[001] The present invention relates to an epoxy resin adhesive, to a method of manufacturing a wind turbine blade using the epoxy resin adhesive, and to the use of such an epoxy resin adhesive.

[002] The present invention in particular relates to a two-component epoxy resin adhesive comprising a resin component comprising epoxy resin and a curing agent component, also known in the art as a hardener, comprising a curing agent system for curing the epoxy resin after the resin component and the curing agent component have been mixed together. The present invention has particular application for use as an adhesive for bonding together large structural parts, for example structural elements of wind turbine blades during manufacture of the wind turbine blade, or parts of marine vessels or civil engineering structures, which can provide an adhesive bond having high structural strength and high fatigue strength.

[003] It is well known to use epoxy resin adhesives, particularly in combination with fibre- reinforced resin composite materials for the manufacture of structural parts in a variety of industrial sectors. For example, epoxy resin adhesives are used for adhesively bonding together large structural parts, such as structural elements of wind turbine blades during manufacture of the wind turbine blade. The adhesive bond is required to have high structural strength and high fatigue strength.

[004] Many formulations for two-component amine-cured epoxy adhesives currently exist on the market. Such two-component amine-cured epoxy adhesives comprise a resin component comprising epoxy resin and a hardener component comprising an amine curing agent system for curing the epoxy resin after the resin component and the hardener component have been mixed together. These two-component amine-cured epoxy adhesives are the primary choice of those skilled in the art for structural bonding of wind turbine blade parts or elements during manufacture.

[005] These two-component amine-cured epoxy adhesives are typically applied for use in bonding wind turbine blades as follows:

1. The blade producer purchases a container of an epoxy adhesive resin and a separate container of amine “hardener” for the adhesive resin.

2. These two components are mixed together at ambient temperature, often using a semiautomated mixing machine. 3. Once mixed, the amine component starts to react with the epoxy component and the adhesive starts to cure.

4. The blade producer needs to apply the mixed adhesive to the blade shells, for bonding two blade shells together, and/or a spar and or shear web, for bonding a spar to a shear web, these elements being composed of composite material. It is important that the adhesive does not cure during this stage and remains sufficiently fluid to allow uniform application of the adhesive over the entire area to be bonded, prior to closure of the bonded joint.

5. Once the adhesive is applied, the two blade shells, and/or the spar and shear web, are brought into contact to form the bond.

6. The blade assembly is then heated to complete the cure.

7. Upon cure, the blade is cooled, refinished and can enter service life.

[006] A corresponding sequence of steps may be used to make other components or products, for example in the marine or civil engineering industries.

[007] The adhesive bond between the blade shells or between the shear web and spars is required to exhibit high structural strength, in particular high tensile strength and high flexural strength, and high fatigue strength. The adhesive bond is also required to exhibit high toughness, which for example provides high impact strength.

[008] It is known to add toughening components to the epoxy resin adhesive, in order to enhance the toughness and fatigue strength of the adhesive bond. Historically, the toughening component typically comprised glass particles. More recently, rubber toughening components have been used instead of glass particles.

[009] The adhesive bond is required to provide good mechanical properties, as described above. Also, particularly when manufacturing a wind turbine blade, the density of the adhesive bond is desired to be low, since clearly it is desirable for a wind turbine blade having given mechanical properties and dimensions to have reduced weight. Rubber toughening components have been used in adhesives for wind turbine blades in place of glass particles in order to reduce the weight of the adhesive bond. However, rubber toughening components are more expensive than glass particles; therefore in order to achieve the combination of good mechanical properties and low density adhesive bonds, the cost of the epoxy resin adhesive for achieving a given adhesive bond has increased by the use of rubber toughening components instead of glass particles. [010] There is therefore a need in the art for an amine-cured epoxy resin adhesive that can be used to form an adhesive bond which exhibits the combination of good mechanical properties and low density, and yet can be produced at low cost.

[Oi l] The present invention aims at least partially to overcome this technical problem which is encountered by the use of known amine-cured epoxy resin adhesives.

[012] The present invention aims to provide an epoxy resin adhesive which can achieve the technical advantage of the combination of good mechanical properties and low density in a resultant an adhesive bond, and yet can be produced at reduced cost, for example as compared to known amine-cured epoxy resin adhesives, in order to overcome this technical problem with known amine-cured epoxy resin adhesives.

[013] The present invention also aims to provide an epoxy resin adhesive which can achieve a good balance between mechanical properties of the adhesive bond, low density of the adhesive bond and cost, particularly when the adhesive is used to bond together large structural parts, for example wind turbine blades, composed of composite material.

[014] Accordingly, in a first aspect, the present invention provides a curable epoxy resin adhesive according to claim 1.

[015] In a second aspect, the present invention provides a method of manufacturing a wind turbine blade according to claim 34.

[016] In a third aspect, the present invention provides a use of a curable epoxy resin adhesive according to claim 35.

[017] Preferred features of these aspects of the present invention are defined in the dependent claims.

[018] The present inventors have addressed the above-described problem of known amine- cured epoxy resin adhesives and have unexpectedly found that by providing a combination of two specific first and second toughening components, in particular a combination of glass particles and a rubber toughener, an adhesive bond can be formed which has substantially equivalent mechanical properties but with the added advantage of lower density, and slightly higher cost, as compared to a known known amine-cured epoxy resin adhesive having a toughening agent composed of glass particles, or alternatively that the adhesive bond has improved mechanical properties and slightly higher density but with the added advantage of lower cost, as compared to a known known amine-cured epoxy resin adhesive having a toughening agent composed of rubber toughener. [019] Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:-

Figure l is a graph which schematically shows the measured tensile stress and tensile strain of cured epoxy resin adhesives in accordance with Examples of the present invention and Comparative Examples; and

Figure 2 is a graph which schematically shows the measured flexural stress and flexural strain of cured epoxy resin adhesives in accordance with Examples of the present invention and Comparative Examples.

[020] In accordance with the present invention there is provided a curable epoxy resin adhesive. [021] The present invention has particular application for use as an adhesive for bonding together large structural parts, for example structural elements of wind turbine blades during manufacture of the wind turbine blade, or parts of marine vessels or civil engineering structures, for which an adhesive bond having good mechanical properties, in particular high mechanical strength and high fatigue strength, as well as good impact strength, is required. When forming an adhesive bond between adjacent structural parts of a wind turbine blade, typically the adjacent structural parts comprise a spar car and a shear web of a wind turbine blade or opposed edges of opposite outer shells of a wind turbine blade

[022] Epoxy resins exhibit excellent adhesive properties and mechanical properties. Therefore the epoxy resins used in the preferred embodiments of the present invention can easily meet the adhesive bonding requirements to enable an epoxy resin adhesive layer to bond strongly to materials used in wind blade manufacture, such as a fibre-reinforced resin matrix composite material, for example an epoxy resin matrix composite reinforced with glass or carbon fibres, or a core material, for example the surface of a polymeric cellular foam (e.g. PET structural foam), balsa or honeycomb (e.g. Nomex ® honeycomb) core material.

[023] The curable epoxy resin adhesive in accordance with the present invention comprises an epoxy resin component and a curing agent component. The epoxy resin component and the curing agent component are in a separated form. When a mixture of the epoxy resin component and the curing agent component is provided at a curing temperature of the adhesive, the epoxy resin component is caused to cure by the curing agent component.

[024] Preferably, the epoxy resin component and the curing agent component are selected to allow a long working time at ambient temperature, such as 20 °C, for the uncured mixture of the resin and hardener components, in combination with a fast curing time at an elevated curing temperature. A longer working time has particular utility when bonding large surface areas. The shorter cure time reduces the overall curing cycle time, which can achieve cost savings both for operational production costs and for capital investment costs. The selection of suitable combinations of epoxy resins and amine curing agents to achieve a desired working time and cure time is known to those skilled in the art.

[025] The curable epoxy resin adhesive comprises an epoxy resin component and a curing agent component. The epoxy resin component and the curing agent component are made and sold in a separated form. During use of the adhesive, a mixture of the epoxy resin component and the curing agent component is formed, as is well known in the art of thermosetting resins, with the epoxy resin component and the curing agent component being mixed at a predetermined ratio, typically a weight ratio, so that the curing agent provides the required number of reactive groups to react with the number of epoxide groups present in the amount of epoxy resin to be cured. The mixture is then usually heated to an elevated curing temperature, which causes curing of the epoxy resin component by the curing agent component, to form the cured adhesive. A typical curing temperature for a curable epoxy resin adhesive is within the range of from 50 to 100 °C, for example 70 °C.

[026] The epoxy resin component comprises at least one epoxide-containing resin. The at least one epoxide-containing resin is typically selected from at least one of a bisphenol-based epoxy resin, an epoxy novolac resin, an epoxy cresol novolac resin and an epoxy phenol novolac resin, or a mixture of any two or more thereof. Epoxide-containing reactive diluents may also be used, as is well known to those skilled in the art. Epoxy resins of different epoxide functionality, and/or different epoxide equivalent weight (EEW) may be blended to achieve desired properties in both the curable resin and the cured resin, as is well known in the art. Typically, the epoxy resin component has an overall epoxy equivalent weight (EEW) of from 175 to 300 g/eq. In a preferred embodiment, the epoxy resin component comprises at least 50 wt%, optionally from 50 to 70 wt%, of a diglycidyl ether bisphenol-A (DGEBA) epoxy resin, based on the total weight of the epoxy resin component. Typically, the diglycidyl ether bisphenol-A (DGEBA) epoxy resin is liquid at 25 °C. Typically, the diglycidyl ether bisphenol - A (DGEBA) epoxy resin has an epoxy equivalent weight (EEW) of from 175 to 200 g/eq.

[027] The curing agent component comprises at least one amine curing agent, and typically a mixture of two or more amine curing agents. Typically, the curing agent component is present in the curable epoxy adhesive in a concentration of from 20 to 50 parts by weight, optionally from 25 to 45 parts by weight, further optionally from 30 to 40 parts by weight, curing agent component to 100 parts by weight epoxy resin component to provide the total weight of the curable epoxy adhesive. Too low an amount of the curing agent component may cause a reduced cure of the epoxy resin material, whereas too high an amount may cause an excessively exothermic cure, reduced Tg and reduced mechanical properties. The cured adhesive can have a high Tg2, and therefore has high mechanical properties associated with a high degree of curing and crosslinking. In some embodiments of the present invention, the epoxy resin adhesive may further comprise an accelerator, which may be formulated so that the epoxy resin may be cured at a selected curing temperature and/or to modify the Tg2 of the cured resin. Any known accelerator may optionally be added to the adhesive, as known to those skilled in the art.

[028] The one or more amine curing agents are selected to provide a desired working time and cure time.

[029] As is well known to those skilled in the art, the reactivity of an amine curing agent used for curing epoxy resins, and the final properties of the cured polymer, can be made to vary significantly depending on the number, type, and position of the reactive -NH _X groups within the amine curing agent. In particular, the degree of electronic effects, such as electron delocalisation, and steric hindrance can affect the reactivity of an amine curing agent.

[030] In accordance with some preferred embodiments of the present invention, a first amine curing agent may be considered to be a “base” amine curing agent and additional curing agents may be used in addition to the first curing agent to provide an amine curing agent blend which, when used, can provide the desired working time and final properties of the cured polymer resin.

[031] In accordance with some preferred embodiments of the present invention, the curing agent component comprises an amine curing agent which comprises a polyoxyalkylene diamine having the formula NH2-(Z)x-R-NH2 where R is an aliphatic or cycloaliphatic constituent, which is either unsubstituted or substituted with at least one functional group, Z is an oxyalkylene moiety and x has a value of greater than 1 and less than 10.

[032] Typically, the at least one amine curing agent comprises a polyoxyalkylene diamine having the formula NH2-(Z)x-R-NH2, Z is an oxypropylene moiety, and/or x has a value within the range of from 2 to 5, and/or R is an alkyl moiety having from 2 to 5 carbon atoms. In a preferred embodiment, the poly oxyalkylene diamine having the formula NH2-(Z)x-R-NH2 has a number average molecular weight Mn of from 175 to 300, for example about 230.

[033] Another suitable amine curing agent is available in commerce from Huntsman Advanced Materials under the trade name Jeffamine® D-230. This known amine curing agent is a difunctional primary amine, in particular a polyoxypropylenediamine having the formula: x 2.5 which has a number average molecular weight Mn of about 230. This polyoxypropylenediamine has an active hydrogen equivalent weight (AHEW) of 60 g/eq.

[034] In some embodiments of the present invention, the at least one amine curing agent comprises a di(aminoalkyl) benzene, wherein each alkyl group has from 1 to 3 carbon atoms and the alkyl groups are the same or different in each aminoalkyl functional group. For example, in some preferred embodiments of the present invention the amine curing agent comprises xylylenediamine, such as 1,3-Bis(aminomethyl)benzene (/^-xylylenediamine) or 1,4-Bis(aminomethyl)benzene ( /-xylylenediamine), a di(aminoethyl) benzene such as 1,3- Bis(aminoethyl)benzene or 1,4-Bis(aminoethyl)benzene, or a di(aminopropyl) benzene such as 1,3-Bis(aminopropyl)benzene or 1,4-Bis(aminopropyl)benzene, or any mixture of two or more thereof.

[035] The at least one amine curing agent may comprise xylylenediamine, such as 1,3- Bis(aminomethyl)benzene (m-xylylenedi amine) or 1,4-Bis(aminomethyl)benzene (p- xylylenediamine), A preferred amine curing agent is 1,3-Bis(aminomethyl)benzene (m- xylylenediamine) which is widely available in commerce under CAS Number 1477-55-0. This amine curing agent is also called m-XDA, or sometimes MXDA.

[036] Another preferred amine curing agent is available in commerce from Bitrez under the trade name Aramine ^(RTM) 31-706; this amine curing agent is a modified TETA (tri ethylene tetramine), in particular an amine adduct of triethylene tetramine.

[037] Finally, yet another preferred amine curing agent is a cycloaliphatic amine-based curing agent, for example isophorone diamine.

[038] Typically, the curing agent component comprises from 50 to 85 wt%, optionally from 55 to 70 wt%, of the at least one amine curing agent, based on the total weight of the curing agent component.

[039] In accordance with the present invention, the epoxy resin component comprises, in addition to the at least one epoxide-containing resin, a first toughening component, and the curing agent component comprises, in addition to the at least one amine curing agent, a second toughening component. The first and second toughening components are each selected from the group consisting of (a) glass particles and (b) a rubber toughener. The first and second toughening components are different selections from the group whereby the mixture of the epoxy resin component and the curing agent component comprises the combination of (a) the glass particles and (b) the rubber toughener.

[040] In one preferred embodiment of the present invention, the first toughening component comprises the glass particles and the second toughening component comprises the rubber toughener. Preferably, the epoxy resin component comprises from 10 to 25 wt% of the glass particles, based on the total weight of the epoxy resin component, and the curing agent component comprises from 4 to 10 wt% of the rubber toughener, based on the total weight of the curing agent component. In one particularly preferred embodiment, the epoxy resin component comprises from 15 to 20 wt% of the glass particles, based on the total weight of the epoxy resin component, and the curing agent component comprises from 6 to 8 wt% of the rubber toughener, based on the total weight of the curing agent component.

[041] In an alternative preferred embodiment of the present invention, the first toughening component comprises the rubber toughener and the second toughening component comprises the glass particles. Preferably, the epoxy resin component comprises from 4 to 10 wt% of the rubber toughener, based on the total weight of the epoxy resin component, and the curing agent component comprises from 10 to 25 wt% of the glass particles, based on the total weight of the curing agent component. In one particularly preferred embodiment, the epoxy resin component comprises from 6 to 8 wt% of the rubber toughener, based on the total weight of the epoxy resin component, and the curing agent component comprises from 15 to 20 wt% of the glass particles, based on the total weight of the curing agent component.

[042] Therefore typically the epoxy resin component and the curing agent component comprise respective different tougheners. i.e. glass particles or a rubber toughener. By providing different tougheners in the different respective epoxy resin and curing agent components, these two different epoxy resin and curing agent components can be easily formulated, because only one toughener is mixed into the respective component. In the resultant mixture of the epoxy resin and curing agent components, the two tougheners can each be homogeneously mixed throughout the entire mixture, and the cured resin comprises a blend of both tougheners homogeneously mixed throughout the entire cured resin.

[043] However, alternatively, in less preferred embodiments of the present invention, the epoxy resin component and the curing agent component may each comprise a plurality of the different tougheners. i.e. the combination of both glass particles and rubber toughener. By providing a plurality of different tougheners in each of the epoxy resin and curing agent components, these two components are less easy to formulate, because at least two tougheners are mixed into the respective component. In the resultant mixture of the epoxy resin and curing agent components, the two tougheners are homogeneously mixed throughout the entire mixture, and the cured resin comprises a blend of both tougheners homogeneously mixed throughout the entire cured resin.

[044] In preferred embodiments of the present invention, the combination of the epoxy resin component and the curing agent component comprises from 9 to 16 wt% of the combination of the glass particles and the rubber toughener, based on the total weight of the combination of the epoxy resin component and the curing agent component.

[045] Typically, in some preferred embodiments of the present invention, the epoxy resin component and the curing agent component are present in a weight ratio of 100 parts by weight of the epoxy resin component to from 25 to 45 parts by weight of the curing agent component. [046] Preferably, the glass particles are glass fibre particles which have an average, which is a mean average by number, fibre length within the range of from 150 to 350 microns, preferably from 180 to 250 microns, more preferably from 200 to 240 microns. Preferably, the glass fibre particles have a round cross-section and an average, which is a mean average by number, diameter within the range of from 7 to 16 microns, more preferably from 9 to 14 microns.

[047] The particle size is measured using a FlowCAM ® PV-Series imaging-based particle analyser, which is a dynamic imaging system available in commerce from Lawson Scientific Limited, Leighton Buzzard, UK.

[048] In particularly preferred embodiment of the present invention, the glass particles comprise E-glass fibre particles sold as a glass fibre filler having the product code FG 400/060 which are available in commerce from Sparkford, Winchester, Hampshire, UK. These E-glass fibre particles have an average, which is a mean average by number, fibre length of 230 microns, a round cross-section, and an average, which is a mean average by number, diameter within the range of from 9 to 14 microns.

[049] Preferably, the rubber toughener comprises rubber particles which have an average, which is a mean average by number, particle size within the range of from 15 to 1500 nm. The rubber particle size is measured using Scanning Electron Microscopy (SEM), by taking an SEM image of a plurality of the rubber particles, measuring the maximum dimension of a statistically significant number of the imaged rubber particles, and calculating from the measured values a mean average particle size, such technique being well-known to those skilled in the art. [050] In preferred embodiments of the present invention, the rubber toughener comprises a core/ shell impact modifier, for example having an acrylic shell and an acrylate rubber core, typically a butyl acrylate rubber core.

[051] The curing agent component and/or the epoxy resin component may comprise additional ingredients, such as fillers, diluents, rheology modifiers, etc. The rheology modifier may typically comprise at least one of a thermoplastic resin and an inorganic particulate thickener or a mixture thereof. Inorganic fillers such as calcium carbonate, fumed silica, wetting additives, air release additives, coupling agents, etc may additionally or alternatively be present in any combination as is well known to those skilled in the art of epoxy resin adhesives.

[052] In preferred embodiments of the present invention, at least one or each of the epoxy resin component and the curing agent component further comprises a rheology modifier system, which comprises inorganic particles having an average, which is a mean average by number, primary particle size within the range of less than 1 micron, preferably from 2 to 100 nm. The particle size is measured using a FlowCAM ® PV-Series imaging-based particle analyser, which is a dynamic imaging system available in commerce from Lawson Scientific Limited, Leighton Buzzard, UK.

[053] Typically, the inorganic particles of the rheology modifier system comprise fumed silica, calcium carbonate or any mixture thereof.

[054] Typically, the inorganic particles of the rheology modifier system have an average, which is a mean average by number, primary particle size within the range of less than 1 micron, preferably from 2 to 100 nm.

[055] In a preferred embodiment, the fumed silica has an average, which is a mean average by number, primary particle size within the range of less than 1 micron, preferably from 2 to 100 nm, and more preferably has a primary particle size within the range of from 5 to 50 nm. Preferably, the fumed silica is a hydrophobic fumed silica which has been surface treated with a polysiloxane, for example a polydimethylsiloxane.

[056] In a preferred embodiment, the calcium carbonate has an average (d50) primary particle size within the range of from 2 to 4 microns, for example about 3 microns.

[057] The present invention further provides a method of manufacturing a wind turbine blade, the method comprising the steps of: i. providing the curable epoxy resin adhesive according to the present invention as described hereinabove; ii. mixing together the epoxy resin component and the curing agent component to form a curable mixture; and iii. providing the curable mixture between adj acent structural parts of a wind turbine blade, wherein the adjacent structural parts comprise a spar car and a shear web of a wind turbine blade and/or opposed edges of opposite outer shells of a wind turbine blade; and iv. heating the curable mixture at a curing temperature to cure the epoxy resin component by the curing agent component thereby to bond together the adjacent structural parts of the wind turbine blade.

[058] The present invention further provides a use of the curable epoxy resin adhesive according to the present invention as described hereinabove, for bonding together structural parts of a wind turbine blade, a marine vessel or a civil engineering structure.

[059] The preferred embodiments of the present invention will now be described further with reference to the following non-limiting Examples.

Example 1

[060] A curable epoxy resin, comprising an epoxy resin component and a curing agent component having the respective compositions shown in Table 1 as separate components, was provided.

[061] The epoxy resin component comprised a typical blend of epoxide resins and reactive diluents to form a liquid epoxide-containing composition suitable for forming an adhesive with flow properties. The curing agent component comprised a typical blend of amine curing agents to provide what the skilled person would typically recognise as a reasonable working time and a reasonable cure time for any given component to be manufactured.

[062] Two tougheners are provided, and in this Example glass particles were present in the epoxy resin component and a rubber toughener was present in the curing agent component.

[063] The glass particles were glass fibre particles, composed of E-glass fibre particles sold as a glass fibre filler having the product code FG 400/060 which are available in commerce from Sparkford, Winchester, Hampshire, UK. These E-glass fibre particles have an average, which is a mean average by number, fibre length of 230 microns, a round cross-section, and an average, which is a mean average by number, diameter within the range of from 9 to 14 microns. In these E-glass fibre particles, the fibre length distribution, measured using a FlowCAM ® PV-Series imaging-based particle analyser, which is a dynamic imaging system available in commerce from Lawson Scientific Limited, Leighton Buzzard, UK, was as follows: >30% by number with fibre length <125 microns, >70% by number with fibre length <250 microns, 99% by number with fibre length <1000 microns, the longest fibre length was 1300 microns, and the fibre length distribution was measured on three samples of 100 fibres, and the average, i.e. mean, fibre length was calculated by number. [064] The rubber toughener was an acrylic core-shell impact modifier based on a butyl-acrylate rubber core and is available in commerce from Dow Chemicals, Inc., USA, under the trade name Paraloid ® EXL 2314 and has an average, which is a mean average by number, particle size of 150-1500 nm. The rubber particle size is measured using Scanning Electron Microscopy (SEM), by taking an SEM image of a plurality of the rubber particles, measuring the maximum dimension of a statistically significant number of the imaged rubber particles, and calculating from the measured values a mean average particle size, such technique being well-known to those skilled in the art.

[065] The epoxy resin component and the curing agent component also each comprised two rheology modifiers, in the form of fumed silica and calcium carbonate.

[066] The fumed silica was a hydrophobic fumed silica which has been surface treated with polydimethylsiloxane and is available in commerce from Cabot Corporation, Mass., USA, under the trade name CAB-O-SIL ® TS-720. The fumed silica had a primary particle size within the range of from 5 to 50 nm.

[067] The calcium carbonate had an average (d50) primary particle size of about 3 microns and is available in commerce from Nexchem Ltd, Leicester, UK, under the trade name Snowcal ® 30B75.

[068] The epoxy resin component and the curing agent component were provided in a mix ratio, by weight, of 100:30 as shown in Table 1.

[069] The epoxy resin component and the curing agent component were homogeneously mixed to form a mixture of the curable resin composition. In the mixture, both of the first and second toughening components, namely the milled glass particles and the rubber toughener, were homogeneously dispersed throughout the curable epoxy resin adhesive mixture.

[070] The curable resin mixture was then cured in air by holding a sample of the mixture at room temperature (20°C) for 2 hours and then heating the sample at an elevated curing temperature of 70°C for 2 hours. The sample had a thickness of 4 mm.

[071] The cured resin was then tested to determine the mechanical properties of the cured resin. In particular, the tensile strength, tensile modulus and tensile strain which were measured under the test protocol of ISO 527-2, and the flexural strength, flexural modulus and flexural strain which were measured under the test protocol of ISO 178. The Charpy Impact Strength according to ISO 179 and the K1C Plane Strain Fracture Toughness according to ASTM D5045 were also measured. Multiple samples of the cured resin were tested and the standard deviation of the measured parameters was calculated. The density of the cured resin was also measured, [072] The results are shown in Table 2 and illustrated in Figure 1, showing the tensile strength and tensile strain results, and Figure 2 showing the flexural strength and flexural strain results. [073] It may be seen from Table 1 and Figures 1 and 2 that when the adhesive of Example 1 is cured at 70°C, the mechanical properties of the cured resin of Example 1 are acceptable for use as an adhesive for bonding structural parts such as parts of a wind turbine blade. The cured resin density is also sufficiently low to be acceptable for use as an adhesive for bonding structural parts, such as parts of a wind turbine blade.

Comparative Example 1

[074] In Comparative Example 1, the same base epoxy resin component and milled glass particles as the first toughening component were used as in Example 1. However, the curing agent component was modified to comprise milled glass particles as the second toughening component, rather than the rubber toughener. In other words, in Comparative Example 1 the milled glass particles were the sole toughening component in the cured resin, and different proportions of the sole toughening component were provided by both the epoxy resin component and the curing agent component.

[075] The mechanical properties and density of the cured resin were tested as in Example 1 and the results are shown in Table 2 and illustrated in Figures 1 and 2.

[076] A comparison of the mechanical properties of the cured resins of Example 1 and Comparative Example 1 shows that the use of a combination of milled glass particles and rubber toughener, as compared to the use of only milled glass particles, can provide the technical effect that the total toughener content and the density of the cured resin can be decreased while maintaining substantially the same mechanical properties, in particular tensile properties (strength, strain and modulus) and flexural properties (strength, strain and modulus). The achievement of these substantially similar mechanical properties at reduced cured resin density is unexpected, and a technical advantage, because this would reduce the total weight of adhesive required to bond the structural parts of a given wind turbine blade. Although the measured toughness parameters (Charpy Impact and K1C) are lower in Example 1 than Comparative Example 1, nevertheless the measured values are acceptable for use in a structural adhesive.

[077] From the various mechanical properties which were measured, the fatigue strength of the cured epoxy resin of Example 1 would be expected to be higher than the fatigue strength of the cured epoxy resin of Comparative Example 1, because the addition of the rubber toughener would be expected to make a greater contribution to fatigue strength than the substituted milled glass particles.

[078] Moreover comparing the compositions of Example 1 and Comparative Example 1, a small amount of rubber toughener effectively substitutes for a larger proportion a milled glass particles to achieve similar mechanical properties and lower density. The current commercial cost of the rubber toughener, on a per weight basis, is higher than the current commercial cost of the milled glass particles, on a per weight basis. On balance, the lower density is achieved at a slight increase in the total material cost of the curable resin adhesive. The present inventors have determined that the lower adhesive density is achieved in a very cost-effective manner and that the technical advantage of the reduced weight in the adhesive-bonded structure gives unexpected total weight savings in a very cost-effective manner.

Example 2

[079] A curable epoxy resin, comprising an epoxy resin component and a curing agent component having the respective compositions shown in Table 1 as separate components, was provided.

[080] As compared to Example 1, the epoxy resin component comprised a slightly modified typical blend of epoxide resins and reactive diluents to form a liquid composition suitable for forming an curable epoxy resin adhesive with flow properties. The curing agent component comprised the same base curing agent composition that was used in Comparative Example 1. [081 ] Two tougheners were again provided, and in Example 2 the rubber toughener (rather than the milled glass particles as in Example 1) was present in the epoxy resin component and the milled glass particles (rather than the rubber toughener as in Example 1) were present in the curing agent component.

[082] The cured resin was then tested as in Example 1 to determine the mechanical properties and density of the cured epoxy resin adhesive.

[083] The results are shown in Table 2 and illustrated in Figures 1 and 2.

[084] It may be seen from Table 1 and Figures 1 and 2 that when the adhesive of Example 2 is cured at 70°C, the mechanical properties of the cured resin of Example 2 are again acceptable for use as an adhesive for bonding structural parts such as parts of a wind turbine blade. The cured resin density is also sufficiently low to be acceptable for use as an adhesive for bonding structural parts, such as parts of a wind turbine blade.

Comparative Example 2 [085] In Comparative Example 2, the same base epoxy resin component and rubber toughener as the first toughening component was used as in Example 2. However, the curing agent component was modified to comprise rubber toughener as the second toughening component, rather than the milled glass particles. In other words, in Comparative Example 2 the rubber toughener was the sole toughening component in the cured resin, and different proportions of the sole toughening component were provided by both the epoxy resin component and the curing agent component.

[086] The mechanical properties and density of the cured resin were tested as in Example 1 and the results are shown in Table 2 and illustrated in Figures 1 and 2.

[087] A comparison of the mechanical properties of the cured resins of Example 2 and Comparative Example 2 shows that the use of a combination of milled glass particles and rubber toughener, as compared to the use of only rubber toughener, the mechanical properties, in particular tensile properties (strength, strain and modulus) and flexural properties (strength, strain and modulus), can be substantially increased while slightly increasing the density of the cured resin. The achievement of these substantially increased mechanical properties at slightly increased cured resin density is again unexpected, and a technical advantage, because this would potentially permit a lower total weight of the adhesive to be used for a given bond strength despite the slight increase in adhesive density, and thereby reduce the total weight of adhesive required to bond the structural parts of a given wind turbine blade. The measured toughness parameters (Charpy Impact and K1C) in Example 2 are acceptable for use in a structural adhesive.

[088] From the various mechanical properties which were measured, the fatigue strength of the cured epoxy resin of Example 2 would be expected to be acceptable for use in a structural adhesive.

[089] Moreover comparing the compositions of Comparative Example 2 and Example 2, a small amount of rubber toughener is effectively substituted by a larger proportion a milled glass particles to achieve improved mechanical properties and a slight increase in density. On balance, the slight increase in density is offset by a decrease in the total material cost of the curable resin adhesive. The present inventors have determined that the improved mechanical properties are achieved in a very cost-effective manner and that the technical advantage of the improved mechanical properties in the adhesive-bonded structure gives unexpected total weight savings in a very cost-effective manner. [090] In summary, by using the toughener system in accordance with the present invention, a high strength, low weight and cost-effective curable epoxy resin adhesive is provided which provides a combination of improved properties as compared to known curable epoxy resin adhesive, and has particular advantages when used to achieve structural bonds between structural parts, in particular in the manufacture of wind turbine blades.

[091] Various modifications to the preferred embodiments of the present invention, as defined in the appended claims, will be apparent to those skilled in the art.

Table 1 Table 2