UV-curable filler compositions

Abstract

The present invention provides a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair system, the UV-curable filler composition comprising a vinyl ester oligomer, a reactive diluent monomer mixture, a photoinitiator and an adhesion promoter. The UV- curable filler composition is particularly suitable for use in a composite repair process, such as for wind blade repair. The present invention also relates to a multi-part repair system comprising the UV-curable filler composition and a UV-curable coating composition, and to cured products made from the UV- curable filler composition.

Description

[0001] UV-CURABLE FILLER COMPOSITIONS

[0002] Field of the Invention

[0003] The present invention relates to a UV-curable filler composition, and to uses and cured products thereof.

[0004] Background

[0005] Wind turbines are becoming increasingly ubiquitous as environmentally safe and relatively inexpensive alternative energy sources. However, wind turbines are susceptible to both damage in the environment, such as from surface erosion, wear or lightning, and during and after in-factory manufacturing. In some cases, the damage sustained may be surface-level or cosmetic, which has an adverse effect on the efficiency and lifetime of the wind turbine. In more extreme cases, the damage sustained may be structural, which has an adverse effect on the operation, and so can increase the risk of a complete failure or breakdown of the wind turbine.

[0006] To maintain the upkeep of wind turbines so that they continue to be reliable and efficient, it is necessary to develop and optimise manufacturing and repair techniques. The repair may be post-production, infactory repair and in-field repair.

[0007] Conventional techniques for wind turbine blade repair typically employ a filler composition, which is for applying onto damaged areas, such as gaps, voids or imperfections, caused by both environmental damage and during and after in-factory manufacturing. These techniques involve a section-by-section repair approach, in which only a portion of the wind blade to be repaired is cured at any given time. Typically, between each section and stage of the repair, the blade is only permitted to rotate at an angle between 30 to 45 degrees to avoid resin sagging and drippage. As a result, the repair of each section of the blade may in some cases require six hours, such as up to 8 hours, before the next section and stage of the repair is permitted to proceed.

[0008] Furthermore, conventional techniques also require a step of post-curing, which involves transferring the entire blade into an autoclave or oven at elevated temperatures, such as 80 °C or more. Naturally, such curing procedures require a long thermal curing process, necessitating long man-hours for wind blade repair processes. Moreover, high-temperature post-curing may also lead to undesirable strain in the materials due to different expansion coefficients in the respective fibre-reinforced composite materials composed in a wind turbine.

[0009] For these reasons, current methods used to repair wind blades necessitate time, cost and human labour, including specialist repair methods and facilities. As such, the manufacture and repair of wind blade parts is both complex and expensive.

[0010] It is known that UV curing may provide a route to a more efficient repair process due to the potentially faster curing. However, known UV curing processes alone are limited to thin laminate curing because of the poor penetration of UV light required for a deeper cure. This poses a significant drawback in wind blade repair as some parts of a wind blade would otherwise require a light penetration of 70 mm for the deeper cure. Such repair processes employing a UV cure are described in the art.

[0011] For example, US 2014 / 0077420 discloses a UV-curable composition including UV curable vinyl ester resin, a phenoxyethyl methacrylate adhesion promoter and a photoinitiator, in which the composition is used for wind blade repair and exhibits bonding strength to composite structures with the addition of cobalt naphthenate. The composition described in US 2014 / 0077420 uses a standard vinyl ester resin comprising styrene as a reactive diluent monomer.

[0012] In conclusion, conventional curing methods typically require significant amounts of time and energy to perform wind blade repair, including practical implications as a result of the inherent size, complexity and magnitude of wind blades.

[0013] Consequently, there remains a need within the art for improvements in the filler compositions for repair applications, which provide a faster, more complete and more cost-effective process, and without a high temperature input. The cured product must provide excellent mechanical, thermal and adhesion properties to support high performance applications such as its use in wind blade parts.

[0014] The present invention has been devised in light of the above considerations.

[0015] Summary of the Invention

[0016] In a first aspect of the invention, there is provided a UV-curable filler composition for use with a UV- curable coating composition in a multi-part repair resin system, UV-curable filler composition comprising:

[0017] (A) a vinyl ester resin composition comprising:

[0018] (a) a vinyl ester oligomer;

[0019] (b) a reactive diluent monomer mixture comprising a first reactive diluent monomer having at least one polymerisable functional group and a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0020] (B) a photoinitiator; and

[0021] (C) an adhesion promoter for use in UV curing.

[0022] Surprisingly, the inventors have found that the UV-curable filler composition of the first aspect can achieve a faster and more complete cure and, importantly, while also maintaining excellent mechanical, thermal and adhesion properties.

[0023] Firstly, the inventors have found that the use of a photoinitiator within a UV-curable filler composition which comprises the combination of the vinyl ester oligomer and the reactive diluent monomer mixture provides an improved curing time without requiring other initiators, such as cobalt accelerators and peroxides. Therefore, the health and safety issues that are typically associated with curing systems employing cobalt accelerators and peroxides are minimised.

[0024] Secondly, the inventors have found that the use of a photoinitiator within a UV-curable filler composition which comprises the combination of the vinyl ester oligomer and the reactive diluent monomer mixture allows for a more complete UV-induced cure. Hence, the need for a post-cure step as described in the art is not required, and the requirements of time and energy necessary for performing the repair procedures are significantly reduced. The combination of a faster cure and a more complete cure, which is generally independent of temperature, allows for a much faster turnaround of parts, as well as a larger repair window.

[0025] Thirdly, the inventors have found that, once cured, the UV-curable filler composition of the first aspect having reactive diluent monomers which each have a different number of polymerisable functional groups, and an adhesion promoter which is suitable for use in UV curing, also maintains excellent mechanical properties, such as tensile modulus, tensile strength, tensile elongation, Barcol hardness and Shore D hardness, excellent thermal properties, such as Tg(glass-transitional temperature), and excellent adhesion properties, such as adhesion pull-off strength. In particular, the provision of reactive diluent monomers which each have a different number of polymerisable functional groups, such as the combination of a monofunctional reactive diluent monomer and a difunctional reactive diluent monomer, or the combination of a difunctional reactive diluent monomer and a trifunctional reactive diluent monomer, is demonstrated in the Examples section to provide these excellent mechanical properties.

[0026] A second aspect of the invention is a method of preparing the UV-curable filler composition of the first aspect, the method comprising combining the vinyl ester resin composition, as a vinyl ester oligomer and a reactive diluent monomer mixture, the photoinitiator, and the adhesion promoter. Optionally, the reactive diluent monomer mixture is notionally split in the method, such that it comprises a first monomer portion and a second monomer portion, as described herein, so that the monomer portions are combined sequentially into the reaction mixture in the method.

[0027] A third aspect of the invention is a method of curing the UV-curable filler composition of the first aspect, the method comprising a step of exposing the UV-curable filler composition to UV light to initiate a cure via a free radical polymerisation.

[0028] A fourth aspect of the invention is the use of the UV-curable filler composition of the first aspect in a composite repair process, such as a wind turbine repair process, such as a wind blade repair process, a tower repair process or a nacelle repair process.

[0029] A fifth aspect of the invention is a cured product of the UV-curable filler composition of the first aspect. The cured product may be comprised in an article or an object, to which the invention also relates.

[0030] Preferably, in the fifth aspect, the cured product has a tensile modulus of at least 3000 MPa, a tensile strength of at least 39 MPa, a tensile elongation of at least 1 .3 % , a Barcol hardness of at least 25, a Shore D hardness of at least 70 D, a Tgof at least 60 °C and an adhesion pull-off strength of at least 5.0 MPa, wherein the tensile modulus, tensile strength, tensile elongation, Barcol hardness, Shore D hardness, Tgand adhesion-pull off strength are each measured in accordance with the methods provided in the methods section.

[0031] A sixth aspect of the invention is a wind blade part comprising the cured product of the third aspect.

[0032] Suitably, the UV-curable filler composition of the first aspect is intended to be used together with a UV- curable coating composition. In a typical resin repair process, a filler composition is employed to fill gaps, voids or imperfections, such as those caused by environmental damage, on the surface of a cured product, such as a wind blade part. A coating composition is a surface layer which is applied over the cured resin, such as over the UV-curable filler composition, for the purposes of enhancing its weathering properties, such as to provide improved stability against UV light, and its aesthetic appearance, such as to provide a particular colour pigmentation.

[0033] A seventh aspect of the invention is a multi-part repair system comprising a plurality of parts, which in combination contains the UV-curable filler composition of the first aspect together with a UV-curable coating composition, wherein the UV-curable coating composition comprises a pigment.

[0034] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0035] Preferred embodiments of the invention may also include any one or more of the following preferred features. Preferred features mentioned in relation to the first aspect of the invention may apply equally to the other aspects.

[0036] Detailed Description of the Invention

[0037] As used herein, the term “(meth)acrylate” refers to both or any one of “acrylate” and “methacrylate” and this well-known abbreviation will be used throughout.

[0038] As used herein, the term “chain-extending reagent” refers to a compound which, when incorporated into the vinyl ester oligomer by reaction, increases the length and molecular weight of the molecular chain.

[0039] As used herein, the term “monomer” refers to a small molecule that may chemically react and become chemically bonded with itself or with other monomers to form a polymer.

[0040] As is well understood in the field of polymer chemistry, the term “vinyl ester oligomer” refers to an oligomer obtained from the esterification of an epoxy resin with (meth)acrylates and their derivatives. An oligomer is obtained by reaction of molecules having lower molecular weights. The oligomer may in some instances contain a small number of repeating sequences of the lower molecular weight molecules, for example two repeating sequences, three repeating sequences or four repeating sequences. The oligomer may in some instances contain five or more repeating sequences. The vinyl ester oligomer typically forms one of two components of the vinyl ester resin composition, the other being the reactive diluent monomer mixture, as described herein.

[0041] In some embodiments, the vinyl ester oligomer is derived from, or is the reaction product of:

[0042] (i) an epoxy compound comprising at least two epoxy groups;

[0043] (ii) a chain-extending reagent; and

[0044] (iii) a compound selected from an a,p-unsaturated monocarboxylic acid, a (meth)acrylate ester or a (meth)acrylate anhydride.

[0045] In some embodiments, the epoxy group of the epoxy compound is capable of reacting with a suitable functional group of the chain-extending reagent, such as a hydroxyl group or a carboxylic acid group, via an epoxide ring-opening reaction.

[0046] In some embodiments, the epoxy group is bisphenol A diglycidyl ether.

[0047] In some embodiments, the chain-extending reagent is an aromatic chain-extending reagent.

[0048] In some embodiments, the chain-extending reagent comprises at least two hydroxyl groups or at least two carboxylic acid groups.

[0049] In some embodiments, the chain-extending reagent is bisphenol A.

[0050] In some embodiments, the a,£-unsaturated monocarboxylic acid is methacrylic acid.

[0051] In some embodiments, the (meth)acrylate ester is a methacrylate ester, such as glycidyl methacrylate.

[0052] In some embodiments, the (meth)acrylate anhydride is a methacrylic anhydride.

[0053] Thus, for example, the vinyl ester oligomer may be prepared by reacting two moles of an epoxy compound, such as bisphenol A diglycidyl ether, with one mole of a chain-extending reagent, such as bisphenol A, to form a first lower molecular weight vinyl ester oligomer. The first lower molecular weight vinyl ester oligomer may be epoxy terminated and / or carboxylic acid terminated. The resulting vinyl ester oligomer could then be reacted with two moles of a compound which acts as an end-capping reagent, such as (meth)acrylic acid, to give a second higher molecular weight vinyl ester oligomer, which would have a theoretical structure A-(X-Y-X)n-A, in which A is the end-capping reagent, X is the epoxy compound, Y is the chain-extending reagent and n is an integer representing the number of X-Y-X sequences. For example, n is selected from 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10 or more. Unless otherwise stated, the second higher molecular weight vinyl ester oligomer is the vinyl ester oligomer of the present invention as described herein.

[0054] As is well understood by those skilled in the art, the preparation of products of this nature inevitably results in a mixture of molecular structures with differing molecular weights, having a variety of values of n. The relative proportions of epoxy compound (X) and chain-extending reagent (Y) determines the average value of n for what is in practice, a mixture of similar oligomer structures. In some embodiments, the vinyl ester oligomer has a number average molecular weight Mn of at least 1100, such as at least 1100, such as at least 1200, such as at least 1300, such as at least 1400, such as at least 1500, such as at least 1600, such as at least 1700, such as at least 1800, such as at least 1900, such as at least 2000, as measured by gel permeation chromatography in accordance with the method provided in the methods section.

[0055] In some embodiments, the vinyl ester has a number average molecular weight Mn of at most 5000, such as at most 4500, such as at most 4000, such as at most 3500, such as at most 3000, as measured by gel permeation chromatography in accordance with the method provided in the methods section.

[0056] Examples of suitable vinyl ester oligomers typically have a number average molecular weight Mn of between 1000 to 5000, such as between 1200 to 5000, such as between 1400 to 4500, such as between 1500 to 4500, such as between 2000 to 4500, such as between 2000 to 4000, such as between 2000 to 3500, such as between 2000 to 3000, as measured by gel permeation chromatography in accordance with the method provided in the methods section.

[0057] In some embodiments, the vinyl ester oligomer is present, based on the total weight of the vinyl ester resin composition, in an amount of at least 20 wt%, such as at least 25 wt%, such as at least 30 wt%, such as at least 35 wt% , such as at least 40 wt% .

[0058] In some embodiments, the vinyl ester oligomer is present, based on the total weight of the vinyl ester resin composition, in an amount of at most 70 wt%, such as at most 65 wt%, such as at most 60 wt%, such as at most 55 wt%, such as at most 50 wt%.

[0059] In some embodiments, the vinyl ester oligomer is present, based on the total weight of the vinyl ester resin composition, in an amount from 20 to 70 wt%, such as from 25 to 65 wt%, such as from 30 to 60 wt%, such as from 35 to 55 wt%, such as from 40 to 50 wt%.

[0060] In some embodiments, the vinyl ester oligomer is present, based on the total weight of the UV-curable filler composition, in an amount of at least 10 wt%, such as at least 16 wt%, such as at least 18 wt%, such as at least 20 wt%, such as at least 22 wt%, such as at least 24 wt%, such as at least 26 wt%.

[0061] In some embodiments, the vinyl ester oligomer is present, based on the total weight of the UV-curable filler composition, in an amount of at most 40 wt%, such as at most 38 wt%, such as at most 36 wt%, such as at most 34 wt%, such as at most 32 wt%, such as at most 30 wt%, such as at most 28 wt%.

[0062] In some embodiments, the vinyl ester oligomer is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 40 wt%, such as from 16 to 38 wt%, such as from 18 to 36 wt%, such as from 20 to 34 wt%, such as from 22 to 32 wt%, such as from 24 to 30 wt%, such as from 26 to 28 wt%.

[0063] Reactive diluent monomer

[0064] A reactive diluent monomer of the UV-curable filler composition may include any reactive diluent monomer known in the art, and is comprised as part of a reactive diluent monomer mixture. The term “reactive monomer diluent” refers to any reactive monomer which is capable of reducing the viscosity of a resin, the reactive monomer being in liquid form at room temperature and being curable with the vinyl ester oligomer of the vinyl ester resin composition. Thus, the reactive diluent monomer mixture typically forms one of two components of the vinyl ester resin composition, the other being the vinyl ester oligomer, as described herein.

[0065] A reactive diluent monomer may have a different number of functionalities, such as a monofunctional reactive diluent monomer, a difunctional reactive diluent monomer, a trifunctional reactive diluent monomer or a tetrafunctional reactive diluent monomer.

[0066] By “monofunctional”, it is meant that the monomer has a functionality of about 1 , i.e. one bonding site. By “difunctional”, it is meant that the monomer has a functionality of about 2, i.e. two bonding sites. By “trifunctional”, it is meant that the monomer has a functionality of about 3, i.e. three bonding sites. By “tetrafunctional”, it is meant that the monomer has a functionality of about 4, i.e. four bonding sites. By “polyfunctional”, it is meant that the monomer has a functionality of more than 1 , such as 2, such as 3. An example of a bonding site is a polymerisable functional group such as a vinyl functional group or a (meth)acrylate functional group.

[0067] The composition and characteristics of the UV-curable filler composition may be modified by selecting specific reactive diluent monomers and by adjusting the relative amount of each reactive diluent monomer.

[0068] The reactive diluent monomer mixture of the present invention comprises a reactive diluent monomer mixture comprising a first reactive diluent monomer having at least one polymerisable functional group and a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer.

[0069] In some embodiments, the first reactive diluent monomer comprises a polyfunctional reactive diluent monomer, as described herein.

[0070] In some embodiments, the polyfunctional reactive diluent monomer comprises at least one vinyl functional group, such as comprising at least one (meth)acrylate functional group. In some embodiments, each functionality of the polyfunctional reactive diluent monomer is each a vinyl functional group. In some embodiments, each functionality of the polyfunctional reactive diluent monomer is each a (meth)acrylate functional group.

[0071] Suitably, the polyfunctional reactive diluent monomer may be a difunctional reactive diluent monomer or a trifunctional reactive diluent monomer.

[0072] In some embodiments, the one or more polyfunctional reactive diluent monomers includes a trifunctional reactive diluent monomer, such as a trivinyl reactive diluent monomer, such as a tri(meth)acrylate reactive diluent monomer. In some embodiments, the one or more polyfunctional reactive diluent monomers includes a difunctional reactive diluent monomer, such as a divinyl reactive diluent monomer, such as a di(meth)acrylate reactive diluent monomer.

[0073] In some embodiments, the one or more polyfunctional reactive diluent monomers includes one or more trifunctional reactive diluent monomers. In some embodiments, the one or more polyfunctional reactive diluent monomers further comprises one or more difunctional reactive diluent monomers.

[0074] In some embodiments, the one or more trifunctional reactive diluent monomers is present in an amount which is greater than the amount of the one or more difunctional reactive diluent monomers.

[0075] In some embodiments, the one or more trifunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 4.0 to 18 wt%, such as from 5.0 to 17 wt%, such as from 6.0 to 16 wt%, such as from 7.0 to 15 wt%, such as from 8.0 to 14 wt%, such as from 9.0 to 13 wt%, such as from 10 to 12 wt%.

[0076] In some embodiments, the one or more trifunctional reactive diluent monomers is present, based on the total weight of the vinyl ester resin composition, in an amount from 10 to 24 wt%, such as from 11 to 23 wt%, such as from 12 to 22 wt%, such as from 13 to 21 wt%, such as from 14 to 20 wt%, such as from 15 to 19 wt%, such as from 16 to 18 wt%.

[0077] In some embodiments, the one or more difunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 3.0 to 15 wt%, such as from 4.0 to 14 wt%, such as from 5.0 to 13 wt%, such as from 6.0 to 12 wt%, such as from 7.0 to 13 wt%, such as from 8.0 to 10 wt%.

[0078] In some embodiments, the one or more difunctional reactive diluent monomers is present, based on the total weight of the vinyl ester resin composition, in an amount from 10 to 20 wt%, such as from 11 to 19 wt%, such as from 12 to 18 wt%, such as from 13 to 17 wt%, such as from 14 to 16 wt%, such as from 13 to 15 wt%.

[0079] In some embodiments, the second reactive diluent monomer comprises a monofunctional reactive diluent monomer.

[0080] In some embodiments, the one or more monofunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 14 to 26 wt%, such as from 16 to 24 wt% , such as from 18 to 22 wt% .

[0081] In some embodiments, the one or more monofunctional reactive diluent monomers is present, based on the total weight of the vinyl ester resin composition, in an amount from 26 to 39 wt%, such as from 28 to 37 wt%, such as from 30 to 35 wt%.

[0082] Suitable examples of monofunctional reactive diluent monomers include hydroxyl ethyl (meth)acrylate, hydroxyl propyl (meth)acrylate, butyl (meth)acrylate, vinyl toluene, hexyl (meth)acrylate and cyclohexyl (meth)acrylate, a-methyl styrene, f-butyl styrene, phenoxyethyl (meth)acrylate, tetrahydro furfuryl, (meth)acrylate, allyl (meth)acrylate, tetrahydro furfuryl (meth)acrylate, hydroxyl ethyl (meth)acrylate, hydroxyl propyl (meth)acrylate, benzyl methacrylate and vinyl methyl oxazolidinone.

[0083] Suitable examples of difunctional reactive diluent monomers include PEG200 di(meth)acrylate, 1 ,4- butanediol di(meth)acrylate, 1 ,3-butanediol di(meth)acrylate, 2,3-butanedioldi(meth)acrylate, 1 ,6- hexanediol di(meth)acrylate and its isomers, diethyleneglycol di(meth)acrylate, triethyleneglycol di(meth)acrylate, glycerol di(meth)acrylate, trimethylolpropane di(meth)acrylate, neopentyl glycol di(meth)acrylate, dipropyleneglycol di(meth)acrylate, tripropyleneglycol di(meth)acrylate, PPG250 di(meth)acrylate, tricyclodecane dimethylol di(meth)acrylate, 1 ,10-decanediol di(meth)acrylate, tetraethylene glycol dimethacrylate and 4-(vinyloxy)butyl methacrylate.

[0084] Suitable trifunctional reactive diluent monomers include trimethylolpropane tri(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, propoxylated glycerine tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, and tris(2-hydroxyethyl)isocyanurate tri(meth)acrylate.

[0085] Suitable tetrafunctional reactive diluent monomers include pentaerythritol tetra(meth)acrylate, ethoxy modified products of pentaerythritol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethoxy modified products of ditrimethylolpropane tetra(meth)acrylate, and condensation reaction products of trimethylolethane, acrylic acid and succinic anhydride.

[0086] The reactive diluent monomer mixture may also include vinyl aromatics, such as a styrene monomer or a styrene derivative monomer. Suitable styrene derivative monomers, if used, include alpha-methyl styrene, vinyl toluene, ethyl styrene, 2-chlorostyrene, 3-chlorostyrene, 4-chlorostyrene, 2-bromostyrene, 3-bromostyrene, 4-bromostyrene, alpha-bromostyrene, beta-bromostyrene and tert-butyl styrene.

[0087] In some preferred embodiments, the styrene monomer is present in an amount, based on the total weight of the UV-curable filler composition, in an amount of less than 5 wt% of styrene monomer, preferably less than 3 wt% of styrene monomer, more preferably less than 1 wt% of styrene monomer.

[0088] In some preferred embodiments, the UV-curable filler composition is substantially free of styrene. As used herein, the term “substantially free of styrene” means that the UV-curable filler compositions are formulated without the inclusion of any styrene monomer. For example, the filler UV-curable filler composition comprises less than 0.05 wt% of styrene monomer, preferably less than 0.03 wt% of styrene monomer, more preferably less than 0.01 wt% of styrene monomer.

[0089] In some embodiments, the reactive diluent monomer mixture is present, based on the total weight of the vinyl ester resin composition, in an amount of at least 35 wt%, such as at least 37 wt%, such as at least 39 wt% at least 41 wt%, such as at least 43 wt%, such as at least 45 wt%, such as at least 47 wt%, such as at least 49 wt%, such as at least 51 wt%, such as at least 53 wt%, such as at least 55 wt%, such as at least 57 wt%, such as at least 59 wt%, such as at least 61 wt%, such as at least 63 wt%.

[0090] In some embodiments, the reactive diluent monomer mixture is present, based on the total weight of the vinyl ester resin composition, in an amount of at most 85 wt%, such as at most 83 wt% at most 81 wt%, such as at most 79 wt%, such as at most 77 wt%, such as at most 75 wt%, such as at least 73 wt%, such as at most 71 wt%, such as at most 69 wt%, such as at most 67 wt%, such as at most 65 wt%.

[0091] In some embodiments, the reactive diluent monomer mixture is present, based on the total weight of the vinyl ester resin composition, in an amount from 35 to 85 wt%, such as from 37 to 85 wt%, such as from 39 to 83 wt%, 41 to 81 wt%, such as 43 to 79 wt%, such as 45 to 77 wt%, such as 47 to 75 wt%, such as from 49 to 73 wt%, such as from 51 to 71 wt%, such as from 53 to 73 wt%, such as from 55 to 71 wt %, such as from 57 to 69 wt%, such as from 59 to 67 wt%, such as 61 to 67 wt%, such as 63 to 65 wt%.

[0092] In some embodiments, the reactive diluent monomer mixture is present, based on the total weight of the UV-curable filler composition, in an amount of at least 10 wt%, such as at least 15 wt%, such as at least 20 wt%, such as at least 22 wt%, such as at least 24 wt%, such as at least 26 wt%, such as at least 28 wt%, such as at least 30 wt%, such as at least 32 wt%, such as at least 34 wt%, such as at least 36 wt%, such as at least 38 wt% .

[0093] In some embodiments, the reactive diluent monomer mixture is present, based on the total weight of the UV-curable filler composition, in an amount of at most 70 wt%, such as at most 65 wt%, such as at most 60 wt%, such as at most 58 wt%, such as at most 56 wt%, such as at most 54 wt%, such as at most 52 wt%, such as at most 50 wt%, such as at most 48 wt%, such as at most 46 wt%, such as at most 44 wt%, such as at most 42 wt%, such as at most 40 wt%.

[0094] In some embodiments, the reactive diluent monomer mixture is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 70 wt%, such as from 15 to 65 wt%, such as from 20 to 60 wt%, such as from 22 to 58 wt%, such as from 24 to 56 wt%, such as from 26 to 54 wt%, such as from 28 to 52 wt%, such as from 30 to 50 wt%, such as from 32 to 48 wt %, such as from 34 to 46 wt%, such as from 36 to 44 wt%, such as from 38 to 42 wt%, such as from 38 to 40 wt%.

[0095] Photoinitiator

[0096] Photoinitiators tend to initiate, enable, facilitate or catalyse chemicals reactions that result in or improve the rate or amount of polymerisation in a given composition when it is subjected to appropriate amounts of actinic radiation. A photoinitiator is a compound that undergoes a photoreaction upon absorption of actinic radiation (particularly in the visible and / or UV spectral region), whereupon reactive species are created. These reactive species are capable of catalysing, initialising or carrying out chemical reactions that result in significant changes in the physical properties of suitable formulations. Hence, the photoinitiator is a compound that can transform the physical energy of light into suitable chemical energy in the form of reactive intermediates. Common types of photoinitiators include cationic photoinitiators and free-radical photoinitiators.

[0097] The photoinitiator is for initiating a free radical polymerisation reaction in the UV-curable filler composition. In some embodiments, the photoinitiator is for absorbing UV light at a wavelength from 200 to 420 nm. In some embodiments, the photoinitiator is for absorbing UV light at a wavelength from 365 to 405 nm, such as from 375 to 405 nm, such as from 385 to 405 nm, such as from 390 to 405 nm.

[0098] In some embodiments, the photoinitiator is selected from a phosphine oxide (such as a benzoyl phosphine oxide), an aryl ketone, a benzophenone, a hydroxylated ketone, a ketal, a metallocene, or a combination thereof.

[0099] In some embodiments, the photoinitiator is selected from 2,4,6-trimethylbenzoyl diphenylphosphine oxide, ethyl(2,4,6-trimethylbenzoyl)-phenyl phosphinate, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide, 2-methyl-1-[4-(methylthio)phenyl]-2- morpholinopropanone-1 ,2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone, 2- dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholin-4-yl-phenyl)-butan-1-one, 4-benzoyl-4'-methyl diphenyl sulphide, 4,4'-bis(diethylamino) benzophenone, 4,4'-bis(N,N'-dimethylamino) benzophenone [Michler's ketone], benzophenone, 4-methyl benzophenone, 2,4,6-trimethyl benzophenone, dimethoxybenzophenone, acetophenone, chlorinated acetophenone, dialkoxyacetophenones, dialkylhydroxyacetophenones, dialkylhydroxyacetophenone esters, benzoin acetate, benzoin, benzoin alkyl ethers, dimethoxybenzoin, dibenzylketone, 1-hydroxycyclohexyl phenyl ketone, phenyl (1- hydroxyisopropyl)ketone, 2-hydroxy-1 -[4-(2-hydroxyethoxy)phenyl]-2-methyl-1 -propanone, 4- isopropylphenyl(1-hydroxyisopropyl)ketone, oligo-[2-hydroxy-2-methyl-1-[4-(1 -methylvinyl)phenyl] propanone], camphorquinone, 4,4'-bis(diethylamino) benzophenone, benzil dimethyl ketal, bis(eta 5-2-4- cyclopentadien-1-yl)bis[2,6-difluoro-3-(1 H-pyrrol-1-yl)phenyl]titanium, benzoylcyclohexanol, acyloxime esters, acylphosphine oxides, acylphosphonates, ketosulfides, dibenzoyldisulfides, diphenyl dithiocarbonate, and any combination thereof.

[0100] Further photoinitiators include: benzoylphosphine oxides, such as, for example, 2,4,6-trimethylbenzoyl diphenylphosphine oxide (Lucirin TPO from BASF) and 2,4,6-trimethylbenzoyl phenyl, ethoxy phosphine oxide (Lucirin TPO-L from BASF), bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819 or BAPO from Ciba), 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1 (Irgacure 907 from Ciba), 2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl) phenyl]-1-butanone (Irgacure 369 from Ciba), 2- dimethylamino-2-(4-methyl-benzyl)-1-(4-morpholin-4-yl-phenyl)-butan-1-one (Irgacure 379 from Ciba), 4- benzoyl-4'-methyl diphenyl sulphide (Chivacure BMS from Chitec), 4,4'-bis(diethylamino) benzophenone (Chivacure EMK from Chitec), 4,4'-bis(N,N'-dimethylamino) benzophenone (Michler's ketone), camphorquinone, and metallocenes such as bis(eta 5-2-4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1 H- pyrrol-1 -yl)phenyl] titanium (Irgacure 784 from Ciba), or a mixture thereof.

[0101] In some embodiments, the photoinitiator is a phosphine oxide. In some embodiments, where a plurality of photoinitiators are used, the photoinitiators comprise a phosphine oxide.

[0102] In some embodiments, the photoinitiator is bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Irgacure 819 from Ciba) or methyl benzoylformate (Irgacure 1173 or Omnirad 1173 from IGM Resins), or a combination thereof. In some embodiments, the photoinitiator is present, based on the total weight of the UV-curable filler composition, in an amount of at least 0.01 wt%, such as at least 0.02 wt%, such as at least 0.03 wt%, such as at least 0.04 wt%, such as at least 0.05 wt%.

[0103] In some embodiments, the photoinitiator is present, based on the total weight of the UV-curable filler composition, in an amount of at most 2.0 wt%, such as at most 1 .0 wt%, such as at most 0.5 wt%, such as at most 0.4 wt%, such as at most 0.3 wt%.

[0104] In some embodiments, the photoinitiator is present, based on the total weight of the UV-curable filler composition, in an amount from 0.01 to 2.0 wt%, such as from 0.02 to 0.5 wt%, such as from 0.05 to 0.3 wt%.

[0105] Adhesion promoter

[0106] Adhesion promoters are substances which tend to enhance the adhesive bonding between two different materials, such as the adhesive bonding of a filler composition to a substrate, such as, but not limited to, glass, metal, polyester, polyethylene or epoxy, or a composite or laminate comprising a combination thereof, such as a glass-fibre composite or laminate. Hence, the adhesion promoter is able to improve the adhesiveness of the UV-curable filler composition of the present invention, and is suitable for UV curing applications.

[0107] In some embodiments, the adhesion promoter is an acid-functionalised adhesion promoter. In some embodiments, the functionalised acid of the adhesion promoter is preferably carboxylic acids functionalised with a functional group selected from an acrylate, a methacrylate, an acryl amide, a methacryl amide, a styrene group, a vinyl ether, a vinyl ester, an allyl ether, an allyl ester, a maleimide and an itaconate. In some embodiments, the functional group is selected from the group consisting of an acrylate, a methacrylate, an acryl amide and a methacryl amide. In some embodiments, the functional group is selected from the group consisting of an acrylate and a methacrylate.

[0108] In some embodiments, the adhesion promoter is a (meth)acrylate adhesion promoter. In some embodiments, the adhesion promoter is a di(meth)acrylate adhesion promoter. In some embodiments, the adhesion promoter is a tri(meth)acrylate adhesion promoter.

[0109] Suitably, the adhesion promoter may be selected from 2-(methacryloyloxy)ethyl phosphate (SR9050 available from Sartomer), tris[2-(methacryloyloxy)ethyl] phosphate (SR9051 available from Sartomer), tris[2-(acryloyloxy)ethyl] phosphate (SR9053 available from Sartomer), KAYAMER® series available from Nippon Kayaku Co., Ltd., and Phosmer® series available from Uni-Chemical Co., Ltd.

[0110] In some embodiments, the adhesion promoter is a zirconium adhesion promoter. Suitable zirconium adhesion promoters may include zirconium acetylacetonate, zirconium butoxyacetylacetonate, zirconium bisacetylacetonate, and zirconium ethylacetoacetate.

[0111] In some embodiments, the adhesion promoter is an aluminium adhesion promoter. Suitable aluminium adhesion promoters may include ethyl acetoacetate aluminium diisopropylate, aluminium tris(ethy I acetoacetate), alkyl acetoacetate aluminium diisopropylate, aluminium tris (acetylacetonate) and aluminium monoacetylacetonate bis(ethylacetoacetate).

[0112] In some embodiments, the adhesion promoter is present, based on the total weight of the UV-curable filler composition, in an amount of at least 1 .0 wt%, such as from 2.0 wt%.

[0113] In some embodiments, the adhesion promoter is present, based on the total weight of the UV-curable filler composition, in an amount of at most 15 wt%, such as at most 14 wt%, such as at most 13 wt%, such as at most 12 wt%, such as at most 11 wt%, such as at most 10 wt%, such as at most 9.0 wt%, such as at most 8.0 wt%, such as at most 7.0 wt%, such as at most 6.0 wt%, such as at most 5.0 wt%, such as at most 4.0 wt%.

[0114] In some embodiments, the adhesion promoter is present, based on the total weight of the UV-curable filler composition, in an amount of less than 15 wt%. In some embodiments, the adhesion promoter is present, based on the total weight of the vinyl ester resin composition, in an amount of less than 15 wt%.

[0115] In some embodiments, the adhesion promoter is present, based on the total weight of the UV-curable filler composition, in an amount from 1 .0 to 10 wt%, such as from 1 .0 to 9.0 wt%, such as from 1 .0 to 8.0 wt%, 1 .0 to 7.0 wt%, 2.0 to 6.0 wt%, such as from 2.0 to 5.0 wt%, such as from 2.0 to 4.0 wt%.

[0116] Further components

[0117] The UV-curable filler composition may further comprise catalysts, curing accelerators, particulate fillers, air release agents, wetting agents, thermal initiators, clays, inhibitors, stabilisers, dyes, pigments, and the like. Further possible components may also include antioxidants, acid scavengers, thickeners, flame retardants, silane coupling agents, resin particles, waxes, core-shell particle impact modifiers, soluble polymers and block polymers.

[0118] The UV-curable filler composition may contain a catalyst. When the UV-curable filler composition comprises the vinyl ester resin composition as described herein, the catalyst may be any catalyst that may facilitate the reaction between the epoxy compound and the chain-extending reagent, in particular the epoxy group and the carboxylic acid group, and the reaction between the lower molecular vinyl ester oligomer and the end-capping reagent.

[0119] Examples of suitable catalysts that can be used herein include tertiary amines such as triethylamine, N,N- dimethyl paratoluidine, N',N'-dimethylbenzylamine, N,N-dimethylaniline, N,N-diethylaniline, N,N-bis(2- hydroxyethyl) paratoluidine, 2,4,6-tris(dimethylaminomethyl)phenol, and diazabicyclooctane; quaternary ammonium salts such as trimethylbenzylammonium chloride and methylthiethylammonium chloride; phosphines such as triphenylphosphine (TPP) and tributylphosphine; imidazoles such as 2- methylimidazole, 1 ,2-dimethylimidazole, and 2-ethyl-4-methylimidazole; and triphenylstibine. In some embodiments, the catalyst is triphenyl phosphine (TPP).

[0120] Further examples of suitable catalysts include metal catalysts, such as a chromium complex, for example a chromium(lll) complex or a chromium(VI) complex, and a zinc complex, for example a zinc(l) complex or a zinc(ll) complex. Examples of suitable catalysts include the NACURE® series of catalysts from King Industries, Inc., for example NACURE XC-259, the K-PURE® series of catalysts, also from King Industries, Inc., for example K-Pure CXC-1765, and the HYCAT® series of catalysts from Dimension Technologies Chemical Systems, Inc., for example HYCAT 2000S, HYCAT 3000S and HYCAT OA.

[0121] The catalyst may be used, based on the total weight of the UV-curable filler composition, in an amount of from 0.05 wt% to 0.5 wt%.

[0122] The UV-curable filler composition may further comprise a curing accelerator. In some embodiments, the curing accelerator is an amine curing accelerator. Examples of curing accelerators include LM7401 from Arkema and Photomer 4967 from IGM Resins.

[0123] The UV-curable filler composition may further comprise a particulate filler. Particulate fillers are typically filler powders in which the particles are usually of micron size, such as less than 100 pm in size, or smaller, such as nanoparticles. Examples of particulate fillers include both organic and inorganic particulate fillers. The particulate filler may possess a surface functionality or not, the surface functionality comprising a polymerisation group that is capable of (co)polymerisation with one or more of the epoxy compound, chain-extending reagent and end-capping reagent.

[0124] In some embodiments, the inorganic particulate filler comprises SiO2, AIO2, ZnO2, SnO2, Am-SnO2, ZrO2, Sb-SnO2, AI2O3 or carbon black, wherein the inorganic particulate filler has a refractive index of less than 2.90. In some embodiments, the organic particulate filler comprises polyurethane particles, polystyrene particles, poly(methyl methacrylate) particles or polycarbonate particles. Further examples of particulate fillers include, but are not limited to, particulates of glass fibres, glass microspheres, silicas, talc, boron fibres, carbon fibres, clays, waxes, graphite, graphene, carbon nanotubes, and aluminium trihydrate.

[0125] In some embodiments, the UV-curable filler composition comprises the one or more fillers in an amount of at least 0.1 wt%, or at least 1 wt%, or at least 5 wt%, or at least 10 wt%, or at least 15 wt%, or at least 20 wt%, or at least 30 wt%, or at least 40 wt%, or at least 50 wt%, based on the total weight of the UV- curable filler composition.

[0126] In some embodiments, the UV-curable filler composition comprises the one or more fillers in an amount of up to 90 wt%, or up to 80 wt%, or up to 70 wt%, or up to 60 wt%, or up to 50 wt%, or up to 40 wt%, or up to 30 wt%, or up to 20 wt%, based on the total weight of the UV-curable filler composition.

[0127] In some embodiments, the UV-curable filler composition comprises the one or more fillers in an amount of up to 50 wt%, based on the total weight of the UV-curable filler composition. In some embodiments, the UV-curable filler composition comprises the one or more fillers in an amount from 0.1 to 50 wt% or from 1 to 50 wt%, based on the total weight of the UV-curable filler composition.

[0128] When the filler present in the UV-curable filler composition is silica, additional sag and rheology may be applied to the UV-curable filler composition. Examples of silica include fumed silica such as Cabosil® TS 720 available from Cabot Corporation, hydrophilic fumed silica such as Aerosil® 130, Aerosil® 130 V, Aerosil® 150 V and Aerosil® 200 SP from Evonik, and precipitated silica such as Lo-Vel®TMor Hi Sil® silicas available from PPG Industries. In some embodiments, the silica is present, based on the total weight of the UV-curable filler composition, in an amount at least 8.5 wt%, such as at least 8.0 wt%, such as at least 7.5 wt%, such as at least 7.0 wt%, such as at least 6.5 wt%, such as at least 6.0 wt%.

[0129] In some embodiments, the silica is present, based on the total weight of the UV-curable filler composition, in an amount of at most 0.5 wt%, such as at most 1 .5 wt%, such as at most 2.0 wt% such as at most 2.5 wt%, such as at most 3.0 wt%, such as at most 3.5 wt%, such as at most 4.0 wt%.

[0130] In some embodiments, the silica is present, based on the total weight of the UV-curable filler composition, in an amount from such as from 2.0 to 8.0 wt%, such as from 2.5 to 7.5 wt%, such as from 3.0 to 7.0 wt%, such as from 3.5 to 6.5 wt%, such as from 4.0 to 6.0 wt%.

[0131] The UV-curable filler composition may further comprise an air release agent. Air release agents may suitably be used to prevent or destroy air bubbles which may be trapped during the application and curing processes.

[0132] Examples of air release agents include BYK-A 066, BYK-A 077, BYK-A 500, BYK-A 501 , BYK-A 515, BYK-A 555, BYK-A 501 , and BYK-A 555 from BYK®.

[0133] In some embodiments, the air release agent is present, based on the total weight of the UV-curable filler composition, in an amount of at least 0.01 wt%, such as at least 0.1 wt%, such as at least 0.5 wt%, such as at least 0.6 wt%, such as at least 0.7 wt%, such as at least 0.8 wt%, such as at least 0.9 wt%, such as at least 1 .0 wt%.

[0134] In some embodiments, the air release agent is present, based on the total weight of the UV-curable filler composition, in an amount of at most 3.0 wt%, such as at most 2.5 wt%, such as at most 2.0 wt%, such as at most 1 .5 wt%.

[0135] In some embodiments, the air release agent is present, based on the total weight of the UV-curable filler composition, in an amount from such as 0.1 to 3.0 wt%., such as 0.4 to 3.0 wt%, such as 0.6 to 2.5 wt%, 0.8 to 2.0 wt%, such as 1 .0 to 1 .5 wt%.

[0136] The UV-curable filler composition may further comprise a wetting agent. Wetting agents may be used to stabilise and encapsulate any inorganic particulate filler as described herein which may be present in the UV-curable filler composition, and also to function as a viscosity suppressant. The amount of wetting agent present in the UV-curable filler composition is typically based on the amount of particulate filler present in the UV-curable filler composition.

[0137] Examples of wetting agents include agents based on polyacrylate, such as FKA® 4500, and EFKA® 4530 from Ciba Specialty Chemicals; agents based on polyester, such as Solsperse® 22000, 24000SC, 26000, 27000 from Avecia, BYK-W 908, BYK-W 940, BYK-W 940 SG, BYK-W 980, BYK-W 985 and BYK-W 996 from BYK; agents based on polyether such as Disparlon® DA234 and DA325 from Kusumoto Chemicals; agents based on polyurethane, such as EFKA® 4046, EFKA® 4047 from Ciba Specialty Chemicals. In some embodiments, the wetting agent is present, based on the total weight of the particulate filler in the UV-curable filler composition, in an amount of at least 0.2 wt%, such as at least 0.4 wt%, such as at least 0.6 wt%, such as at least 0.8 wt%, such as at least 1 .0 wt%, such as at least 1 .2 wt%.

[0138] In some embodiments, the wetting agent is present, based on the total weight of the particulate filler in the UV-curable filler composition, in an amount of at most 3.0 wt%, such as at most 2.8 wt%, such as at most 2.6 wt%, such as at most 2.4 wt%, such as at most 2.2 wt%, such as at most 2.0 wt%.

[0139] In some embodiments, the wetting agent is present, based on the total weight of the particulate filler in the UV-curable filler composition, in an amount from 0.2 to 3.0 wt%., such as 0.4 to 2.8 wt%, such as 0.6 to 2.6 wt%, such as 0.8 to 2.4 wt%, such as 1 .0 to 2.2 wt%, such as 1 .2 to 2.0 wt%.

[0140] The UV-curable filler composition may further comprise a clay. In some embodiments, the clay is an organoclay. As disclosed herein the phrase “organoclay” refers to an organically surface treated clay. An organically surface treated clay may also be known as an organically modified clay or an organically modified phyllosilicate or an organophilic phyllosilicate.

[0141] Typically, an organoclay is an organophilic cation-modified clay derived from a clay mineral, generally of the smectite group, for example, bentonite, montmorillonite, hectorite, saponite or the like, by replacing the inorganic exchangeable cations, generally alkali or alkaline earth metal cations, which occur in the clay mineral, by organic cations (typically quaternary alkylammonium ions) each comprising at least one hydrocarbon radical which has sufficient carbon atoms to render the surface of the cation-exchanged clay hydrophobic.

[0142] The organoclay may be a mixed mineral organoclay in which the organoclay comprises a first type of clay mineral and a second type of clay mineral, wherein the first and second types of clay mineral have different shaped particles. For instance, the mixed mineral organoclay may be a mixture of clay particles having platelet structures, ribbon-shaped structures, pipe-shaped structures and / or needle-like structures. In particular, the mixed mineral organoclay may be a mixture of a first clay mineral having platelet shaped particles and a second clay mineral having needle-like particles.

[0143] Preferably, the organoclay is selected from an organically modified smectite clay, an organically modified bentonite clay or a mixed mineral organoclay.

[0144] Preferably, the organoclay is a mixed mineral organoclay.

[0145] Preferably, the mixed mineral organoclay comprises benzalkonium sepiolite and benzalkonium montmorillonite.

[0146] Preferably, the organoclay is an organically modified bentonite clay and the organically modified bentonite clay comprises stearalkonium bentonite.

[0147] In some embodiments, the clay is present, based on the total weight of the UV-curable filler composition, in an amount of at least 0.1 wt%, such as at least 0.2 wt%, such as at least 0.3 wt%, such as at least 0.4 wt%. In some embodiments, the clay is present, based on the total weight of the UV-curable filler composition, in an amount of at most 2.0 wt%, such as at most 1 .5 wt%, such as at most 2.0 wt%, such as at most 1 .5 wt%, such as at most 1 .0 wt%, such as at most 0.5 wt%.

[0148] In some embodiments, the clay is present, based on the total weight of the UV-curable filler composition, in an amount from such as 0.1 to 2.0 wt%, such as 0.2 to 1 .5 wt%, 0.3 to 1 .0 wt%, such as 0.4 to 0.5 wt%.

[0149] The UV-curable filler composition may further comprise a thermal initiator. Thermal initiators tend to initiate, enable, facilitate or catalyse chemicals reactions that result in or improve the rate or amount of polymerisation in a given composition when it is subjected to appropriate temperatures. A thermal initiator is a compound that undergoes decomposition at specific temperatures, whereupon reactive species are created. These reactive species are capable of catalysing, initialising or carrying out chemical reactions that result in significant changes in the physical properties of suitable formulations. Hence, the thermal initiator is a compound that can yield an appreciable amount of reactive intermediates at elevated temperatures. Common types of thermal initiators include peroxide thermal initiators and azo compound thermal initiators.

[0150] The thermal initiator may be any common thermal initiator. A plurality of thermal initiators may be used. The thermal initiator may be used for initiating a further free radical polymerisation reaction in the UV- curable filler composition, wherein the thermal peroxide is heat-activable from the initiation of a free radical polymerisation by the free radical photoinitiator.

[0151] Therefore, the thermal initiator is activated by heat, such as heat generated from the initiation using the photoinitiator present in the UV-curable filler composition. In other words, the thermal initiator is heat- activatable. The heat generated is sufficient to raise the temperature of at least a portion of the UV- curable filler composition, such as to a temperature above the 1-hour half-life temperature, or above the 10-hour half-life temperature or the self-accelerating decomposition temperature of the thermal initiator.

[0152] An important indicator of activity of a thermal initiator is its half-life (tic). It is the time required to reduce the original thermal initiator content of a solution by 50% at a given temperature. The activity may also depend on the solvent system used.

[0153] Most commonly, the decomposition rate of a thermal initiator may be characterised by its 1-hour half-life (ti / 2) temperature or its 10-half half-life (tie) temperature in a particular solvent. That is, the 1-hour halflife temperature is the temperature at which an original thermal initiator content of a solution is reduced by 50% after 1 hour. The 10-hour half-life temperature is the temperature at which an original thermal initiator content of a solution is reduced by 50% after 10 hours.

[0154] In some embodiments, the thermal initiator has a 1-hour half-life temperature of 120 °C or less, such as a 1-hour half-life temperature of 110 °C or less, such as a 1-hour half-life temperature of 100 °C or less, such as a 1-hour half-life temperature of 95 °C or less, such as a 1-hour half-life temperature of 90 °C or less, such as a 1-hour half-life temperature of 85 °C or less, such as a 1-hour half-life temperature of 80 °C or less, such as a 1-hour half-life temperature of 75 °C or less, such as a 1-hour half-life temperature of 70 °C or less, such as a 1 -hour half-life temperature of 65 °C or less, such as a 1 -hour half-life temperature of 60 °C or less, such as a 1-hour half-life temperature of 55 °C or less, such as a 1-hour half-life temperature of 50 °C or less, such as a 1-hour half-life temperature of 45 °C or less, such as a 1- hour half-life temperature of 40 °C or less, wherein the 1-hour half-life temperature is measured in chlorobenzene.

[0155] In some embodiments, the thermal initiator has a 10-hour half-life temperature of 100 °C or less, such as a 10-hour half-life temperature of 95 °C or less, such as a 10-hour half-life temperature of 90 °C or less, such as a 10-hour half-life temperature of 85 °C or less, such as a 10-hour half-life temperature of 80 °C or less, such as a 10-hour half-life temperature of 75 °C or less, such as a 10-hour half-life temperature of 70 °C or less, such as a 10-hour half-life temperature of 65 °C or less, such as a 10-hour half-life temperature of 60 °C or less, such as a 10-hour half-life temperature of 55 °C or less, such as a 10-hour half-life temperature of 50 °C or less, such as a 10-hour half-life temperature of 45 °C or less, such as a 10-hour half-life temperature of 40 °C or less, wherein the 10-hour half-life temperature is measured in chlorobenzene.

[0156] In some embodiments, the thermal initiator has a 1-hour half-life temperature of 95 °C or less as measured in chlorobenzene.

[0157] In some embodiments, the thermal initiator has a 10-hour half-life temperature of 75 °C or less as measured in chlorobenzene.

[0158] Another indicator of activity of a thermal initiator is its self-accelerating decomposition temperature (SADT). The self-accelerating decomposition temperature (SADT) of a thermal initiator is the lowest ambient temperature at which the thermal initiator in a typical vessel or package will undergo a selfaccelerating decomposition within one week. The self-accelerating decomposition temperature (SADT) is in accordance with the Heat Accumulation Storage Test (HAST).

[0159] In some embodiments, the thermal initiator has a self-accelerating decomposition temperature of 100 °C or less, such as 95 °C or less, such as 90 °C or less, such as 85 °C or less, such as 80 °C or less, such as 75 °C or less, such as 70 °C or less, such as 65 °C or less, such as 60 °C or less, such as 55 °C or less, such as 50 °C or less, such as 45 °C or less, such as 40 °C or less, such as 35 °C or less, such as 30 °C or less, such as 25 °C or less, wherein the self-accelerating decomposition temperature is measured in accordance with the Heat Accumulation Storage Test (HAST).

[0160] In some embodiments, the thermal initiator has a self-accelerating decomposition temperature of 40 °C or less as measured in accordance with the Heat Accumulation Storage Test (HAST).

[0161] In some embodiments, the thermal initiator is a peroxide thermal initiator. In some embodiments, where a plurality of thermal initiators is used, the thermal initiators comprise a peroxide thermal initiator.

[0162] In some embodiments, the thermal initiator is selected from tert-amyl peroxybenzoate, 4,4-azobis(4- cyanovaleric acid), 1 ,1 '-azobis(cyclohexanecarbonitrile), 2,2'-azobisisobutyronitrile (AIBN), benzoyl peroxide, 2,2-bis(tert-butylperoxy)butane, 1 ,1-bis(tert-butylperoxy)cyclohexane, 2,5-bis(tert-butylperoxy)- 2,5-dimethylhexane, 2,5-bis(tert-butylperoxy)-2,5-dimethyl-3-hexyne, bis(1-(tert-butylperoxy)-1- methylethyl)benzene, 1 ,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, tert-butyl hydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butylperoxy isopropyl carbonate, cumene hydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroyl peroxide, 2,4-pentanedione peroxide, peracetic acid, potassium persulfate, and any combination thereof.

[0163] Further thermal initiator include, for example, Diisobutyryl peroxide (TRIGONOX 187 from Nouryon), Cumyl peroxyneodecanoate (TRIGNOX 99 from Nouryon), 1 ,1 ,3,3-Tetramethylbutyl peroxyneodecanoate (TRIGONOX 423 from Nouryon), tert-Amyl peroxyneodecanoate (TRIGONOX 123 from Nouryon), Di-sec- butyl peroxydicarbonate (TRIGONOX SBP from Nouryon), Di(4-tert-butylcyclohexyl) peroxydicarbonate (PERKADOX 16 from Nouryon), Di(2-ethylhexyl) peroxydicarbonate (TRIGONOX EHP from Nouryon), tert-Buty I peroxyneodecanoate (TRIGONOX 23 from Nouryon), Dicetyl peroxydicarbonate (PERKADOX 24 from Nouryon), Dimyristyl peroxydicarbonate (PERKADOX 26 from Nouryon), 1 ,1 ,3,3-Tetramethylbutyl peroxypivalate (TRIGONOX 425 from Nouryon), tert-Amyl peroxypivalate (TRIGONOX 125 from Nouryon), tert-Buty I peroxypivalate (TRIGONOX 25 from Nouryon), Di(3,5,5-trimethylhexanoyl) peroxide (TRIGONOX 36 from Nouryon), Dilauroyl peroxide (LAUROX from Nouryon), 2,5-Dimethyl-2,5-di(2- ethylhexanoylperoxy)hexane (TRIGONOX 141 from Nouryon), Di(4-methylbenzoyl) peroxide (PERKADOX PM-W75 from Nouryon), 1 ,1 ,3,3-Tetramethylbutyl peroxy-2-ethylhexanoate (TRIGONOX 421 from Nouryon), tert-Amyl peroxy-2-ethylhexanoate (TRIGONOX 121 from Nouryon), Dibenzoyl peroxide (PERKADOX L from Nouryon), tert-Butyl peroxy-2-ethylhexanoate (TRIGONOX 21 S from Nouryon), tert-Butyl peroxyisobutyrate (TRIGONOX 41 from Nouryon), 1 ,1-Di(tert- amylperoxy)cyclohexane (TRIGONOX 122 from Nouryon), 1 ,1-Di(tert-butylperoxy)-3,3,5- trimethylcyclohexane (TRIGONOX 29 from Nouryon), 1 ,1-Di(tert-butylperoxy)cyclohexane (TRIGONOX 22 from Nouryon), tert-Amylperoxy 2-ethylhexyl carbonate (TRIGONOX 131 from Nouryon), tert-Butyl peroxy-3,5,5-trimethylhexanoate (TRIGONOX 42 from Nouryon), 2,2-Di(tert-butylperoxy)butane (TRIGONOX D-C50* from Nouryon), tert-Butylperoxy isopropyl carbonate (TRIGONOX BPIC-C75* from Nouryon),), tert-Amyl peroxybenzoate (TRIGONOX 127 from Nouryon), tert-Butyl peroxyacetate (TRIGONOX F-C50 from Nouryon), tert-Butyl peroxybenzoate (TRIGONOX C from Nouryon), Dicumyl peroxide (PERKADOX BC-FF from Nouryon), Di(tert-butylperoxyisopropyl)benzene (PERKADOX 14S from Nouryon), 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane (TRIGONOX 101 from Nouryon), Di-tert-butyl peroxide (TRIGONOX B from Nouryon), 3,6,9-Triethyl-3,6,9-trimethyl-1 ,4,7-triperoxonane (TRIGONOX 301 from Nouryon), 1 ,2,4,5,7,8-Hexoxonane, 3,6,9-trimethyl-3,6,9-tris (Et and Pr) (TRIGONOX 501 from Nouryon), 1 ,1 ,3,3-Tetramethylbutyl hydoperoxide (TRIGONOX TMBH-L from Nouryon), Cumyl hydroperoxide (TRIGONOX K from Nouryon), tert-Butyl hydroperoxide (TRIGONOX A from Nouryon), tert-Amyl hydroperoxide (TRIGONOX TAHP-W85 from Nouryon), and azo compounds, such as, for example, 2,2’-Azodi(isobutyronitrile) (PERKADOX AIBN from Nouryon), 2,2’-Azodi(2-methylbutyronitrile) (PERKADOX AMBN from Nouryon), 1 ,1 ’-Azodi(hexahydrobenzonitrile) (PERKADOX ACCN from Nouryon), or a mixture thereof.

[0164] In some embodiments, the thermal initiator is selected from di(4-tert-butylcyclohexyl) peroxydicarbonate (PERKADOX 16 from Nouryon) or tert-Butyl peroxy-2-ethylhexanoate (TRIGONOX 21 S from Nouryon), or a mixture thereof. In some embodiments, the thermal initiator is present, based on the total weight of the UV-curable filler composition, in an amount of at least 0.01 wt%, such as at least 0.1 wt%, such as at least 0.2 wt%, such as at least 0.3 wt%, such as at least 0.4 wt%, such as at least 0.5 wt%.

[0165] In some embodiments, the thermal initiator is present, based on the total weight of the UV-curable filler composition, in an amount of at most 2.0 wt%, such as at most 1 .5 wt%, such as at most 1 .0 wt%.

[0166] In some embodiments, the thermal initiator is present, based on the total weight of the UV-curable filler composition, in an amount from 0.01 to 2.0 wt%, such as from 0.5 to 1 .0 wt%.

[0167] The UV-curable filler composition may contain inhibitors and stabilisers, which help to prolong the shelf life of the product in storage and to control the working time of the adhesive in use. Such inhibitors are well known to those skilled in the art and may comprise quinones, hydroquinones, substituted phenols and the like. Examples of specific inhibitors include, but are not limited to, 2,6-di-tert-butyl-a- dimethylamino-p-cresol, butylated hydroxytoluene (BHT), butylated hydroxyanisole, hydroquinone, toluhydroquinone, para-methoxyphenol, mono tert-butyl hydroquinone, tert-butyl catechol, oxalic acid, and phenothiazine.

[0168] In addition to inhibitors that may be added directly to the UV-curable filler composition during its preparation, some of the raw materials employed, especially the a,p-unsaturated monocarboxylic acid, (meth)acrylate ester or (meth)acrylate anhydride which may be used to produce a vinyl ester resin composition, may also contain inhibitors introduced by the manufacturer / supplier. Hence, on account of the variety of materials that may be chosen, some with in situ inhibitors, and the variety of initiators, promoters and inhibitors that may be used to prepare a composition, the selection of the complete inhibitor package is generally the final step in the formulating process. Preferred combinations of initiator and promoter for the curing system are those suitable for curing at and / or below ambient temperatures.

[0169] The inhibitor may be present, based on the total weight of the UV-curable filler composition, in an amount of less than 1 .0 wt%, such as less than 0.5 wt%, such as less than 0.1 wt%, such as less than 0.09 wt%, such as less than 0.08 wt%, such as less than 0.07 wt%, such as less than 0.06 wt%.

[0170] Where the UV-curable filler composition contains inhibitors and stabilisers, the UV-curable filler composition may further contain diluents for said inhibitors and stabilisers.

[0171] In some embodiments, the UV-curable filler composition comprises one or more dyes. In some embodiments, the dye is a blue dye. In some embodiments, the UV-curable filler composition comprises the dye in an amount of less than 0.002 wt%, such as less than 0.001 wt%, based on the total weight of the UV-curable filler composition.

[0172] Method of preparing the vinyl ester resin composition

[0173] The second aspect of the invention relates to methods of preparing the vinyl ester resin composition of the UV-curable filler composition of the first aspect. Thus, in a second aspect of the invention, there is provided a method of preparing a vinyl ester resin composition of the first aspect, the method comprising the following steps of:

[0174] (i) reacting an epoxy compound and a chain-extending reagent, to obtain a first lower molecular weight vinyl ester oligomer;

[0175] (ii) mixing the reaction mixture comprising the first lower molecular weight vinyl ester oligomer obtained in step (i) with an a,p-unsaturated monocarboxylic acid, a (meth)acrylate ester or a (meth)acrylate anhydride to obtain a vinyl ester oligomer with reactive unsaturated groups; and

[0176] (iii) blending the reaction mixture comprising the vinyl ester oligomer with reactive unsaturated groups with a reactive diluent monomer mixture to obtain a vinyl ester resin composition.

[0177] In some embodiments, the step (iii) is split into two additional steps of:

[0178] (iv) blending the reaction mixture comprising the vinyl ester oligomer with reactive unsaturated groups with a first monomer portion of the reactive diluent monomer mixture to obtain an unfinished vinyl ester resin composition; and

[0179] (v) blending the unfinished vinyl ester resin composition with a second monomer portion of the reactive diluent monomer mixture to obtain a finished vinyl ester resin composition.

[0180] In some embodiments, the first monomer portion in step (iv) comprises a first reactive diluent monomer having at least one polymerisable functional group, and the second monomer portion in step (v) comprises a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer.

[0181] In some embodiments, the epoxy compound may be produced from one or more precursor(s) and / or the chain-extending reagent may be produced from one or more precursor(s). The epoxy compound may be made in situ from precursors, such as before combination with the chain-extending reagent. Thus, in some embodiments, the method of making the vinyl ester resin composition comprises combining the epoxy compound and the chain-extending reagent or precursor(s) thereof.

[0182] In some embodiments, the reaction temperature of steps (i) to (iii) is at most 170 °C, preferably at most 125 °C, more preferably at most 120 °C.

[0183] In some embodiments, the reaction temperature of steps (i) to (iii) is at least 110 °C.

[0184] In some embodiments, the reaction temperature of step (i) is between 120 °C to 130 °C.

[0185] In some embodiments, the temperature of step (ii) is between 100 °C to 125 °C, preferably between 105 °C to 120 °C, more preferably between 110 °C to 120 °C, more preferably about 115 °C.

[0186] In some embodiments, the reaction temperature of step (iii) is at most 90 °C, preferably at most 85 °C, more preferably at most 80 °C.

[0187] In some embodiments, the reaction is monitored by acid value (AV) which corresponds to the remaining amount of unreacted carboxylic acid in the reaction mixture. In some embodiments, step (i) is performed under an inert atmosphere, for example, a nitrogen atmosphere or an argon atmosphere.

[0188] In some embodiments, the vinyl ester resin composition is present, based on the total weight of the UV- curable filler composition, in an amount of at least 40 wt%, such as at least 42 wt%, such as at least 44 wt%, such as at least 46 wt%, such as at least 48 wt%, such as at least 50 wt%, such as at least 52 wt%, such as at least 54 wt% , such as at least 56 wt% , such as at least 58 wt% .

[0189] In some embodiments, the vinyl ester resin composition is present, based on the total weight of the UV- curable filler composition, in an amount of at most 80 wt%, such as at most 78 wt%, such as at most 76 wt%, such as at most 74 wt%, such as at most 72 wt%, such as at most 70 wt%, such as at most 68 wt%, such as at most 66 wt%, such as at most 64 wt%, such as at most 62 wt%.

[0190] In some embodiments, the vinyl ester resin composition is present, based on the total weight of the UV- curable filler composition, in an amount from 40 to 80 wt%, such as from 42 to 78 wt%, such as from 44 to 76 wt%, such as from 46 to 74 wt%, such as from 48 to 72 wt%, such as from 50 to 70 wt%, such as from 52 to 68 wt%, such as from 54 to 66 wt%, such as from 56 to 64 wt%, such as from 58 to 62 wt%.

[0191] Methods of curing

[0192] The third aspect of the invention relates to methods of curing the UV-curable filler composition of the first aspect.

[0193] Thus, in a fourth aspect of the invention, there is provided a method of curing the UV-curable filler composition of the first aspect, the method comprising the step of (i) exposing the UV-curable filler composition to UV light to initiate a cure via a free radical polymerisation.

[0194] In some embodiments, the UV-curable filler composition is cured at a wavelength of from 200 to 420 nm, such as from 200 to 405 nm. Suitably, the UV-curable filler composition is cured by a source of UV-A light at a wavelength of 315 to 400 nm, UV-B light at a wavelength of 280 to 315 nm, or UV-C light at a wavelength of 200 to 280 nm.

[0195] In some embodiments, the UV light is UV LED light. Therefore, the UV light may be at a wavelength of from 365 to 405 nm, such as from 375 to 405 nm, such as from 385 to 405 nm, such as from 390 to 405 nm.

[0196] In some embodiments, the UV light is at a wavelength from 380 to 420 nm, such as from 390 to 405 nm.

[0197] In some embodiments, the UV light is at an intensity of at least 0.1 W cm2, such as at least 0.2 W cm2, such as at least 0.3 W cm2, such as at least 0.4 W cm2, such as at least 0.5 W cm2, such as at least 0.6 W cm2, such as at least 0.7 W cm2, such as at least 0.8 W cm2, such as at least 0.9 W cm2, such as at least 1 .0 W cm2.

[0198] In some embodiments, the UV light is at an intensity of at most 3.0 W cm2, such as at most 2.8 W cm2, such as at most 2.6 W cm2, such as at most 2.4 W cm2, such as at most 2.2 W cm2, such as at most 2.0 W cm2, such as at most 1 .8 W cm2, such as at most 1 .6 W cm2, such as at most 1 .4 W cm2, such as at most 1 .2 W cm2, such as at most 1.0 W cm2.

[0199] In some embodiments, the UV light is at an intensity of from 0.1 to 3.0 W cm2, such as at most 0.2 to 2.8 W cm2, such as 0.3 to 2.6 W cm2, such as from 0.4 to 2.4 W cm2, such as from 0.5 to 2.2 W cm2, such as from 0.6 to 2.0 W cm2, such as from 0.7 to 1 .8 W cm2, such from 0.7 to 1 .6 W cm2, such from 0.7 to 1 .4 W cm2, such from 0.7 to 1 .2 W cm2, such from 0.8 to 1 .2 W cm2, such as from 0.8 to 1 .0 W cm2.

[0200] In some embodiments, the duration of step (i) is 120 seconds or less, such as 90 seconds or less, such as 60 seconds or less, such as 30 seconds or less.

[0201] In some embodiments, a thermal initiator, such as a thermal peroxide, may be used in the UV-curable filler composition according to the invention. Suitably, the UV-curable filler composition can undergo a curing process in which polymerisation takes place when exposed to the photoinitiator and the thermal initiator. Firstly, the photoinitiator can initiate the curing process when exposed to UV radiation. An elevated temperature is generated by the photoinitiator as it initiates a further curing process because the free radical polymerisation is an exothermic reaction.

[0202] Suitably, in some embodiments, the method of curing the UV-curable filler composition of the first aspect comprises the steps of:

[0203] (i) exposing the UV-curable filler composition to UV light at a wavelength from 200 to 405 nm to initiate a cure via a free radical polymerisation, and generating heat in the UV-curable filler composition; and

[0204] (ii) exposing the UV-curable filler composition and a thermal initiator to initiate a further cure in the UV-curable filler composition via the free radical polymerisation using the heat generated in step (i).

[0205] In some embodiments, the thermal initiator is exposed to UV light in step (ii). In some embodiments, the UV light source emits heat, such as to increase the temperature of the UV-curable filler composition. Thus, there may also be a relatively small contribution in initiation from the exposure of the thermal initiator to UV light to due heat generation from the UV light.

[0206] In alternative embodiments, the thermal initiator is not exposed to UV light in step (ii). Therefore, the UV light source may be removed or switched off after step (i), such that the UV-curable filler composition is also not exposed to UV light in step (ii). In such embodiments, the thermal initiator initiates a free radical polymerisation in the UV-curable filler composition and is induced purely from the heat generation in step (i).

[0207] The temperature achieved in step (ii) from the heat generation in step (i) is a temperature at which the thermal initiator is activated. When the heat generated is sufficient to raise the temperature of at least a portion of the UV-curable filler composition to activate the thermal initiator , such as above the 1-hour half-life temperature, the 10-hour half-life temperature or the self-accelerating decomposition temperature of the thermal initiator , a deeper cure may be achieved without requiring any further penetration of UV light.

[0208] In some embodiments, the heat generated in step (i) imparts a temperature of at least 70 °C, such as at least 75 °C, such as at least 80 °C, such as at least 85 °C, such as at least 90 °C, such as at least 95 °C, such as at least 100 °C, such as at least 105 °C, such as at least 110 °C, such as at least 115 °C, such as at least 120 °C, such as at least 125 °C, such as at least 130 °C, such as at least 135 °C, such as at least 140 °C, such as at least 145 °C, such as at least 150 °C, to the UV-curable filler composition.

[0209] In some embodiments, the heat generated in step (i) imparts a temperature from 70 to 150 °C, such as from 80 to 150 °C, such as from 90 to 150 °C.

[0210] In some embodiments, the duration of step (i) is 20 minutes or less, such as 18 minutes or less, such as 16 minutes or less, such as 14 minutes or less, such as 12 minutes or less, such as 10 minutes or less.

[0211] In some embodiments, the duration of step (i) is 120 seconds or less, such as 90 seconds or less, such as 60 seconds or less, such as 30 seconds or less.

[0212] In some embodiments, the duration of step (i) is 3 minutes or more, such as 4 minutes or more, such as 5 minutes or more, such as 6 minutes or more, such as 7 minutes or more, such as 8 minutes or more, such as 9 minutes or more, such as 10 minutes or more.

[0213] In some embodiments, the duration of step (i) is from 3 to 20 minutes, such as from 4 to 18 minutes, such as from 5 to 18 minutes, such as from 6 to 16 minutes, such as from 7 to 14 minutes, such as from 8 to 12 minutes.

[0214] In some embodiments, the duration of step (i) is 10 minutes or less.

[0215] In some embodiments, the duration of step (ii) is 30 minutes or less, such as 28 minutes or less, such as 26 minutes or less, such as 24 minutes or less, such as 22 minutes or less, such as 20 minutes or less, such as 18 minutes or less, such as 16 minutes or less, such as 14 minutes or less, such as 12 minutes or less.

[0216] In some embodiments, the duration of step (ii) is 12 minutes or more, such as 14 minutes or more, such as 16 minutes or more, such as 18 minutes or more, such as 20 minutes or more, such as 22 minutes or more, such as 24 minutes or more, such as 26 minutes or more, such as 28 minutes or more, such as 30 minutes or more.

[0217] In some embodiments, the duration of step (ii) is from 12 to 24 minutes, such as from 14 to 22 minutes, such as from 16 to 20 minutes.

[0218] The invention also relates to cured products of the UV-curable filler composition of the first aspect, as described below. The fourth aspect of the invention relates to the use of the UV-curable filler composition of the first aspect in a composite repair process.

[0219] In this aspect, there may be provided an application of a repair patch, comprising the UV-curable filler composition of the first aspect, for applying on a substrate to cover a damaged portion or surface of a substrate. The damaged portion of the substrate is typically grinded down and cleaned out. In some embodiments, the repair patch may comprise a plurality of prepregs comprising the UV-curable filler composition of the first aspect. Therefore, a layered stack of a plurality of prepregs may be applied to form a multilaminar repair patch on the substrate. In some embodiments, the lamination is performed by hand (hand lay-up). Manual smoothing and levelling pressure over the repair patch surface may be employed to spread the resin composition, smooth out and high spots and fill in any low spots in the surface, and to achieve lamination of adjacent prepreg layers with the desired outer surface profile.

[0220] The UV-curable filler composition is cured by UV light. In some embodiments, the UV-curable filler composition is cured according to the method of curing of the third aspect.

[0221] In a fifth aspect of the invention, there is provided a cured product of the UV-curable filler composition of the first aspect. The cured product may have properties as described further below.

[0222] The UV-curable filler composition is particularly suitable for a wind turbine repair process. Thus, in some embodiments, the composite repair process relates to a wind turbine repair process. In some embodiments, the wind turbine repair process is a wind blade repair process, a tower repair process or a nacelle repair process.

[0223] The UV-curable filler composition allows for a faster and more complete cure over other curing systems, such as cobalt accelerators and initiators. The UV-curable filler composition is operable at a wider temperature range. Hence the need for a post-curing step, such as a typical thermal post-curing step, is entirely eliminated. This allows for a significantly more rapid turnaround of repairs. The application of the repair patch for the purposes of repairing a damaged portion of the wind blade part may be performed as a post-production application of a wind blade part or as an in-field application.

[0224] In a sixth aspect of the invention, there is provided a wind turbine part comprising the cured product of the fifth aspect. The wind turbine part may be a wind blade part, a tower part or a nacelle part.

[0225] In a seventh aspect, there is provided a multi-part repair system comprising a plurality of parts, which in combination contains the UV-curable filler composition of the first aspect, together with a UV-curable coating composition, wherein the UV-curable coating composition comprises a pigment.

[0226] Mechanical and adhesion properties

[0227] The following properties relate to cured products of the UV-curable filler composition. For example, the UV-curable filler composition may be cured by the methods described herein. The sample preparation and testing methodology are discussed further below. Typically, when cured, the UV-curable filler compositions according to the first aspect have the following mechanical properties: a tensile modulus of at least 3000 MPa, such at least 3200 MPa, such as at least 3400 MPa, such as at least 3600 MPa, such as at least 3800 MPa, such as at least 4000 MPa, such as at least 4200 MPa, such as at least 4400 MPa, such as at least 4600 MPa, such as at least 4800 MPa, wherein the tensile modulus is measured in accordance with the method provided in the methods section; and / or a tensile strength of at least 39 MPa, such as at least 40 MPa, such as at least 41 MPa, such as at least 42 MPa, such as at least 43 MPa, such as at least 44 MPa, such as at least 45 MPa, wherein the tensile strength is measured in accordance with the method provided in the methods section; and / or a tensile elongation of at least 1 .3%, such as at least 1 .4%, such as at least 1 .5%, such as at least 1 .6%, wherein the tensile elongation is measured in accordance with the method provided in the methods section; and / or a Barcol hardness of at least 25, such as at least 26, such as at least 27, such as at least 28, such as at least 29, such as at least 30, such as at least 31 , such as at least 32, wherein the Barcol hardness is measured in accordance with the method provided in the methods section; and / or a Shore D hardness of at least 70 D, such as at least 71 D, such as at least 72 D, such as at least 73 D, such as at least 74 D, such as at least 75 D, such as at least 76 D, such as at least 77 D, such as at least 78 D, such as at least 79 D, such as at least 80 D, such as at least 81 D, such as at least 82 D, such as at least 83 D, such as at least 84 D, such as at least 85 D, wherein the Shore D hardness is measured in accordance with the method provided in the methods section; and / or a Tgof at least 60 °C, such as at least 65 °C, such as at least 70 °C, such as at least 75 °C , such as at least 80 °C , such as at least 85 °C , such as at least 90 °C, such as at least 95 °C, wherein the Tgis measured in accordance with the method provided in the methods section; and / or an adhesion pull-off strength to epoxy-glass fibre laminate of at least 5.0 MPa, such as at least 6.0 MPa, such as at least 7.0 MPa, such as at least 8.0 MPa, such as at least 9.0 MPa, such as at least 10 MPa, such as at least 11 MPa, wherein the adhesion pull-off strength is measured in accordance with the method provided in the methods section. an adhesion pull-off strength to polyester-glass fibre laminate of at least 5.0 MPa, such as at least 6.0 MPa, such as at least 7.0 MPa, such as at least 8.0 MPa, such as at least 9.0 MPa, such as at least 10 MPa, such as at least 11 MPa, wherein the adhesion pull-off strength is measured in accordance with the method provided in the methods section. Further embodiments

[0228] In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise:

[0229] (A) a vinyl ester resin composition comprising:

[0230] (a) a vinyl ester oligomer; and

[0231] (b) a reactive diluent monomer mixture comprising a first reactive diluent monomer having at least one polymerisable functional group and a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0232] (B) a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm, such as from 380 to 420 nm; and

[0233] (C) an adhesion promoter for use in UV curing; wherein reactive diluent monomer mixture is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 28 wt%, such as from 12 to 26 wt%, such as from 14 to 24 wt%, such as from 16 to 22 wt%, such as from 18 to 20 wt%.

[0234] In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise:

[0235] (A) a vinyl ester resin composition comprising:

[0236] (c) a vinyl ester oligomer; and

[0237] (d) a reactive diluent monomer mixture comprising a polyfunctional reactive diluent monomer as a first reactive diluent monomer, and a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0238] (B) a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm, such as from 380 to 420 nm; and

[0239] (C) an adhesion promoter for use in UV curing; wherein one or more polyfunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 28 wt%, such as from 12 to 26 wt%, such as from 14 to 24 wt%, such as from 16 to 22 wt%, such as from 18 to 20 wt%.

[0240] In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise: (A) a vinyl ester resin composition comprising:

[0241] (a) a vinyl ester oligomer; and

[0242] (b) a reactive diluent monomer mixture comprising a polyfunctional reactive diluent monomer as a first reactive diluent monomer, wherein the polyfunctional reactive diluent monomer comprises a vinyl functional group, such as a (meth)acrylate functional group, and a second reactive diluent monomer having at least one polymerisable functional group wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0243] (B) a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm, such as from 380 to 420 nm; and

[0244] (C) an adhesion promoter for use in UV curing; wherein the one or more polyfunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 28 wt%, such as from 12 to 26 wt%, such as from 14 to 24 wt%, such as from 16 to 22 wt%, such as from 18 to 20 wt%.

[0245] In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise:

[0246] (A) a vinyl ester resin composition comprising:

[0247] (a) a vinyl ester oligomer; and

[0248] (b) a reactive diluent monomer mixture comprising a polyfunctional reactive diluent monomer as a first reactive diluent monomer, wherein the one or more polyfunctional reactive diluent monomers is one or more polyvinyl reactive diluent monomers, such as one or more poly(meth)acrylate reactive diluent monomers, and a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0249] (B) a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm, such as from 380 to 420 nm; and

[0250] (C) an adhesion promoter for use in UV curing; wherein the one or more polyvinyl reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 28 wt%, such as from 12 to 26 wt%, such as from 14 to 24 wt%, such as from 16 to 22 wt%, such as from 18 to 20 wt%. In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise:

[0251] (A) from 40 to 80 wt%, such as from 52 to 68 wt%, such as from 56 to 64 wt%, such as from 58 to 62 wt %, based on the total weight of the UV-curable filler composition, a vinyl ester resin composition comprising:

[0252] (a) from 16 to 38 wt%, such as from 20 to 34 wt%, such as from 24 to 30 wt%, based on the total weight of the UV-curable filler composition, a vinyl ester oligomer; and

[0253] (b) from 10 to 70 wt%, such as from 20 to 60 wt%, such as from 30 to 50 wt%, based on the total weight of the UV-curable filler composition, a reactive diluent monomer mixture comprising a polyfunctional reactive diluent monomer as a first reactive diluent monomer , wherein the polyfunctional reactive diluent monomer comprises a vinyl functional group, such as a (meth)acrylate functional group, and a second reactive diluent monomer having at least one polymerisable functional group wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0254] (B) from 0.01 to 5.0 wt%, such as from 0.01 to 1 .0 wt%, such as from 0.01 to 0.5 wt%, based on the total weight of the UV-curable filler composition, a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm, such as from 380 to 420 nm; and

[0255] (C) from less than 15 wt%, such as from 1 .0 to 10 wt%, such as from 1 .0 to 5.0 wt%, based on the total weight of the UV-curable filler composition, an adhesion promoter for use in UV curing; wherein the one or more polyfunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 28 wt%, such as from 12 to 26 wt%, such as from 14 to 24 wt%, such as from 16 to 22 wt%, such as from 18 to 20 wt%.

[0256] In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise:

[0257] (A) from 52 to 68 wt%, such as from 56 to 64 wt%, based on the total weight of the UV-curable filler composition, a vinyl ester resin composition comprising:

[0258] (a) from 20 to 34 wt%, such as from 24 to 30 wt%, based on the total weight of the UV-curable filler composition, a vinyl ester oligomer; and

[0259] (b) from 20 to 60 wt%, such as from 30 to 50 wt%, based on the total weight of the UV-curable filler composition, a reactive diluent monomer mixture comprising a polyfunctional reactive diluent monomer as a first reactive diluent monomer, wherein the polyfunctional reactive diluent monomer comprises a vinyl functional group, such as a (meth)acrylate functional group, and a second reactive diluent monomer having at least one polymerisable functional group wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer; (B) from 0.01 to 1 .0 wt%, such as from 0.01 to 0.5 wt%, based on the total weight of the UV-curable filler composition, a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm, such as from 380 to 420 nm; and

[0260] (C) from 1 .0 to 10 wt%, such as from 1 .0 to 5.0 wt%, based on the total weight of the UV-curable filler composition, an adhesion promoter for use in UV curing; wherein the one or more polyfunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 14 to 24 wt%, such as from 16 to 22 wt%, such as from 18 to 20 wt%.

[0261] In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise:

[0262] (A) from 56 to 64 wt%, based on the total weight of the UV-curable filler composition, a vinyl ester resin composition comprising:

[0263] (a) from 24 to 30 wt%, based on the total weight of the UV-curable filler composition, a vinyl ester oligomer; and

[0264] (b) from 30 to 50 wt%, based on the total weight of the UV-curable filler composition, a reactive diluent monomer mixture comprising a polyfunctional reactive diluent monomer as a first reactive diluent monomer, wherein the polyfunctional reactive diluent monomer comprises a vinyl functional group, such as a (meth)acrylate functional group, and a second reactive diluent monomer having at least one polymerisable functional group wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0265] (B) from 0.01 to 0.5 wt%, based on the total weight of the UV-curable filler composition, a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm, such as from 380 to 420 nm; and

[0266] (C) from 1 .0 to 5.0 wt%, based on the total weight of the UV-curable filler composition, an adhesion promoter for use in UV curing; wherein the one or more polyfunctional reactive diluent monomers is present, based on the total weight of the UV-curable filler composition, in an amount from 16 to 22 wt%, such as from 18 to 20 wt%.

[0267] In further embodiments, a UV-curable filler composition for use with a UV-curable coating composition in a multi-part repair resin system according to the first aspect may comprise:

[0268] (A) a vinyl ester resin composition comprising:

[0269] (a) a vinyl ester oligomer; and (b) a reactive diluent monomer mixture comprising a first reactive diluent monomer having at least one polymerisable functional group and a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;

[0270] (B) a photoinitiator for absorbing UV light at a wavelength from 200 to 420 nm; and

[0271] (C) an adhesion promoter for use in UV curing; such that, when cured, the composition has a tensile elongation of at least 1 .3%, such as at least 1 .4%, such as at least 1.5%, such as at least 1.6%, wherein the tensile elongation is measured in accordance with the method provided in the methods section.

[0272] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0273] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0274] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0275] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0276] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0277] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%. Examples

[0278] Materials

[0279] EPIKOTE® 828 is a diglycidyl ether of bisphenol A (DGEBA) prepared from the reaction between bisphenol A and epichlorohydrin obtained from Westlake Epoxy.

[0280] Bisphenol A (BPA) was obtained from Tokyo Chemical Company.

[0281] Triphenyl phosphine (TPP) was obtained from Sigma Aldrich Company Ltd.

[0282] Mono-monomer 1 (MM-1) is a mono(meth)acrylate monomer obtained from GEO Speciality Chemicals.

[0283] Di-monomer 1 (DM-1) is a di(meth)acrylate monomer obtained from Sartomer.

[0284] Tri-monomer 1 (TM-1) is a tri(meth)acrylate monomer obtained from Sartomer.

[0285] Mono-monomer 2 (MM-2) is a monovinyl monomer obtained from BASF.

[0286] Di-monomer 2 (DM-2) is a di(meth)acrylate monomer obtained from Evonik.

[0287] Tri-monomer 2 (TM-2) is a tri(meth)acrylate monomer obtained from Arkema.

[0288] PI-1 is a phosphine oxide photoinitiator obtained from IGM Resins.

[0289] AP-1 is an acidic triacrylate adhesion promoter obtained from Arkema.

[0290] Methods

[0291] Tensile testing

[0292] Tensile stress and tensile strain properties were measured according to ISO 527-4 at 5 mm / min.

[0293] Barcol hardness

[0294] Barcol hardness was measured according to ASTM D2538 with an average taken from three measurements of a sample.

[0295] Shore D hardness

[0296] Shore D hardness was measured according to ISO 868 with an average taken from three measurements of a sample.

[0297] Dynamic mechanical thermal analysis The term, glass-transition temperature (Tg), is well known in the art, indicating the temperature below which an amorphous material behaves as a glassy solid and above which the same material turns into a viscous or rubber-like state.

[0298] Dynamic mechanical thermal analysis (DMTA) is the cyclic deformation of a polymer or composite to measure the viscoelastic response of material under certain conditions. A temperature sweep is undertaken which varies the temperature throughout the test, while other parameters are kept constant. Specimens should be 60 x 10 x 2 mm according to ISO 6271-2. The temperature sweep is conducted to measure Tgaccording to the following protocol: The sample was set up by fixing the sample at the upper clamp of the SRF. Before attaching the sample to the lower clamp of the SRF, the NF displayed in the control panel must be Zero (NF = 0 N ± 0.01 N). Frequency is 1 Hz; Perform LVER Test to ascertain for each material type; Heating rate: 2 K / min (2°C / min); A normal force (NF) tension of -0.2 N is set in order to keep the sample stretched during the entire test. For elastomers: the NF should not exceed -0.003 N mm2; for thermoplastics and thermosets: the NF should not exceed -0.1 N mm2. When thermoset materials are in the rubbery plateau region, the NF should not exceed -0.03 N mm2. While the sample gets softer with increasing temperature and the linear region is getting wider, the normal force was reduced the set strain was increased towards higher temperatures. G’ analysis is conducted according to ISO 11357-1 (2016-05) and ISO 11357-2 (2-14-07).

[0299] Adhesion pull-off strength

[0300] In the preparation for conducting the adhesion pull-off strength tests, the laminate / substrate, the UV- curable filler composition of the first aspect, and the dolly were each prepared as follows. The surface of the laminate I substrate (epoxy-glass fibre laminate or polyester-glass fibre laminate) was sanded with 40 grit sandpaper, dusted off, cleaned with isopropyl alcohol or acetone and dried. A sample of the UV- curable filler composition of the first aspect was applied, cured according to the curing method as described below, and was allowed to cool to ambient temperature (20 to 30 °C). The resulting cured product of the UV-curable filler composition was sanded with 40 grit sandpaper, dusted off, cleaned with isopropyl alcohol or acetone and dried. 4 dollies were each sanded with 40 grit sandpaper, dusted off, cleaned with isopropyl alcohol or acetone and dried. Each of the prepared dollies were adhered onto the surface of the prepared cured product using the adhesive Araldite 2015, and the adhesive was allowed to cure for a minimum of 25 hours at ambient temperature (20 to 30 °C). The cured product of the UV- curable filler composition on the surface of the laminate was abraded back using a hole saw to the point at which until the laminate is exposed around the dolly.

[0301] The adhesion pull-off strength was measured according to the following protocol. A pull-off device was attached to the dolly and an increasing load was applied. The adhesion pull-off strength of the cured product was recorded in MPa based on the maximum force observed before the point of adhesive failure for each dolly tested. The point of adhesive failure occurs when the cured product is separated from the laminate. A minimum of 4 samples were tested and the results averaged. The adhesion pull-off strength is cited with reference to the type of substrate. Number molecular

[0302] The number average molecular weight Mn was measured using gel permeation chromatography in accordance with the following measurement conditions: GPC setup: Viscotex GPC Max, Column Oven, Viscotek VE3580RI detector; GPC software: OmniSec 4.5; Column: Phenomenex Phenogel 5pm Linear / Mixed Guard column 30 x 4.6mm, Phenogel 5pm Linear (2) column 300 x 4.6mm & Phenogel 5pm 50A column 300 x 4.6mm; Mobile phase: THF; Sample; Solvent: THF with 0.2% Toluene as flow marker; Flow rate: 0.35ml / min; Column Temperature: 40°C; Injection volume: 20pL; Detector Temperature: 35°C; Sample Prep: ~0.025g dissolved in 5.0ml Solvent, filtered through a 0.45pm PTFE syringe filter; Calibration Standards: Agilent Polystyrene High EasyVials; Elution time: 30 minutes.

[0303] General vinyl ester resin and UV-curable filler

[0304] The vinyl ester resin composition, was prepared according to the following general protocol:

[0305] In the first stage, under nitrogen atmosphere, an epoxy compound (EPIKOTE® 828), a chain-extending reagent (BPA) and a suitable amount of catalyst (TPP) were weighed into a round bottomed flask with an overhead mechanical stirrer, a gas feed, a temperature probe and in inlet. The reaction mixture was then heated to a temperature of between 120 °C to 130 °C under an inert atmosphere and monitored via melt viscosity and EEW (epoxy equivalent weight) value.

[0306] In the second stage, the temperature was set to 115 °C and a first set of inhibitors were added into the reaction mixture. A gas mixture of 50:50 air itrogen was then continuously fed into the reactor. The reaction mixture was monitored via melt viscosity and EEW (epoxy equivalent weight) value.

[0307] In the third stage, the end-capping reagent was weighed together with a suitable amount of catalyst (TPP). The temperature of the reaction mixture was set to 100 °C to 125 °C. The mixture containing the end-capping reagent and catalyst is then drip fed over the course of one hour into the reaction mixture. When the acid value is approaching zero, such as less than 14, such as less than 10, such as less than 5, the reaction is proceeded to the fourth stage. The reaction mixture was monitored via melt viscosity and EEW (epoxy equivalent weight) value.

[0308] In the fourth stage, a second set of inhibitors was added to the reaction mixture. The temperature was set to 80 °C and then a first portion of the reactive diluent monomer mixture comprising one or more polyfunctional reactive diluent monomers (mono-monomer, di-monomer and tri-monomer) and a third set of inhibitors were added to the reaction mixture under vigorous stirring. The reaction mixture was then cooled to a temperature of between 20 °C to 25 °C (room temperature). Once cooled, the reaction mixture was decanted from the reaction vessel to obtain a first vinyl ester resin composition VE A. Therefore, the first vinyl ester resin composition VE A comprises a vinyl ester oligomer, a monomonomer, a di-monomer and a tri-monomer (see Tables 1 , 2 and 3 below, in which the weight of the mono-monomer, di-monomer and tri-monomer is each relative to the total weight of the UV-curable filler composition). In the fourth stage, a second portion of reactive diluent monomer mixture comprising one or more polyfunctional reactive diluent monomers (polyfunctional monomer (i) and polyfunctional monomer (ii)), which may be the same as or different to those of the first portion, was added to the vinyl ester resin composition VE A to form a second vinyl ester resin composition. Therefore, the second vinyl ester resin composition comprises VE A (a vinyl ester oligomer, a mono-monomer, a di-monomer and a tri-monomer) and the polyfunctional reactive diluent monomers (see Tables 1 , 2 and 3 below).

[0309] The second vinyl ester resin composition was then added with a photoinitiator and an adhesion promoter as shown in the Tables below, together with further components of 1 .5 wt% of an air release agent, 1 .9 wt% of a wetting agent, 0.5 wt% of a clay, 12.5 wt% of a talc, 10 wt% of an aluminium hydroxide, 4.5 wt% of a silica to form the UV-curable filler composition.

[0310] Curing method

[0311] Each UV-curable filler composition was cured using a 395 nm UV LED lamp at 1.1 GHz at an intensity of 60% (around 0.8 W cm2) for 2 minutes. The maximum curing depth achieved for each UV-curable filler composition was 6.5 mm.

[0312] Table 1 - Inventive Example 1 and Reference Example 1

[0313] Inventive Examples 1 each a UV-curable filler composition according to the first aspect of the invention. Reference Example 1 corresponds to a commercially available styrene-based non-UV-curable filler composition which does not comprise a photoinitiator and an adhesion promoter. Once cured according to the curing method as described above, Inventive Example 1 and Reference Example 1 were tested for the tensile modulus, tensile strength, tensile elongation, Barcol hardness, Shore D hardness, Tgand adhesion pull-off strength, as described in the methods section above.

[0314] As shown in Table 1 , an excellent combination of mechanical, thermal and adhesion properties is achieved in each resulting cured product of Inventive Example 1. Importantly, the combination of mechanical, thermal and adhesion properties achieved by the cured product of Inventive Examples 1 to 3 is superior, or at least similar, compared to the properties achieved by the cured product of Reference Example 1 , while at the same time being able to achieve a rapid and complete UV-cure. For example, Inventive Example 1 , compared to Reference Example 1 , achieves a significantly improved mechanical properties such as tensile modulus (4986.02 MPa vs > 3000 MPa), Barcol hardness (32 vs > 25) and Shore D hardness (85 D vs > 60 D), significantly improved thermal properties such as Tg(67.2 °C vs > 55 °C), and significantly improved adhesion properties such as adhesion pull-off strength (epoxy-glass fibre laminate) (11.15 MPa vs > 5 MPa) and adhesion pull-off strength (polyester glass fibre laminate) (8.89 MPa vs > 3.5 MPa).

[0315] Table 2 - Inventive Examples 2 to 6

[0316] Inventive Examples 2 to 6 is each a UV-curable filler composition according to the first aspect of the invention. Each of Inventive Examples 3 to 7 comprise the same mono-monomer, di-monomer, trimonomer, photoinitiator and adhesion promoter, differing only by the (poly)functional monomer (i) and the amount thereof. Once cured according to the curing method as described above, each of Inventive Examples 2 to 6 were tested for the tensile modulus, tensile strength, tensile elongation, Barcol hardness, Shore D hardness, Tfland adhesion-pull off strength, as described in the methods section above.

[0317] Inventive Examples 2 to 6 were each prepared to demonstrate how the amounts of two polyfunctional reactive diluent monomers (i) and (ii) of the second monomer portion, can influence the mechanical properties, in particular the tensile elongation, of the resulting cured product. The (poly)functional reactive diluent monomers (i) used in Inventive Examples 2 to 6 are MM-2 (a monofunctional vinyl monomer) and DM-2 (a difunctional (meth)acrylate monomer).

[0318] It has been found that decreasing the amount of MM-2 from 4.5 wt% (Inventive Example 2) to 3.0 wt% (Inventive Example 3) had the effect of significantly increasing the tensile modulus at a slight detriment to the tensile elongation. It has also been found that decreasing the amount of DM-2 from 4.5 wt% (Inventive Example 4) to 3.0 wt% (Inventive Example 5) had the effect of significantly increasing the tensile elongation and a slight detriment to the tensile modulus. Additionally, further decreasing the amount of DM-2 to 2.0 wt% (Inventive Example 6) had the effect of additionally increasing the tensile elongation with an improved tensile modulus.

[0319] Table 3 - Inventive Examples 7 to 12

[0320] Inventive Examples 7 to 12 is each a UV-curable filler composition according to the first aspect of the invention. Each of Inventive Examples 7 to 12 comprise the same mono-monomer, di-monomer, trimonomer, photoinitiator and adhesion promoter, differing only by the polyfunctional monomer (I) and the polyfunctional monomer (ii), and the amount of the polyfunctional monomer (i) and the amount of the polyfunctional monomer (ii). Thus, each of Inventive Examples 7 to 12 differ by the amount of the polyfunctional reactive diluent monomer. Once cured according to the curing method as described above, each of Inventive Examples 7 to 12 were tested for the tensile modulus, tensile strength, tensile elongation, Barcol hardness, Shore D hardness, Tgand adhesion pull-off strength, as described in the methods section above.

[0321] Inventive Examples 7 to 12 were each prepared to demonstrate how the selection of two polyfunctional reactive diluent monomers (i) and (ii) of the second monomer portion, and amounts thereof can influence the mechanical properties of the resulting cured product. And it has been found that a UV-curable filler composition comprising an amount of a trifunctional vinyl monomer (TM-2) that is greater than the amount of the lower functionality difunctional monomer (DM-2), provides a cured product with the combination of excellent mechanical and thermal properties. Importantly, the properties are achieved from UV curing at ambient temperature, and without the use of cobalt accelerators, peroxide initiators and high temperatures.

Claims

Claims:1 . A UV-curable filler composition comprising:(A) a vinyl ester resin composition comprising:(a) a vinyl ester oligomer;(b) a reactive diluent monomer mixture comprising a first reactive diluent monomer having at least one polymerisable functional group and a second reactive diluent monomer having at least one polymerisable functional group, wherein the first reactive diluent monomer has a different number of polymerisable functional groups to the second reactive diluent monomer;(B) a photoinitiator; and(C) an adhesion promoter for use in UV curing.

2. The UV-curable filler composition of claim 1 , wherein the vinyl ester oligomer is present, based on the total weight of the UV-curable filler composition, in an amount from 16 to 38 wt%, such as from 18 to 36 wt%, such as from 20 to 34 wt%, such as from 22 to 32 wt%, such as from 24 to 30 wt%, such as from 26 to 28 wt% .

3. The UV-curable filler composition of claims 1 or 2, wherein the reactive diluent monomer mixture is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 70 wt%, such as from 15 to 70 wt%, such as from 15 to 65 wt%, such as from 20 to 65 wt%, such as from 20 to 60 wt%, such as from 25 to 60 wt%, such as from 25 to 55 wt%, such as from 25 to 50 wt%, such as from 30 to 50 wt%, such as from 35 to 50 wt%, such as from 35 to 45 wt%.

4. The UV-curable filler composition of any one of claims 1 to 3, wherein the first reactive diluent monomer comprises a polyfunctional reactive diluent monomer.

5. The UV-curable filler composition of any one of claims 1 to 3, wherein the first reactive diluent monomer is present, based on the total weight of the UV-curable filler composition, in an amount from 10 to 28 wt%, such as from 12 to 26 wt%, such as from 14 to 24 wt%, such as from 16 to 22 wt%, such as from 18 to 20 wt%.

6. The UV-curable filler composition of any one of claims 1 to 4, wherein the first reactive diluent monomer comprises a vinyl functional group, such as comprising a (meth)acrylate functional group.

7. The UV-curable filler composition of any one of claims 1 to 5, wherein the first reactive diluent monomer comprises a trifunctional reactive diluent monomer.

8. The UV-curable filler composition of claim 7, wherein the trifunctional reactive diluent monomer is present, based on the total weight of the UV-curable filler composition, in an amount from 6.0 to 16 wt%, such as from 8.0 to 14 wt%, such as from 10 to 12 wt%.

9. The UV-curable filler composition of claim 7 or 8, wherein the one or more trifunctional reactive diluent monomers is a tri(meth)acrylate monomer.

10. The UV-curable filler composition of any one of claims 1 to 8, wherein the first reactive diluent monomer comprises a difunctional reactive diluent monomer.11 . The UV-curable filler composition of claim 10, wherein the difunctional reactive diluent monomer is present, based on the total weight of the UV-curable filler composition, in an amount from 4.0 to 14 wt%, such as from 6.0 to 12 wt%, such as from 8.0 to 10 wt%.

12. The UV-curable filler composition of claim 10 or 11 , wherein the difunctional reactive diluent monomers is a di(meth)acrylate monomer.

13. The UV-curable filler composition of any one of claims 1 to 12, wherein the second reactive diluent monomer comprises a monofunctional reactive diluent monomer; optionally wherein the second reactive diluent monomer is present, based on the total weight of the UV-curable filler composition, in an amount from 14 to 26 wt%, such as from 16 to 24 wt%, such as from 18 to 22 wt%.

14. The UV-curable filler composition any one of claims 1 to 13, wherein the second reactive diluent monomer comprises a monovinyl monomer.

15. The UV-curable filler composition of any one of claims 1 to 14, wherein the vinyl ester oligomer has a number average molecular weight Mn of at least 2000, as measured by gel permeation chromatography in accordance with the methods provided in the methods section.

16. The UV-curable filler composition of any one of claims 1 to 15, wherein the photoinitiator is for absorbing UV light at a wavelength from 200 to 420 nm, such as from 365 to 405 nm, such as from 375 to 405 nm, such as from 385 to 405 nm, such as from 390 to 405 nm; optionally wherein the photoinitiator is present, based on the total weight of the UV-curable filler composition, in an amount from 0.01 to 2.0 wt%, such as from 0.02 to 0.5 wt%, such as from 0.05 to 0.3 wt%.

17. The UV-curable filler composition of any one of claims 1 to 16, wherein the adhesion promoter is present, based on the total weight of the UV-curable filler composition, in an amount of less than 15 wt%, such as from 1 .0 to 15 wt%, such as from 1 .0 to 10 wt%, such as from 1 .0 to 5.0 wt%, such as from 2.0 to 4.0 wt%.

18. The UV-curable filler composition of any one of claims 1 to 17, wherein the photoinitiator is a photoinitiator for absorbing UV light at a wavelength from 380 to 420 nm.

19. The UV-curable filler composition of any one of claims 1 to 18, wherein the adhesion promoter is an acid-based adhesion promoter, such as a trifunctional acid-based adhesion promoter, such as a tri(meth)acrylate acid-based adhesion promoter.

20. The UV-curable filler composition of any one of claims 1 to 19, further comprising an air release agent.

21. The UV-curable filler composition of any one of claims 1 to 20, further comprising an organoclay, such as a mixed mineral organoclay.

22. The UV-curable filler composition of any one of claims 1 to 21 , further comprising a particulate filler, such as aluminium trihydrate, talc or fumed silica, or a combination thereof.

23. Use of the UV-curable filler composition of any one of claims 1 to 22 in a wind turbine repair process, such as a wind blade repair process, a tower repair process or a nacelle repair process.

24. A cured product of the UV-curable filler composition of any one of claims 1 to 22.

25. A multi-part repair system comprising a plurality of parts, which in combination contains the UV- curable filler composition of any one of claims 1 to 22, together with a UV-curable coating composition, wherein the UV-curable coating composition comprises a pigment.

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