in the production of wind turbine blades, since such blades are often rather large.
  • the composite structure may be a spar or a spar cap of a rotor blade.
  • the composite structure may be a root section of a rotor blade.
  • These structures are load bearing, why it is important that they are sufficiently cured and have low thermal stresses in the material. These structures may also have a rather large material thicknesses.
  • the rotor blade might be the rotor blade of a wind turbine.
  • the present invention relates to a fibre composite structure comprising fibres and a thermosetting resin, wherein said thermosetting resin comprises thermally conductive nanoparticles allowing a thermally percolating effect within the composite structure, and wherein the fibre composite structure has a material thickness of more than 10 millimetres.
  • the composite structure may be a wind turbine blade, or a part of a wind turbine blade, such as a spar for use in a wind turbine blade.
  • the nanoparticles of the resin may be unevenly distributed throughout the composite structure. Thus, they may be present in a higher concentration in a certain part of the structure, such as a thick part where an increased thermal conductivity might be especially beneficial during production, and in a lower concentration in other parts of the structure. In this way, the total amount of particles used in the production of a composite structure may be kept down, which is advantageous if e.g. the particles are expensive or negatively affect the mechanical properties of the structure.
  • the present invention relates to a wind turbine, which comprises a fibre composite structure in accordance with the discussion above.
  • Fig 1 is a schematic side view of a wind turbine blade.
  • Fig 2 is a schematic cross-sectional view, in high magnification, of cured resin, which includes nanoparticles, in the blade of fig 1.
  • thermal percolation or “thermally percolating effect” describes the effect that is achieved, in accordance with the present invention, when the thermally conductive nanoparticles interact with each other to form a thermally conductive path within the resin.
  • the nanoparticles are in direct contact with each other to form a chain, which chain is the conductive path.
  • a thermally percolating effect may also be achieved even if there is no chain formed, or the chain is discontinuous, whereby heat may travel between two nanoparticles, or chain fragments, by heat diffusion or radiation. See also fig 2.
  • the invention is particularly advantageous for use in the production of large composite structures.
  • large composite structures are wind turbine blades, or spars for use in wind turbine blades, which blades can currently be as large as 44 meters long, and have a diameter of 1.5 meters and a material thickness of about 80 millimetres at the root end of the blade. It is envisioned that even larger blades will be produced in the near future, such as blades being up to 80 meters long. The use of the present invention is thus a great advantage in the production of wind turbine blades.
  • a large composite structure exemplified by a typical wind turbine blade 1 comprises a spar, or beam, 2 and a shell member 3, supported by the spar 2 and forming an air foil.
  • the blade 1 comprises a root section 4, adapted to be attached to a hub of a wind turbine rotor.
  • the spar 2, the shell member 3 or the entire blade 3 may be in accordance with the present invention, thus being a composite structure comprising fibres and a resin comprising thermally conductive nanoparticles.
  • a thermally conductive path 10 is formed by thermally conductive nanoparticles 11 , here exemplified by needle-shaped carbon nanotubes, in a resin matrix 12, such as a thermosetting epoxy resin.
  • Fig 2 illustrates that even though the nanoparticles 11 are randomly intermixed with the resin 12, they may, provided that the particle concentration is sufficiently high, form chains that may function as conductive paths 10.
  • the composite structure of the present invention may have an essentially uniform nanoparticle concentration throughout the structure, or the structure may comprise parts having a higher nanoparticle concentration as compared with other parts of the same structure.
  • Carbon black nanoparticles are intermixed with a thermosetting epoxy resin to a concentration of 2% by weight.
  • the resin, including the nanoparticles is infused into a glass fibre structure in a heatable mould, by means of an applied vacuum, to form a wind turbine rotor blade spar being 40 meters long and having a root section diameter of 1.5 meters.
  • the resin/fibre composite structure i.e. the spar
  • the mould is heated to 12O 0 C, whereby the surface of the structure, which is in contact with the heated mould, acquires a temperature of about 12O 0 C, and the heat is conducted to the interior of the spar via heat conductive paths formed by the carbon black nanoparticles in the epoxy resin such that the interior of the spar acquires a temperature which is sufficiently high to cure the resin much faster than if the nanoparticles were not present in the resin.
  • the spar is subsequently used as the backbone of a wind turbine rotor blade, and the root of the spar is fitted in the hub of a wind turbine rotor during assembly of a wind turbine comprising a tower, a nacelle rotatably mounted on said tower, and said rotor.
  • the invention has above mainly been described with reference to a few embodiments.

Abstract

The invention relates to a method of curing a fibre composite structure, comprising: providing a composite structure, said structure comprising fibres and a thermosetting resin, and heating said composite structure, wherein said thermosetting resin comprises thermally conductive nanoparticles providing a thermally percolating effect within the composite structure during heating, and wherein the fibre composite structure has a material thickness of more than 10 millimetres. The invention also relates to a fibre composite structure comprising fibres and a thermosetting resin, wherein said thermosetting resin comprises thermally conductive nanoparticles allowing a thermally percolating effect within the composite structure, and wherein the fibre composite structure has a material thickness of more than 10 millimetres; as well as to a wind turbine comprising such a structure.

Description

METHOD OF CURING A COMPOSITE STRUCTURE
Field of the Invention
The present invention relates to an improved method of heat curing a fibre composite structure. The present invention also relates to a composite structure, and to a wind turbine comprising a composite structure.
Background of the Invention
When curing a composite structure, comprising fibres and a thermosetting resin, the structure is heated to a temperature sufficient to cure the thermosetting resin and then cooled down. During heating, a temperature gradient is formed over the structure material, since the heat is applied to the surface of the structure, e.g. by means of an oven or a heated mould, and the heating of the interior of the structure is dependent on heat travelling from the surface to the interior. In a corresponding way, a heat gradient is also formed during cooling of the composite structure. This is a problem in the art since the entire structure will not have the same curing conditions, the interior of the structure will, e.g., not reach the same temperature as the surface or will not hold a temperature sufficient for curing for the same period of time as the surface. This problem is especially pronounced for larger composite structures, having larger material thickness, where thus larger heat gradients are formed. This problem is not relevant for curing of composite structures that do not include a thermosetting resin, i.e. where the curing is not dependent on heating.
Summary of the Invention According to one aspect, the present invention relates to a method of curing a fibre composite structure, comprising: providing a composite structure, said structure comprising fibres and a thermosetting resin; and heating said composite structure; wherein said resin comprises thermally conductive nanoparticles providing a thermally percolating effect within the composite structure during heating; and wherein the fibre composite structure has a material thickness of more than 10 millimetres. In accordance with the invention, a fibre/resin composite is cured, fully or partly, by heating the composite, since the composite comprises a thermosetting resin.
The resin comprises thermally conductive nanoparticles, which allows for thermal percolation within the composite structure. The thermally conductive nanoparticles thus improve the heat conduction within the structure, e.g. by facilitating for heat to travel from a surface of the structure towards an interior of the structure. By means of these particles, the heat gradient over the composite structure is smaller, resulting in a more even temperature throughout the structure during heating, as well as during cooling. This is a great advantage since the more even the temperature is, the more even the curing is throughout the structure, whereby a more uniform structural strength throughout the structure is obtained. The curing process, and thus the structural strength of the final composite structure, may thus be better controlled.
As used in this description the term "thermal percolation", or "thermally percolating effect", describes the effect that is achieved, in accordance with the present invention, when the thermally conductive nanoparticles interact with each other to form a thermally conductive path within the resin. For curing to take place, the structure temperature should exceed a certain temperature, depending predominantly on the resin used, for a certain time. With an improved heat conductivity of the composite, it is possible to reach that temperature also in the interior of the composite structure, or reach it faster than would otherwise be possible, even if heat is only applied to the surface of the structure. The advantage is more pronounced, the larger the structure is, or the further away from an outer surface any point within the structure is. The improved heat conductivity also facilitates the cooling down of the structure after heating, allowing it to reach ambient temperature faster, thus speeding up the curing process even more. It might, due to the risk of introducing detrimental thermal stress in the material, not be desirable to allow the outer parts of the structure to exceed the temperature which is needed for curing by much, or to allow these parts to be heated to said temperature for much longer than needed for the curing of said parts. However, in order to also achieve sufficient curing of the interior of the structure, both the temperature, and the time for which that temperature is held, may have to exceed the levels needed for curing the outer parts. By improving the heat conductivity of the composite structure in accordance with the present invention, also this problem may be alleviated.
The time for curing a composite structure may thus be reduced with the help of the present invention, which significantly reduces the production costs of composite structures by increasing the throughput of a production line and by allowing production equipment for curing the composite structures to be more efficiently utilized.
The nanoparticles may be any type of nanoparticles able to conduct heat, such as carbon black or carbon nanotubes or a mixture thereof. An advantage with using carbon nanotubes is that, by virtue of their needle shape, formation of paths within the composite structure able to conduct heat may be facilitated, even if the nanotubes are randomly intermixed with the resin, because there is a higher probability that a nanoparticle will be positioned in contact with another nanoparticle in the resin. Thus a lower concentration of nanoparticles may be sufficient to achieve the percolation effect if carbon nanotubes are used. An advantage of carbon black nanoparticles over carbon nanotubes is the significantly lower cost of the carbon black nanoparticles. A mixture of carbon black and carbon nanotubes may have the advantage of combining a relatively low cost with a relatively low particle concentration requirement.
The thermal conductivity of the composite structure is improved even if there are not enough nanoparticles to form continuous conductive paths formed by particles in direct contact with each other throughout the structure, but the thermal conductivity is more markedly improved if such paths are formed. In order to achieve such paths, thereby improving the thermal percolation, the concentration of nanoparticles in the resin should not be too low. The concentration needed depends e.g. on the properties of the particles, whereby a higher concentration might be needed e.g. if carbon black is used as compared with if carbon nanotubes are used, and on the composition of the composite, whereby e.g. the fibre content might influence the needed concentration of particles in the resin. Typically, the particle concentration in the resin is at least 0.005% by weight, but preferably it is above 0.05% by weight, more preferably above 0.5% by weight, most preferably above 1 % by weight, such as above 2, 3 or 4 percent by weight of the resin.
The nanoparticles might, depending on the particles used and the properties of the composite structure, negatively affect the mechanical properties of the composite structure if the concentration is increased. Typically, the concentration of nanoparticles in the resin may thus be below 10% by weight, but preferably it is below 5 percent by weight, such as below 4, 3, 2, 1.9, 1.8, 1.7, 1.6 or 1.5 percent by weight of the resin. In a particular embodiment, the concentration of nanoparticles in the resin may be below 1.9 percent by weight of the resin, especially below 1.5 percent by weight of the resin. The nanoparticle concentration in the resin may thus typically be between 0.005 and 10, such as between 0.05 and 5, such as between 0.5 and 3, or such as between 1 and 2, percent by weight of the thermosetting resin, particularly between 1 and 1.9, such as between 1 and 1.5, percent by weight of the thermosetting resin. In order to meet the conductivity requirements while maintaining adequate mechanical properties of the composite structure the nanoparticle concentration in the resin should preferably be between 1 and 1.5, percent by weight of the thermosetting resin.
The nanoparticles may be randomly intermixed in the resin. Thus the inclusion of the particles in the resin is made simple, and no specific measures need to be taken to ensure a specific distribution or orientation of the particles in the resin.
The thermosetting resin may be any type of resin that is cured, or partly cured, by applying heat, possibly in combination with additional curing measures, such as the addition of a hardener. The resin may be a thermosetting epoxy resin. Such a resin has a latency, allowing curing whenever desired by heating the resin to above a certain temperature, but without the resin being cured below said certain temperature. An epoxy resin also provides a very strong bond when cured and is free of solvents.
The fibres may be any type of fibres, or filaments, possibly in the form of yarns, or tows, for extra strength. The fibres may have the function of reinforcing the resin. Typically, carbon or glass fibres may be used. The fibres may have a length of at least 1 millimetre, such as at least 10 millimetres or at least 100 millimetres. The fibres may have a diameter of at least 1 micrometre, such as at least 5 micrometres or at least 10 micrometres. The fibres, yarns or tows may be arranged in any way within the structure to reinforce said structure, such as monodirectionally in parallel with each other or bidirectionally at e.g. 90 degrees to each other.
The temperature which the structure is heated to is dependent on the curing properties of the resin. As discussed above, there might be a temperature gradient over the composite structure during heating, especially pronounced for thicker structures, resulting in a higher temperature at the surface of the composite structure than in the centre of the structure. Typically, the surface of the structure may be heated to at least 6O0C, preferably to at least 8O0C, more preferably to at least 1000C, such as to about 12O0C. Also, it may be convenient not to heat the fibre composite structure more than necessary to obtain a temperature required to achieve curing conditions throughout the structure. This may reduce the energy needed for the curing, and it may also reduce the risk of heat damaging, or burning, the composite structure. Thus, it may be convenient to heat the surface of the structure to less than 3000C, preferably to less than 2000C, especially to less than 16O0C, during curing of the thermosetting resin of the fibre composite structure.
For the reasons set out above, the surface of the composite structure is preferably heated to a temperature between 100 0C and 160 0C The heating may be made e.g. by means of an oven or by means of a heated mould, depending e.g. on the size and shape of the composite structure. The composite structure may have been formed by means of an infusion process prior to heating.
The composite structure may have a material thickness of more than 10 millimetres, such as more than 20, 30, 40, 50, 60, 70, 80, 90, 100 or 110 millimetres. The problems with heat gradients, as discussed above, are more pronounced with increasing material thickness, why the present invention may be particularly beneficial for composite structures having a large material thickness. Typically, the composite structure may have a material thickness of between 10 and 200 millimetres, such as between 50 and 170 millimetres, especially between 100 and 150 millimetres, particularly between 110 and 130 millimetres, such as about 120 millimetres.
The composite structure may be a wind turbine blade, or a part of a wind turbine blade. Since, as discussed above, the inventive method may be specifically advantageous for larger composite structures, it may be desired to use it in the production of wind turbine blades, since such blades are often rather large.
The composite structure may be a spar or a spar cap of a rotor blade. Alternatively, the composite structure may be a root section of a rotor blade. These structures are load bearing, why it is important that they are sufficiently cured and have low thermal stresses in the material. These structures may also have a rather large material thicknesses. The rotor blade might be the rotor blade of a wind turbine.
According to another aspect, the present invention relates to a fibre composite structure comprising fibres and a thermosetting resin, wherein said thermosetting resin comprises thermally conductive nanoparticles allowing a thermally percolating effect within the composite structure, and wherein the fibre composite structure has a material thickness of more than 10 millimetres.
The composite structure may be a wind turbine blade, or a part of a wind turbine blade, such as a spar for use in a wind turbine blade.
The nanoparticles of the resin may be unevenly distributed throughout the composite structure. Thus, they may be present in a higher concentration in a certain part of the structure, such as a thick part where an increased thermal conductivity might be especially beneficial during production, and in a lower concentration in other parts of the structure. In this way, the total amount of particles used in the production of a composite structure may be kept down, which is advantageous if e.g. the particles are expensive or negatively affect the mechanical properties of the structure.
The discussion above in respect of the inventive method is in applicable parts also relevant for the fibre composite structure. Reference is made to that discussion.
According to another aspect, the present invention relates to a wind turbine, which comprises a fibre composite structure in accordance with the discussion above.
Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached claims as well as from the drawings. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.
Brief Description of the Drawings
The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:
Fig 1 is a schematic side view of a wind turbine blade. Fig 2 is a schematic cross-sectional view, in high magnification, of cured resin, which includes nanoparticles, in the blade of fig 1. Detailed Description of Preferred Embodiments of the Invention
As used in this description the term "thermal percolation", or "thermally percolating effect", describes the effect that is achieved, in accordance with the present invention, when the thermally conductive nanoparticles interact with each other to form a thermally conductive path within the resin. For this effect to be more pronounced, it is most preferred that the nanoparticles are in direct contact with each other to form a chain, which chain is the conductive path. However, a thermally percolating effect may also be achieved even if there is no chain formed, or the chain is discontinuous, whereby heat may travel between two nanoparticles, or chain fragments, by heat diffusion or radiation. See also fig 2.
As discussed above, in the summary part of this description, the invention is particularly advantageous for use in the production of large composite structures. Examples of such large composite structures are wind turbine blades, or spars for use in wind turbine blades, which blades can currently be as large as 44 meters long, and have a diameter of 1.5 meters and a material thickness of about 80 millimetres at the root end of the blade. It is envisioned that even larger blades will be produced in the near future, such as blades being up to 80 meters long. The use of the present invention is thus a great advantage in the production of wind turbine blades.
With reference to fig 1 , a large composite structure exemplified by a typical wind turbine blade 1 comprises a spar, or beam, 2 and a shell member 3, supported by the spar 2 and forming an air foil. The blade 1 comprises a root section 4, adapted to be attached to a hub of a wind turbine rotor. The spar 2, the shell member 3 or the entire blade 3 may be in accordance with the present invention, thus being a composite structure comprising fibres and a resin comprising thermally conductive nanoparticles.
With reference to fig 2, a highly magnified circular part of resin within a composite structure, such as the spar 2 of fig 1 , is described. A thermally conductive path 10 is formed by thermally conductive nanoparticles 11 , here exemplified by needle-shaped carbon nanotubes, in a resin matrix 12, such as a thermosetting epoxy resin. Fig 2 illustrates that even though the nanoparticles 11 are randomly intermixed with the resin 12, they may, provided that the particle concentration is sufficiently high, form chains that may function as conductive paths 10.
The composite structure of the present invention may have an essentially uniform nanoparticle concentration throughout the structure, or the structure may comprise parts having a higher nanoparticle concentration as compared with other parts of the same structure.
The skilled person will appreciate that other types of resin and/or fibres than those explicitly mentioned here may be used in the composite structure of the present invention. The skilled person will also appreciate that the choice of materials, as well as the size and shape of the composite structure, may influence the parameters of the curing process. As a specific example, the temperature to which the composite structure is heated during curing may be dependent on the resin, or resin system, used.
Example
Carbon black nanoparticles are intermixed with a thermosetting epoxy resin to a concentration of 2% by weight. The resin, including the nanoparticles, is infused into a glass fibre structure in a heatable mould, by means of an applied vacuum, to form a wind turbine rotor blade spar being 40 meters long and having a root section diameter of 1.5 meters.
After infusion, the resin/fibre composite structure, i.e. the spar, is heated by means of the mould. The mould is heated to 12O0C, whereby the surface of the structure, which is in contact with the heated mould, acquires a temperature of about 12O0C, and the heat is conducted to the interior of the spar via heat conductive paths formed by the carbon black nanoparticles in the epoxy resin such that the interior of the spar acquires a temperature which is sufficiently high to cure the resin much faster than if the nanoparticles were not present in the resin.
When the spar is fully cured, the heating is discontinued and the composite structure is allowed to cool down, whereby heat from the interior of the spar is conducted via the conductive paths to the surface of the spar, resulting in a more efficient cooling which takes less time than if the nanoparticles were not present in the resin. The spar is subsequently used as the backbone of a wind turbine rotor blade, and the root of the spar is fitted in the hub of a wind turbine rotor during assembly of a wind turbine comprising a tower, a nacelle rotatably mounted on said tower, and said rotor. The invention has above mainly been described with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. It should be specifically noted that the invention is not limited to the field of wind turbines, but may be applicable to a variety of other fields, such as to air planes.

Claims

1. A method of curing a fibre composite structure, comprising: providing a composite structure, said structure comprising fibres and a thermosetting resin; and heating said composite structure; wherein said resin comprises thermally conductive nanoparticles providing a thermally percolating effect within the composite structure during heating; and wherein the fibre composite structure has a material thickness of more than 10 millimetres.
2. The method of claim 1 , wherein the thermally conductive nanoparticles are carbon black nanoparticles.
3. The method of claim 1 , wherein the thermally conductive nano- particles are carbon nanotubes.
4. The method of claim 1 , wherein the thermally conductive nanoparticles are a mixture of carbon black nanoparticles and carbon nanotubes.
5. The method of any one of the preceding claims, wherein the nanoparticles are present in a concentration between 1 and 1.5 percent by weight of the thermosetting resin.
6. The method of any one of the preceding claims, wherein the nanoparticles are randomly intermixed in the thermosetting resin.
7. The method of any one of the preceding claims, wherein the thermosetting resin is a thermosetting epoxy resin.
8. The method of any one of the preceding claims, wherein a surface of the composite structure is heated to in between 1000C and 16O0C.
9. The method of any one of the preceding claims, wherein the composite structure is heated by means of an oven.
10. The method of any one of claims 1-8, wherein the composite structure is heated by means of a heated mould.
11. The method of any one of the preceding claims, wherein the structure has been formed by means of an infusion process prior to the heating.
12. The method of any one of the preceding claims, wherein the composite structure has a material thickness of more than 50 millimetres, preferably more than 100 millimetres.
13. The method of any one of the preceding claims, wherein the composite structure has a material thickness of between 10 and 200 millimetres, preferably between 50 and 170 millimetres, more preferably between 100 and 150 millimetres.
14. The method of any one of the preceding claims, wherein the composite structure is a wind turbine blade or a part of a wind turbine blade.
15. The method of any one of the preceding claims, wherein the composite structure is a spar or a spar cap of a rotor blade.
16. The method of any one of claims 1-14, wherein the composite structure is a root section of a rotor blade.
17. A fibre composite structure comprising fibres and a thermosetting resin, wherein said thermosetting resin comprises thermally conductive nano- particles allowing a thermally percolating effect within the composite structure, and wherein the fibre composite structure has a material thickness of more than 10 millimetres.
18. The structure of claim 17, wherein said structure is a wind turbine blade or a part of a wind turbine blade.
19. The structure of claim 17, wherein said structure is a spar, or a spar cap, intended for use in a wind turbine blade.
20. The structure of claim 17, wherein said structure is a root section for a wind turbine blade.
21. The structure of any one of claims 17-20, wherein the resin of different parts of the structure comprises different concentrations of nano- particles.
22. A wind turbine comprising the structure of any one of claims 17-21.
PCT/EP2009/058274 2008-07-02 2009-07-01 Method of curing a composite structure WO2010000782A1 (en)

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CN102585257A (en) * 2012-02-20 2012-07-18 连云港中复连众复合材料集团有限公司 Heating solidifying method of epoxy resin for preparing fan blade
EP2523856A1 (en) * 2010-01-14 2012-11-21 Saab AB Multifunctional de-icing/anti-icing system of a wind turbine
EP2524133A1 (en) * 2010-01-14 2012-11-21 Saab AB A wind turbine blade having an outer surface with improved properties

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WO2005028174A2 (en) * 2003-06-16 2005-03-31 William Marsh Rice University Fabrication of carbon nanotube reinforced epoxy polymer composites using functionalized carbon nanotubes
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Publication number Priority date Publication date Assignee Title
DE29805833U1 (en) * 1998-03-31 1998-10-08 Holger Mueller Fa Formation of the surface of a rotor blade of a wind turbine
US20040067364A1 (en) * 2002-10-08 2004-04-08 National Aerospace Laboratory Of Japan Carbon nanofiber-dispersed resin fiber-reinforced composite material
WO2005028174A2 (en) * 2003-06-16 2005-03-31 William Marsh Rice University Fabrication of carbon nanotube reinforced epoxy polymer composites using functionalized carbon nanotubes
DE102006048920B3 (en) * 2006-10-10 2008-05-21 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Preparing light-weight component, useful e.g. in vehicle, comprises pre-impregnating semi-fabricated product having e.g. glass and electrically conductive fiber, inserting product into heatable molding tool, applying pressure and hardening

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Publication number Priority date Publication date Assignee Title
EP2523856A1 (en) * 2010-01-14 2012-11-21 Saab AB Multifunctional de-icing/anti-icing system of a wind turbine
EP2524133A1 (en) * 2010-01-14 2012-11-21 Saab AB A wind turbine blade having an outer surface with improved properties
EP2524133A4 (en) * 2010-01-14 2014-08-20 Saab Ab A wind turbine blade having an outer surface with improved properties
EP2523856A4 (en) * 2010-01-14 2015-01-28 Saab Ab Multifunctional de-icing/anti-icing system of a wind turbine
CN102585257A (en) * 2012-02-20 2012-07-18 连云港中复连众复合材料集团有限公司 Heating solidifying method of epoxy resin for preparing fan blade

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