UV-curable coating compositions

Abstract

The present invention provides a multi-part UV-curable coating composition comprising a first part (A) for providing a pigmented layer (A') and a second part (B) for providing a UV-resistant layer (B'). The multi- part UV-curable coating composition is particularly suitable for use in a composite repair process, such as for wind turbine repair. The present invention also relates to a multi-part repair system comprising the multi-part UV-curable coating composition and a UV-curable filler composition, and to methods of curing of, and to cured products made from, the multi-part UV-curable coating composition.

Description

[0001] UV-CURABLE COATING COMPOSITIONS

[0002] Field of the Invention

[0003] The present invention relates to a UV-curable coating composition, and to uses, methods of curing 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] In particular, the composite materials of wind turbines, which are typically made from epoxy or polyester resins reinforced with glass fibre, are susceptible to degradation when exposed to UV light. The consequences of prolonged exposure of UV light on such materials has detrimental effects on the surface finish of the wind blade, culminating in diminished energy generation performance and increased maintenance costs.

[0008] One option to improve the UV stability, and thus the weathering performance and resistance, of the exterior coating of a wind turbine is to employ organic or inorganic UV absorbers. UV absorbers work by absorbing UV light, which may be detrimental to the composite material, and converting it into heat. The heat may then be dissipated through the material. The use of UV absorbers to provide UV stability to such materials is known in the art.

[0009] For example, US10047227B discloses a radiation-curable coating composition comprising polymerisable, unsaturated compounds in an amount from 15 to 98 wt% and an organic UV absorber which is present in an amount from 5 to 35 wt%. The organic UV absorber is characterised by having a maximum absorption coefficient in a wavelength range of less than 390 nm, such as less than 350 nm, and the coating composition relies solely on the organic UV absorber for imparting UV stability to the coating composition. To impart colour, the coating composition also comprises a pigment, wherein the pigment and the organic UV absorber are provided together in a single coating composition.

[0010] Consequently, there remains a need within the art for improvements in coating and coating compositions for repair applications, which provide excellent weathering stability and without introducing limitations which may otherwise affect the curing capability of the composition. The cured product must also provide excellent mechanical and adhesion properties to support its performance in wind turbine applications.

[0011] The present invention has been devised in light of the above considerations.

[0012] Summary of the Invention

[0013] In a first aspect of the invention, there is provided a multi-part UV-curable coating composition comprising:

[0014] (A) a first part for providing a pigmented layer (A’) when cured comprising:

[0015] (i) a pigment; and

[0016] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0017] (i) a UV absorber and a free radical scavenger; wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0018] Surprisingly, the inventors have found that, once cured, the multi-part UV-curable coating composition of the first aspect can achieve excellent weathering performance. This is exhibited by significantly low average values of colour change (dE) after undergoing both xenon and QUV accelerated weathering.

[0019] Firstly, the inventors have found that the use of a pigment in the multi-part UV curable coating composition of the first aspect is capable of maintaining a desirable colour, once cured.

[0020] Secondly, the inventors have found that the use of a UV absorber, a hindered amine light stabiliser, and an anti-oxidant is capable of imparting excellent weathering performance on the UV-curable coating composition, once cured. In particular, the UV-resistant mixture works by having two types of components which impart UV-resistant properties on the coating composition. The UV absorber functions to absorb UV light and convert it into less harmful radiation, such as infrared light. The free radical scavenger, which has a different mechanism of action to the UV absorber, removes free radicals generated from UV exposure without directly interacting with UV light itself. When the two types of components are present together, the components of the UV-resistant mixture provide a significant amount of UV protection to the coating composition, and is demonstrated from a significantly low average value of colour change (dE).

[0021] Thirdly, the inventors have found that, by providing the first part (A) and the second part (B) as distinct and separate components, the desirable weathering properties are obtained, when cured. The first part (A) is for providing a pigmented layer (A’) in a cured product. The second part (B) is for providing a UV- resistant layer (B’) in a cured product. By providing the first part (A) separate from the second part (B), such that the pigment and the UV absorber and free radical scavenger are separated from each other, the UV cure depth is significantly improved. This being advantageous over one-pot compositions comprising the pigment and the UV absorber and free radical scavenger, which display a reduced UV cure depth. In particular, in order to achieve the desirable weathering properties, in use the UV-resistant layer (B’) is positioned on top of the pigmented layer (A’) relative to a substrate.

[0022] In preferred embodiments, the free radical scavenger is a hindered amine light stabiliser or an antioxidant, or a combination thereof. In particular, the hindered amine light stabiliser and an anti-oxidant, which have a different mechanism of action to the UV absorber, each function as free radical scavengers by removing free radicals generated from UV exposure without directly interacting with UV light itself. Typically, hindered amine light stabilisers act on larger radicals, while anti-oxidants act on smaller radicals.

[0023] Suitably, in a second aspect of the invention, there is provided a method of curing the multi-part UV- curable coating composition of the first aspect, the method comprising the steps of:

[0024] (i) applying the first part (A) onto a substrate;

[0025] (ii) exposing the first part (A) to UV light at a wavelength from 200 to 405 nm to initiate a cure via a free radical polymerisation, and obtaining a pigmented layer (A’);

[0026] (iii) applying the second part (B) onto the pigmented layer (A’); and

[0027] (iv) exposing the second part (B) to UV light at a wavelength from 200 to 405 nm to initiate a cure via a free radical polymerisation, and obtaining a UV-resistant layer (B’).

[0028] A third aspect of the invention provides the use of the multi-part UV-curable coating composition of the first aspect in a composite repair process.

[0029] The method of curing of the second aspect employs a stepwise approach by curing the first part (A) before curing the second part (B). Thus, the method involves preparing a cured product comprising a pigmented layer (A’) corresponding to the first part (A) of the multi-part UV-curable coating composition, and a UV-resistant layer (B') corresponding to the second part (B) of the multi-part UV-curable coating composition. That is, the UV-resistant layer (B ) is positioned on top of the pigmented layer (A’) relative to a substrate.

[0030] Surprisingly, the inventors have found that such a method, in which the stepwise approach to curing is employed by splitting up the first part (A) for providing a pigmented layer (A’) and the second part (B) for providing a UV-resistant layer (B'), excellent weathering performance may be achieved. Importantly, the first part (A) comprising the pigment is cured first, so that the pigment and the UV absorber do not affect each other when exposed to light. By providing the first part (A) separate from the second part (B), such that the pigment is separated from the UV absorber and free radical scavenger, the UV cure depth is significantly improved. Otherwise, a one-pot composition comprising the pigment and the UV absorber and free radical scavenger has the effect of reducing the UV cure depth.

[0031] Suitably, in a fourth aspect of the invention, there is provided a cured product of the multi-part UV-curable coating composition of the first aspect. The cured product may be comprised in an article or an object, to which the invention also relates. Preferably, in the fourth aspect, the cured product has a dE value of less than 4.0, as measured using QUV accelerated weathering tests at 1000 hours in accordance with the method provided in the methods section.

[0032] A fifth aspect of the invention is a wind turbine part comprising the cured product of the third aspect.

[0033] The multi-part UV-curable coating composition of the first aspect can be used together with a UV-curable filler composition. In a standard resin repair process, a filler composition is typically employed to fill gaps, voids or imperfections, such as those caused by environmental damage, on the surface of a cured product, such as the surface of a wind turbine. Typically, a coating composition is a decorative protective layer which is applied over the cured resin to enhance its durability, provide additional strength, and improve its aesthetic appearance, such as to provide a glossy, matte or stain finish. The coating composition may be a gelcoat composition or a topcoat composition. When used as a gelcoat composition, the coating composition is typically applied to a mould before it is then applied to a substrate. When used as a topcoat composition, the multi-part UV-curable coating composition is applied directly to a substrate as a final layer.

[0034] Suitably, in a sixth aspect of the invention, there is provided a multi-part repair system comprising a plurality of parts, which in combination contains the multi-part UV-curable coating composition of the first aspect together with a UV-curable filler composition, wherein the UV-curable filler composition comprises a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer mixture.

[0035] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0036] 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.

[0037] Detailed Description of the Invention

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] In some embodiments, the vinyl ester oligomer is derived from, or is the reaction product of:

[0044] (i) an epoxy compound comprising at least two epoxy groups;

[0045] (ii) a chain-extending reagent; and

[0046] (iii) a compound selected from an a,p-unsaturated monocarboxylic acid, a (meth)acrylate ester or a (meth)acrylate anhydride.

[0047] 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.

[0048] In some embodiments, the epoxy group is bisphenol A diglycidyl ether.

[0049] In some embodiments, the chain-extending reagent is an aromatic chain-extending reagent.

[0050] In some embodiments, the chain-extending reagent comprises at least two hydroxyl groups or at least two carboxylic acid groups.

[0051] In some embodiments, the chain-extending reagent is bisphenol A.

[0052] In some embodiments, the a,p-unsaturated monocarboxylic acid is methacrylic acid.

[0053] In some embodiments, the (meth)acrylate ester is a methacrylate ester, such as glycidyl methacrylate.

[0054] In some embodiments, the (meth)acrylate anhydride is a methacrylic anhydride.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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% .

[0061] 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%.

[0062] 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%.

[0063] The vinyl ester oligomer of the present invention is comprised in the first part (A) and the second part (B) of the multi-part UV-curable coating composition. The amount of the vinyl ester oligomer in the first part (A) may be independent of the amount of the vinyl ester oligomer in the second part (B).

[0064] In some embodiments, the vinyl ester oligomer is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition in an amount of at least 20 wt%, such as at least 23 wt%, such as at least 25 wt%, such as at least 27 wt%, such as at least 29 wt%, such as at least 31 wt%, such as at least 33 wt%.

[0065] In some embodiments, the vinyl ester oligomer is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of at most 50 wt%, such as at most 45 wt%, such as at most 43 wt%, such as at most 41 wt%, such as at most 39 wt%, such as at most 37 wt%, such as at most 35 wt%. In some embodiments, the vinyl ester oligomer is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 20 to 50 wt%, such as from 23 to 45 wt%, such as from 25 to 43 wt%, such as from 27 to 41 wt%, such as from 29 to 39 wt%, such as from 31 to 37 wt%, such as from 33 to 35 wt%.

[0066] In some embodiments, the vinyl ester oligomer is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition in an amount of at least 60 wt%, such as at least 70 wt%, such as at least 80 wt%, such as at least 83 wt%, such as at least 85 wt%, such as at least 87 wt% , such as at least 89 wt% .

[0067] In some embodiments, the vinyl ester oligomer is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of at most 95 wt%, such as at most 94 wt%, such as at most 93 wt%, such as at most 92 wt%, such as at most 91 wt%.

[0068] In some embodiments, the vinyl ester oligomer is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 60 to 95 wt%, such as from 70 to 95 wt%, such as from 80 to 95 wt%, such as from 83 to 94 wt%, such as from 85 to 93 wt%, such as from 87 to 92 wt%, such as from 89 to 91 wt%.

[0069] Reactive diluent monomer

[0070] A reactive diluent monomer of the multi-part UV-curable coating composition may include any reactive diluent monomer known in the art, and can be 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 typically forms one of two components of the vinyl ester resin composition, the other being the vinyl ester oligomer, as described herein.

[0071] A reactive diluent monomer may have a different number of functionalities, such as a monofunctional reactive diluent monomer, a difunctional reactive diluent monomer or a trifunctional reactive diluent monomer.

[0072] 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 “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 vinyl functional group or a (meth)acrylate functional group.

[0073] 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. In some embodiments, the reactive diluent monomer of the present invention comprises one or more polyfunctional reactive diluent monomers.

[0074] 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.

[0075] Suitably, the polyfunctional reactive diluent monomer may be a difunctional reactive diluent monomer or a trifunctional reactive diluent monomer.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] In some embodiments, the one or more trifunctional reactive diluent monomers is present, based on the total weight of the multi-part UV-curable coating 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%.

[0081] 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%.

[0082] In some embodiments, the one or more difunctional reactive diluent monomers is present, based on the total weight of the multi-part UV-curable coating 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%.

[0083] 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%. In some embodiments, the reactive diluent monomer further comprises one or more monofunctional reactive diluent monomers. In some embodiments, the one or more monofunctional reactive diluent monomers is present, based on the total weight of the multi-part UV-curable coating composition, in an amount from 14 to 26 wt%, such as from 16 to 24 wt%, such as from 18 to 22 wt%. 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% .

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] The reactive diluent monomer 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.

[0089] In some preferred embodiments, the styrene monomer is present in an amount, based on the total weight of the multi-part UV-curable coating 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.

[0090] In some preferred embodiments, the multi-part UV-curable coating composition is substantially free of styrene. As used herein, the term “substantially free of styrene” means that the multi-part UV-curable coating compositions are formulated without the inclusion of any styrene monomer. For example, the multi-part UV-curable coating 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.

[0091] In some embodiments, the reactive diluent monomer 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%.

[0092] In some embodiments, the reactive diluent monomer 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%.

[0093] In some embodiments, the reactive diluent monomer 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%.

[0094] The reactive diluent monomer mixture of the present invention is comprised in the first part (A) and the second part (B) of the multi-part UV-curable coating composition. The amount of the reactive diluent monomer in the first part (A) may be independent of the amount of the reactive diluent monomer in the second part (B).

[0095] In some embodiments, the reactive diluent monomer is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating 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 32 wt%, such as at least 34 wt%, such as at least 36 wt%, such as at least 38 wt%, such as 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% .

[0096] In some embodiments, the reactive diluent monomer is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of at most 80 wt%, such as at most 75 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%, 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%.

[0097] In some embodiments, the reactive diluent monomer is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 20 to 80 wt%, such as from 25 to 75 wt%, such as from 30 to 70 wt%, such as from 32 to 68 wt%, such as from 34 to 66 wt%, such as from 36 to 64 wt%, such as from 38 to 62 wt%, such as from 40 to 60 wt%, such as from 42 to 58 wt %, such as from 44 to 56 wt%, such as from 46 to 54 wt%, such as from 48 to 52 wt%, such as from 48 to 50 wt% . In some embodiments, the reactive diluent monomer is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of at least 25 wt%, such as at least 30 wt%, such as at least 31 wt%, such as at least 33 wt%, such as at least 35 wt%, such as at least 37 wt%, such as at least 39 wt%, such as 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%.

[0098] In some embodiments, the reactive diluent monomer is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of at most 85 wt%, such as at most 80 wt%, such as at most 75 wt%, such as at 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%, such as at most 63 wt%, such as at most 61 wt%, such as at most 59 wt%, such as at most 57 wt%, such as at most 53 wt%.

[0099] In some embodiments, the reactive diluent monomer is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 25 to 85 wt%, such as from 30 to 80 wt%, such as from 31 to 75 wt%, such as from 33 to 71 wt%, such as from 35 to 69 wt%, such as from 37 to 67 wt%, such as from 39 to 65 wt%, such as from 41 to 63 wt%, such as from 43 to 61 wt %, such as from 45 to 59 wt%, such as from 47 to 57 wt%, such as from 49 to 55 wt%, such as from 51 to 53 wt%.

[0100] The vinyl ester resin composition of the present invention is comprised in the first part (A) and the second part (B) of the multi-part UV-curable coating composition, and comprises the vinyl ester oligomer and the reactive diluent monomer, as described herein. The amount of the vinyl ester resin composition in the first part (A) may be independent of the amount of the vinyl ester resin composition in the second part (B).

[0101] In some embodiments, the vinyl ester resin composition is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 70 to 94 wt%, such as from 74 to 92 wt%, such as from 76 to 90 wt%, such as from 78 to 88 wt%, such from 80 to 86 wt%, such as from 82 to 85 wt%.

[0102] In some embodiments, the vinyl ester resin composition is present in the second part (B) of the multi-part UV-curable coating composition, in an amount from 60 to 95 wt%, such as from 70 to 95 wt%, such as from 80 to 95 wt%, such as from 83 to 94 wt%, such as from 85 to 93 wt%, such as from 87 to 92 wt%, such as from 89 to 91 wt%.

[0103] A pigment is a substance that imparts a particular colour onto a substrate. Typically, pigments work by selectively absorbing wavelengths of visible light, such that certain wavelengths are absorbed while other wavelengths are allowed to pass through or be reflected. The colour of the reflected visible light is the observed colour, and thereby the colour imparted onto a substrate. The pigment of the present invention is comprised in the first part (A) of the multi-part UV-curable coating composition, wherein the first part (A) is for providing a pigmented layer (A’).

[0104] Suitably, the pigment of the present invention is a pigment which is capable of permitting light, such as UV light, to pass through.

[0105] Preferably, the pigment of the first part (A) has a refractive index (Rf) of 2.40 or less, as measured in accordance with ASTM D542. The refractive index of a pigment refers to the extent to which light, such as UV light, is bent or refracted, as it passes through the pigment.

[0106] In some embodiments, the pigment has a refractive index (Rf) of 2.35 or less, such as 2.40 or less, such as 2.25 or less, such as 2.20 or less, such as 2.15 or less, such as 2.10 or less, as measured in accordance with ASTM D542. Without wishing to be bound by theory, it is believed that pigments having lower refractive indices causes decreased light scattering, thereby resulting in the pigmented layer (A) to have decreased opacity and increased translucency. This in turn increases the amount of UV light which can penetrate through the coating, thereby increasing the capability for UV curing.

[0107] The pigment may be selected based on the desired colour which is imparted onto a substrate. The pigment may be a white pigment, a yellow pigment, an orange pigment or a black pigment, or a combination thereof. For example, the pigment, or the combination of pigments, may achieve the colour of white or light grey, such as RAL 7035. In some embodiments, the white pigment is present in an amount which is greater than the other coloured pigments, where present.

[0108] Typically, pigments may be inorganic pigments or organic pigments. In some embodiments, the pigment is an inorganic pigment.

[0109] The inorganic pigment may be a metal oxide pigment or a metal sulfide pigment. Suitable pigments may include, for example, a zinc sulfide pigment.

[0110] In some embodiments, the pigment is a pigment which has undergone an organic (after)treatment.

[0111] In some embodiments, the pigment is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of at least 3.0 wt%, such as at least 3.5 wt%, such as at least 4.0 wt%, such as at least 4.5 wt%, 5.0 wt%, such as at least 5.5 wt%, such as at least 6.0 wt%, such as at least 6.5 wt%, such as at least 7.0 wt%, such as at least 7.5 wt%, such as at least 8.0 wt%, such as at least 8.5 wt%, such as at least 9.0 wt%.

[0112] In some embodiments, the pigment is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of at most 17.0 wt%, such as at most 16.5 wt%, such as at most 16.0 wt%, such as at most 15.5 wt%, such as at most 15.0 wt%, such as at most 14.5 wt%, such as at most 14.0 wt%, such as at most 13.5 wt%, such as at most 13.0 wt%, such as at most 12.5 wt%, such as at most 12.0 wt%, such as at most 11 .5 wt%, such as at most 11 .0 wt%.

[0113] In some embodiments, the pigment is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of from 3.0 to 17.0 wt%, such as from 3.5 to 16.5 wt%, such as from 4.0 to 16.0 wt%, such as from 4.5 to 15.5 wt%, such as from 5.0 to 15.0 wt%, such as from 5.5 to 14.5 wt%, such as from 6.0 to 14.0 wt%, such as from 6.5 to 13.5 wt%, such as from 7.0 to 13.0 wt%, such as from 7.5 to 12.5 wt%, such as from 8.0 to 12.0 wt%, such as from 8.5 to 11 .5 wt%, such as from 9.0 to 11 .0 wt%.

[0114] In some embodiments, the pigment is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 3.0 to 17.0 wt%, such as from 5.0 to 15.0 wt%, such as from 7.0 to 13.0 wt%.

[0115] Photoinitiator

[0116] 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.

[0117] The multi-part UV-curable coating composition comprises, in the first part (A) for providing a pigmented layer (A’) when cured and in the second part (B) for providing a UV-resistant layer (B’) when cured, a photoinitiator.

[0118] The photoinitiator of the first part (A) and of the second part (B) is for initiating a free radical polymerisation reaction in the multi-part UV-curable coating composition. The photoinitiator is also employed for preventing oxygen inhibition and for reducing surface tackiness.

[0119] 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.

[0120] In some embodiments, the photoinitiator is a free-radical photoinitiator. In some embodiments, the free- radical 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.

[0121] In some embodiments, the free-radical 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'- mecthyl 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 (l-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.

[0122] Further free-radical 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 -y l)pheny I] titanium (Irgacure 784 from Ciba), or a mixture thereof.

[0123] 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.

[0124] In some embodiments, the photoinitiator is Irgacure 1173 or Omnirad 1173 (methyl benzoylformate) obtained from IGM Resins, Omnirad 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide) obtained from IGM Resins, or Speedcure MBF (methyl benzoylformate) obtained from Sartomer, or a combination thereof.

[0125] In some embodiments, the photoinitiator is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of at least 0.5 wt%, such as at least 1 .0 wt%, such as at least 1 .5 wt%, such as at least 2.0 wt%, such as at least 2.5 wt%. In such embodiments, the wax may be absent.

[0126] In some embodiments, the photoinitiator is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of at most 6.0 wt%, such as at most 5.0 wt%, such as at most 4.5 wt%, such as at most 4.0 wt%, such as at most 3.5 wt%, such as at most 3.0 wt%. In such embodiments, the wax may be absent.

[0127] In some embodiments, the photoinitiator is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 0.5 to 6.0 wt%, such as from 1 .0 to 5.0 wt%, such as from 1 .5 to 4.5 wt%, such as from 2.0 to 4.0 wt%, such as from 2.5 to 3.5 wt%, such as from 2.5 to 3.0 wt%. In such embodiments, the wax may be absent. In some embodiments, the photoinitiator is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating 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%. In such embodiments, the wax as described herein may be present.

[0128] In some embodiments, the photoinitiator is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount of at most 1.0 wt%, such as at most 0.9 wt%, such as at most 0.8 wt%, such as at most 0.7 wt%, such as at most 0.6 wt%. In such embodiments, the wax as described herein may be present.

[0129] In some embodiments, the photoinitiator is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 0.01 to 1 .0 wt%, such as from 0.1 to 0.8 wt%, such as from 0.5 wt% to 0.6 wt%. In such embodiments, the wax as described herein may be present.

[0130] In some embodiments, the photoinitiator is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of less than 1 .0 wt%, such as less than 0.5 wt%.

[0131] Wax

[0132] A wax may be employed for preventing oxygen inhibition and for reducing surface tackiness.

[0133] In preferred embodiments, the multi-part UV-curable coating composition comprises, in the first part (A) for providing a pigmented layer (A’), a wax.

[0134] Examples of types of waxes include paraffin waxes, polyethylene waxes and silicone-based waxes.

[0135] Examples of the wax are BYK-S 782 and BYK-S 780 from BYK®.

[0136] In some embodiments, the wax is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating 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%, such as at least 0.5 wt%.

[0137] In some embodiments, the wax is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount at most 2.0 wt%, such as at most 1 .8 wt%, such as at most 1 .6 wt%, such as at most 1 .4 wt%, such as at most 1 .2 wt%, such as at most 1 .0 wt%, such as at most 0.8 wt%, such as at most 0.6 wt%.

[0138] In some embodiments, the wax is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 0.1 to 2.0 wt%, such as from 0.5 to 0.6 wt%. UV absorber

[0139] A UV absorber is a substance typically employed to prevent the passage of damaging UV radiation through a material. A UV absorber works by converting UV light, having a wavelength of, for example, 365 nm, which may be considered more damaging to materials due to the formation of free radicals upon exposure, into infrared radiation as its mechanism of action. Typically, UV light is preferentially absorbed by UV light over unsaturated groups within a material.

[0140] The UV absorber of the present invention is comprised in the second part (B) of the multi-part UV-curable coating composition, wherein the second part (B) is for providing a UV-resistant layer (B’).

[0141] In some embodiments, the UV absorber is an organic UV absorber.

[0142] Examples of types of UV absorbers include a benzotriazole-based UV absorber, a benzophenonebased UV absorber, a triazine-based UV absorber, a salicylate-based UV absorber, and a cyanoacrylate- based UV absorber. Of these, particularly preferred UV absorbers are benzotriazole- based UV absorbers, benzophenone-based UV absorbers and triazine-based UV absorbers.

[0143] Examples of benzotriazole-based UV absorbers include the TINUVIN® series of UV absorbers from BASF: Tinuvin P, Tinuvin PS, Tinuvin 109, Tinuvin 99-2, Tinuvin 213, Tinuvin 234 FF, Tinuvin 234 P, Tinuvin 326 FL, Tinuvin 326 P, Tinuvin 360, Tinuvin 384-2, Tinuvin 479 and Tinuvin 571 ; the POWERSORB® series of UV absorbers from TinToll (SINOPCC Group): Powersorb 1000, Powersorb 1130, Powersorb 234, Powersorb 320, Powersorb 326, Powersorb 327, Powersorb 328, Powersorb 329, Powersorb 360, Powersorb 571 and Powersorb 928; and the CHISORB® series from Double Bond Chemical: Chisorb 2260, Chisorb 234, Chisorb 325, Chisorb 327, Chisorb 5431 , Chisorb 5530, Chisorb 5582 and Chisorb P.

[0144] Examples of benzophenone-based UV absorbers include the ADK STAB® series of UV absorbers from ADEKA Corporation: ADK STAB 1413 and ADK STAB LA-51 ; the Chiguard® series of UV absorbers from Chitec Technology: Chiguard BP-1 , Chiguard BP-2, Chiguard BP-3, Chiguard BP-6, Chiguard BP-12 and Chiguard BP-2UV-81 ; and the Chimassorb® series of UV absorbers from BASFK: Chimassorb 81 FL and Chimassorb 81 P.

[0145] Examples of the triazine-based UV absorbers include the TINUVIN® series of UV absorbers from BASF include Tinuvin 400, Tinuvin 405, Tinuvin 460, Tinuvin 477, Tinuvin 479 and Tinuvin 1577; and the the Hostavin® series of UV absorbers from Clariant: Hostavin 3400 LIQ, Hostavin 3070 P and Hostavin 3315 DISP.

[0146] Examples of salicylic acid-based UV absorbers include methyl salicylate, butyl salicylate, octyl salicylate and phenyl salicylate.

[0147] Examples of cyanoacrylate-based UV absorbers include the UVINUL® series of UV absorbers from BASF: Uvinul 3035, Uvinul 3039, and Uvinul 3030.

[0148] In some embodiments, the UV absorber is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating 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%, such as at least 1 .4 wt%, such as at least 1 .6 wt%, such as at least 1 .8 wt%.

[0149] In some embodiments, the UV absorber is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of at most 5.0 wt%, such as at most 4.5 wt%, such as at most 4.0 wt%, such as at most 3.5 wt%, such as at most 3.0 wt%, such as at most 2.5 wt%, such as at most 2.0 wt%.

[0150] In some embodiments, the UV absorber is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 0.2 to 5.0 wt%, such as from 0.4 to 5.0 wt%, such as from 0.6 to 5.0 wt%, such as from 0.8 to 4.5 wt%, such as from 1 .0 to 4.0 wt%, such as from 1 .2 to 3.5 wt%, such as from 1 .4 to 3.0 wt%, such as from 1 .6 to 2.5 wt%, such as from 1 .8 to 2.0 wt%.

[0151] In some embodiments, the UV absorber is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%.

[0152] Free radical scavenger

[0153] The free radical scavenger is typically a species which either prevents free radicals from being formed, or removes them before they can propagate undesirable side reactions. Free radicals are typically generated when a material, such as a polymer, is exposed to UV radiation. Rather than directly absorbing UV radiation, the free radicals are neutralised by free radical scavengers to prevent them from reacting with the polymer chains, thereby inhibiting degradation via photo-oxidation. Thus, free radical scavengers have a different mechanism of action to UV absorbers.

[0154] The free radical scavenger of the present invention is comprised in the second part (B) of the multi-part UV-curable coating composition, wherein the second part (B) is for providing a UV-resistant layer (B’).

[0155] In some embodiments, the free radical scavenger is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of at least 2.0 wt%, such as at least 3.0 wt%, such as at least 3.5 wt%, such as at least 3.8 wt% such as at least 4.0 wt%.

[0156] In some embodiments, the free radical scavenger is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of at most 10 wt%, such as at most 9.0 wt%, such as at most 8.0 wt%, such as at most 5.0 wt%, such as at most 4.4 wt%.

[0157] In some embodiments, the free radical scavenger is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 2.0 to 10 wt%, such as from 2.0 to 9.0 wt%, such as from 3.0 to 8.0 wt%, such as from 3.5 to 5.0 wt%, such as from 3.8 to 4.6 wt% such as from 4.0 to 4.4 wt%. Two examples of free radical scavengers are hindered amine light stabilisers and anti-oxidants, as described herein. Therefore, in preferred embodiments, the free radical scavenger is a hindered amine light stabiliser or an anti-oxidant, or a combination thereof.

[0158] Hindered amine light stabiliser

[0159] A hindered amine light stabiliser is typically employed as a free radical scavenger to prevent the effects of UV exposure. A hindered amine light stabiliser works by scavenging free radicals which are generated when a material, such as a polymer, is exposed to UV radiation, rather than directly absorbing UV radiation. The free radicals are neutralised by hindered amine light stabilisers to prevent them from reacting with the polymer chains, thereby inhibiting degradation via photo-oxidation. Thus, hindered amine light stabilisers have a different mechanism of action to UV absorbers. Hindered amine light stabilisers are characterised by bulky, sterically hindering groups surrounding an amine functional group on the compound, such as wherein the carbon atoms adjacent to the nitrogen atom of the amine group do not have a hydrogen atom attached directly thereto.

[0160] The hindered amine light stabiliser of the present invention is comprised in the second part (B) of the multi-part UV-curable coating composition, wherein the second part (B) is for providing a UV-resistant layer (B’).

[0161] Examples of hindered amine light stabilisers include the TINUVIN® series of hindered amine light stabilisers from BASF: Tinuvin XT 55, Tinuvin NOR 356, Tinuvin NOR 371 , Tinuvin 111 , Tinuvin 123, Tinuvin 494 AR, Tinuvin 622 SF, Tinuvin 765, Tinuvin 770 DF, Tinuvin 783 FDL, Tinuvin 791 FB, Tinuvin PA 123, Tinuvin PA 144, Tinuvin XT 100 FF, Tinuvin 200 FF, Tinuvin XT 835 FF, Tinuvin XT 847 FF, Tinuvin XT 850 FF and Tinuvin 855 FF; and the UVASORB® series of hindered amine light stabiliser from 3V Sigma: Uvasorb HA22, Uvasorb HA29, Uvasorb HA44, Uvasorb HA77 and Uvasorb HA700.

[0162] In some embodiments, the hindered amine light stabiliser is present in the second part (B), based on the total weight of the second part (A) of the multi-part UV-curable coating composition, in an amount of at least 2.0 wt%, such as at least 2.2 wt%, such as at least 2.4 wt%, such as at least 2.6 wt%, such as at least 2.8 wt%, such as at least 3.0 wt%, such as at least 3.2 wt%, such as at least 3.4 wt%, such as at least 3.6 wt%, such as at least 3.8 wt%.

[0163] In some embodiments, the hindered amine light stabiliser is present in the second part (B), based on the total weight of the second part (A) of the multi-part UV-curable coating composition, in an amount of at most 10.0 wt%, such as at most 9.0 wt%, such as at most 8.0 wt%, such as at most 7.8 wt%, such as at most 7.6 wt%, such as at most 7.4 wt%, such as at most 7.2 wt%, such as at most 7.0 wt%, such as at most 6.8 wt%, such as at most 6.6 wt%, such as at most 6.4 wt%, such as at most 6.2 wt%, such as at most 6.0 wt%, such as at most 5.8 wt%, such as at most 5.6 wt%, at most 5.4 wt%, such as at most 5.2 wt%, such as at most 5.0 wt%, such as at most 4.8 wt%, such as at most 4.6 wt%, such as at most 4.4 wt%, such as at most 4.2 wt%. In some embodiments, the hindered amine light stabiliser is present in the second part (B), based on the total weight of the second part (A) of the multi-part UV-curable coating composition, in an amount from 2.0 to 6.0 wt%, such as from 2.2 to 5.8 wt%, such as from 2.4 to 5.6 wt%, such as from 2.6 to 5.4 wt%, such as from 2.8 to 5.2 wt%, such as from 3.0 to 5.0 wt%, such as from 3.2 to 4.8 wt%, such as from 3.4 to 4.6 wt%, such as from 3.6 to 4.4 wt%, such as from 3.8 to 4.2 wt%.

[0164] In some embodiments, the hindered amine light stabiliser is present in the second part (B), based on the total weight of the second part (A) of the multi-part UV-curable coating composition, in an amount from 2.0 to 10.0 wt%, such as from 3.0 to 8.0 wt%, such as from 3.5 to 5.0 wt%.

[0165] Anti-oxidant

[0166] An anti-oxidant is typically a type of free radical scavenger for protecting against thermal degradation and oxidation, thereby to extend the UV stability of materials. Typically, the anti-oxidant is a sterically- hindered species and acts to slow down the propagation of free radical processes.

[0167] The anti-oxidant of the present invention is comprised in the second part (B) of the multi-part UV-curable coating composition, wherein the second part (B) is for providing a UV-resistant layer (B’).

[0168] Anti-oxidants may include amino anti-oxidants and phenolic anti-oxidants. In some embodiments, the anti-oxidant is a phenolic anti-oxidant.

[0169] Examples of anti-oxidants include the Irganox® series of anti-oxidants from BASF: Irganox, Irganox 1010, Irganox 1010 BMBcert, Irganox 1010 ED, Irganox 1035, Irganox 1035 FF, Irganox 1076, Irganox 1076 FD, Irganox 1076 FD BMBcert, Irganox 1076 Melt, Irganox 1098, Irganox 1098 ED, Irganox 1135, Irganox 1141 and Irganox 1150; the Alvinox® series of anti-oxidants from 3V Sigma: Alvinox P, Alvinox FB, Alvinox 100, Alvinox 110 and Alvinox 176.

[0170] In some embodiments, the anti-oxidant is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating 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%, such as at least 0.06 wt%, such as at least 0.07 wt%, such as at least 0.08 wt%, such as at least 0.09 wt%.

[0171] In some embodiments, the anti-oxidant is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of such as at most 0.40 wt%, such as at most 0.35 wt%, such as at most 0.30 wt%, such as at most 0.25 wt%, such as at most 0.20 wt%, such as at most 0.19 wt%, such as at most 0.18 wt%, such as at most 0.17 wt%, such as at most 0.16 wt%, such as at most 0.15 wt%, such as at most 0.14 wt%, such as at most 0.13 wt%, such as at most 0.12 wt%, such as at most 0.11 wt%.

[0172] In some embodiments, the anti-oxidant is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 0.01 to 0.40 wt%, such as from 0.01 to 0.35 wt%, such as from 0.01 to 0.30 wt%, such as from 0.02 to 0.25 wt%, such as from 0.02 to 0.20 wt%, such as from 0.02 to 0.19 wt%, such as from 0.02 to 0.18 wt%, such as from 0.03 to 0.17 wt%, such as from 0.04 to 0.16 wt%, such as from 0.05 to 0.15 wt%, such as from 0.06 to 0.14 wt%, such as from 0.07 to 0.13 wt%, such as from 0.08 to 0.12 wt%, such as from 0.09 to 0.11 wt%.

[0173] In some embodiments, the anti-oxidant is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 0.01 to 0.40 wt%, such as from 0.05 to 0.20 wt%, such as from 0.05 to 0.15 wt%.

[0174] Further components

[0175] The multi-part UV-curable coating composition may further comprise air release agents, wetting agents, inhibitors, stabilisers, optical brighteners, and the like. Further possible components may also include acid scavengers, thickeners, flame retardants, resin particles, soluble polymers and block polymers.

[0176] The multi-part UV-curable coating composition may further comprise air release agents. Air release agents may suitably be used to prevent or destroy air bubbles which may be trapped during the application and curing processes.

[0177] 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 A501 , and BYK A555 from BYK®, and CLiQSart AP, CliQSmart PD-30, CliQSmart PD- 21 and CliQSmart AP20 from CliQ®.

[0178] In some embodiments, the air release agent is present, based on the total weight of the multi-part UV- curable coating 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%.

[0179] In some embodiments, the air release agent is present, based on the total weight of the multi-part UV- curable coating composition, in an amount of at most 5.0 wt%, such as at most 4.5 wt%, such as at most 4.0 wt%, such as at most 3.5 wt%, such as at most 3.0 wt%, such as at most 2.5 wt%, such as at most 2.0 wt%.

[0180] In some embodiments, the air release agent is present, based on the total weight of the multi-part UV- curable coating composition, in an amount from such as 0.1 to 5.0 wt%., such as 0.4 to 4.0 wt%, such as 0.6 to 3.0 wt%, 0.8 to 2.5 wt%, such as 1 .0 to 2.0 wt%.

[0181] The multi-part UV-curable coating composition may further comprise wetting agents. Wetting agents may be used to stabilise and encapsulate any filler in the multi-part UV-curable coating composition, and also to function as a viscosity suppressant.

[0182] 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; 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 multi-part UV- curable coating composition, in an amount of at least 0.01 wt%, such as at least 0.05 wt%, such as at least 0.1 wt%, such as at least 0.2 wt%.

[0183] In some embodiments, the wetting agent is present, based on the total weight of the multi-part UV- curable coating composition, in an amount of 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%.

[0184] In some embodiments, the wetting agent is present, based on the total weight of the multi-part UV- curable coating composition, in an amount from such as 0.01 to 1 .0 wt%, such as from 0.05 to 0.5 wt%, such as from 0.1 to 0.4 wt%, such as from 0.2 to 0.3 wt%.

[0185] The multi-part UV-curable coating 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.

[0186] In addition to inhibitors that may be added directly to the multi-part UV-curable coating composition during its preparation, some of the raw materials employed, especially the a,£-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.

[0187] The inhibitor may be present, based on the total weight of the multi-part UV-curable coating 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%.

[0188] Where the multi-part UV-curable coating composition contains inhibitors and stabilisers, the multi-part UV-curable coating composition may further contain diluents for said inhibitors and stabilisers.

[0189] The multi-part UV-curable coating composition may further comprise one or more optical brightener(s). In some embodiments, the multi-part UV-curable coating composition comprises one or more pigment(s) and one or more optical brightener(s).

[0190] In some embodiments, the optical brightener is a fluorescent brightener. Suitable examples of such brighteners include, but are not limited to, benzoxazole derivatives, bis-benzoxazoles, bis-benzoxazolyl- stilbenes, bis-benzoxazolyl-thiophenes, thiophenediyl benzoxazoles, 2,5-thiophenediylbis-(5-tert-butyl- 1 ,3-benzoxazoles). In some embodiments, the multi-part UV-curable coating composition comprises the optical brightener in an amount of up to 3.0 wt%, or up to 2.5 wt%, or up to 2.0 wt%, or up to 1 .5 wt%, or up to 1 .2 wt%, or up to 1 .0 wt%, or up to 0.7 wt%, or up to 0.5 wt%, or up to 0.3 wt%, or up to 0.1 wt%, based on the total weight of the multi-part UV-curable coating composition.

[0191] In some embodiments, the multi-part UV-curable coating composition comprises the optical brightener in an amount of at least 0.001 wt%, or at least 0.01 wt%, or at least 0.1 wt%, or at least 0.3 wt%, or at least 0.5 wt%, or at least 0.7 wt%, or at least 1 .0 wt%, based on the total weight of the multi-part UV-curable coating composition.

[0192] Methods of curina

[0193] The invention also relates to methods of curing the multi-part UV-curable coating composition of the invention.

[0194] Thus, in a second aspect of the invention, there is provided a method of curing the multi-part UV-curable coating composition of the first aspect, the method comprising the steps of:

[0195] (i) applying the first part (A) onto a substrate;

[0196] (ii) exposing the first part (A) to UV light at a wavelength from 200 to 405 nm to initiate a cure via a free radical polymerisation, and obtaining a pigmented layer (A’);

[0197] (iii) applying the second part (B) onto the pigmented layer (A’); and

[0198] (iv) exposing the second part (B) to UV light at a wavelength from 200 to 405 nm to initiate a cure via a free radical polymerisation, and obtaining a UV-resistant layer (B’) (B’).

[0199] In some embodiments, the substrate is a composite material. In some embodiments, the composite material is a composite laminate material. In some embodiments, the composite material or composite laminate material comprises a glass-fibre material, such as an epoxy-glass-fibre material or a polyester- glass-fibre material.

[0200] The multi-part UV-curable coating composition is cured at a wavelength of from 200 to 405 nm. Suitably, the multi-part UV-curable coating 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.

[0201] 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.

[0202] In some embodiments, the UV light is at a wavelength from 390 to 405 nm. In some embodiments, the UV light is at a wavelength of 395 nm.

[0203] 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.

[0204] 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 cm-2, such as at most 1 .6 W cm2, such as at most 1 .4 W cm'2, such as at most 1 .2 W cm2, such as at most 1 .0 W cm2.

[0205] 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

[0206] 1 .4 W cm2, such from 0.7 to 1 .2 W cm'2, such from 0.8 to 1 .2 W cm2, such as from 0.8 to 1 .0 W cm-2.

[0207] In some embodiments, the multi-part UV-curable coating composition is cured by a mercury lamp, such as a low-pressure mercury lamp, a medium-pressure mercury lamp or a high-pressure mercury lamp.

[0208] In some embodiments, the duration of step (ii) is 8 minutes or less, such as 7 minutes or less, such as 6 minutes or less, such as 5 minutes or less, such as 4 minutes or less, such as 3 minutes or less.

[0209] In some embodiments, the duration of step (ii) is 0.5 minutes or more, such as 1 minute or more, such as

[0210] 1 .5 minutes or more, such as 2 minutes or more.

[0211] In some embodiments, the duration of step (ii) is from 0.5 to 8 minutes, such as from 0.5 to 7 minutes such as from 1 to 6 minutes, such as from 1 .5 to 7 minutes, such as from 2 to 3 minutes.

[0212] In some embodiments, the duration of step (ii) is 2 minutes or less.

[0213] In some embodiments, the duration of step (iv) is 8 minutes or less, such as 7 minutes or less, such as 6 minutes or less, such as 5 minutes or less, such as 4 minutes or less, such as 3 minutes or less.

[0214] In some embodiments, the duration of step (iv) is 0.5 minutes or more, such as 1 minute or more, such as

[0215] 1 .5 minutes or more, such as 2 minutes or more.

[0216] In some embodiments, the duration of step (iv) is from 0.5 to 8 minutes, such as from 0.5 to 7 minutes such as from 1 to 6 minutes, such as from 1 .5 to 7 minutes, such as from 2 to 3 minutes.

[0217] In some embodiments, the duration of step (iv) is 2 minutes or less.

[0218] The invention also relates to cured products of the multi-part UV-curable coating composition of the first aspect, as described below.

[0219] The third aspect of the invention relates to the use of the multi-part UV-curable coating composition of the first aspect in a composite repair process.

[0220] In this aspect, there may be provided an application of a repair patch, comprising the multi-part UV- curable coating 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 multi-part UV-curable coating 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.

[0221] In some embodiments, the multi-part UV-curable coating composition of the first aspect may be used as an in-mould gelcoat composition. In such embodiments, the multi-part UV-curable coating composition is applied to a mould before it is then applied to a substrate. In some embodiments, the multi-part UV- curable coating composition of the first aspect may be used as a topcoat composition. In such embodiments, the multi-part UV-curable coating composition is applied to a substrate.

[0222] The multi-part UV-curable coating composition is cured by UV light. In some embodiments, the multi-part UV-curable coating composition is cured according to the method of curing of the second aspect.

[0223] In a fourth aspect of the invention, there is provided a cured product of the multi-part UV-curable coating composition of the first aspect. The cured product is obtained from the method of curing of the second aspect.

[0224] Thus, the cured product comprises a pigmented layer (A ) corresponding to the first part (A) of the multipart UV-curable coating composition, and a UV-resistant layer (B’) corresponding to the second part (B) of the multi-part UV-curable coating composition, such that in use the pigmented layer (A’) is positioned between a substrate and the UV-resistant layer (B’). The pigmented layer (A’) is intended to impart an aesthetic colour onto the cured product, based on the specific combination of pigments present. The UV- resistant layer (B’) is intended to provide excellent weathering performance to the cured product, based on the combination within the UV-resistant mixture.

[0225] In some embodiments, the thickness of the pigmented layer (A’) and the UV-resistant layer (B’) is each independently from 200 to 600 mm, such as from 200 to 500 nm, such as from 200 to 400 nm. For example, the thickness of the pigmented layer (A’) and the UV-resistant layer (B’) is each 400 mm to correspond with typical wind turbine repair processes.

[0226] The cured product may also have weathering and mechanical properties as described further below.

[0227] The multi-part UV-curable coating 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.

[0228] The multi-part UV-curable coating composition allows for a faster and more complete cure over other curing systems, such as cobalt accelerators and initiators. The multi-part UV-curable coating 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 not required. 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 turbine part may be performed as a post-production application of a wind turbine part or as an in-field application.

[0229] In a fifth 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.

[0230] In a sixth aspect, there is provided a multi-part repair system comprising a plurality of parts, which in combination contains the multi-part UV-curable coating composition of the first aspect, together with a UV-curable filler composition, wherein the UV-curable filler composition comprises a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer.

[0231] Mechanical and adhesion properties

[0232] The following properties relate to cured products of the multi-part UV-curable coating composition. For example, the UV-curable filler resin composition may be cured by the methods described herein. The sample preparation and testing methodology are discussed further below.

[0233] Typically, when cured, the multi-part UV-curable coating compositions according to the first aspect have the following weathering properties: a dE of less than 5.0, such as less than 4.8, such as less than 4.6, such as less than 4.4, such as less than 4.2, such as less than 4.0, wherein the dE is measured using QUV accelerated weathering tests at 1000 hours in accordance with the method provided in the methods section. a dE of less than 5.0, such as less than 4.8, such as less than 4.6, such as less than 4.4, such as less than 4.2, such as less than 4.0, wherein the dE is measured using Xenon accelerated weathering tests at 1000 hours in accordance with the method provided in the methods section.

[0234] Typically, when cured, the multi-part UV-curable coating compositions according to the first aspect have the following mechanical properties: an adhesion pull-off strength of the pigmented layer (A’) on an epoxy-glass fibre laminate of 4.0 MPa or more, such as 4.1 MPa or more, such as 4.2 MPa or more, such as 4.3 MPa or more, such as 4.4 MPa or more, wherein the thickness of the pigmented layer (A’) is 400 pm, and the adhesion pull-off strength is measured in accordance with the method provided in the methods section. an adhesion pull-off strength of the pigmented layer (A’) on a polyester-glass fibre laminate of 2.4 MPa or more, such as 2.5 MPa or more, such as 2.6 MPa or more, such as 2.7 MPa or more, such as 2.8 MPa or more, wherein the thickness of the pigmented layer (A’) is 400 pm, and the adhesion pull-off strength is measured in accordance with the method provided in the methods section. Further embodiments

[0235] In further embodiments, a multi-part UV-curable coating composition according to the first aspect may comprise:

[0236] (A) a first part for providing a pigmented layer (A’) when cured comprising:

[0237] (i) a pigment; and

[0238] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0239] (i) a UV-resistant mixture comprising a UV absorber and a free radical scavenger; wherein the UV absorber is present, based on the total weight of the second part (B), in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%. wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0240] In further embodiments, a multi-part UV-curable coating composition according to the first aspect may comprise:

[0241] (A) a first part for providing a pigmented layer (A’) when cured comprising:

[0242] (i) a pigment; and

[0243] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0244] (i) a UV-resistant mixture comprising a UV absorber, a hindered amine light stabiliser, and an anti-oxidant; wherein the UV absorber is present, based on the total weight of the second part (B), in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%. wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0245] In further embodiments, a multi-part UV-curable coating composition according to the first aspect may comprise:

[0246] (A) a first part for providing a pigmented layer (A’) when cured comprising:

[0247] (i) a pigment having a refractive index (Rf) of 2.40 or less, as measured in accordance with ASTM D542; and

[0248] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0249] (i) a UV-resistant mixture comprising a UV absorber, a hindered amine light stabiliser, and an anti-oxidant; wherein the UV absorber is present, based on the total weight of the second part (B), in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%. wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0250] In further embodiments, a multi-part UV-curable coating composition according to the first aspect may comprise:

[0251] (A) a first part for providing a pigmented layer (A’) when cured comprising:

[0252] (i) a pigment having a refractive index (Rf) of 2.40 or less, such as 2.35 or less, such as 2.30 or less, as measured in accordance with ASTM D542; and

[0253] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0254] (i) a UV-resistant mixture comprising a UV absorber, a hindered amine light stabiliser, and an anti-oxidant; wherein the UV absorber is present, based on the total weight of the second part (B), in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%. wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0255] In further embodiments, a multi-part UV-curable coating composition according to the first aspect may comprise:

[0256] (A) a first part for providing a pigmented layer (A’) when cured comprising:

[0257] (i) a pigment having a refractive index (Rf) of 2.40 or less, such as 2.35 or less, such as 2.30 or less, as measured in accordance with ASTM D542; and

[0258] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0259] (i) a UV-resistant mixture comprising a UV absorber, a hindered amine light stabiliser, and an anti-oxidant; wherein the UV absorber is present, based on the total weight of the second part (B), in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%, and wherein the hindered amine light stabiliser is present, based on the total weight of the second part (B) in an amount from 2.0 to 10.0 wt%, such as from 3.0 to 8.0 wt%, such as from 3.5 to 5.0 wt%. wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0260] In further embodiments, a multi-part UV-curable coating composition according to the first aspect may comprise:

[0261] (A) a first part for providing a pigmented layer (A’) when cured comprising: (i) a pigment having a refractive index (Rf) of 2.40 or less, such as 2.35 or less, such as 2.30 or less, as measured in accordance with ASTM D542; and

[0262] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0263] (i) a UV-resistant mixture comprising a UV absorber, a hindered amine light stabiliser, and an anti-oxidant; wherein the UV absorber is present, based on the total weight of the second part (B), in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%, wherein the hindered amine light stabiliser is present, based on the total weight of the second part (B) in an amount from 2.0 to 10.0 wt%, such as from 3.0 to 8.0 wt%, such as from 3.5 to 5.0 wt%, and wherein the anti-oxidant is present, based on the total weight of the second part (B), in an amount from 0.01 to 0.40 wt%, such as from 0.05 to 0.20 wt%, such as from 0.05 to 0.15 wt%.

[0264] Wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0265] In further embodiments, a multi-part UV-curable coating composition according to the first aspect may comprise:

[0266] (A) a first part for providing a pigmented layer (A’) when cured comprising:

[0267] (i) a pigment having a refractive index (Rf) of 2.40 or less, such as 2.35 or less, such as 2.30 or less, as measured in accordance with ASTM D542; wherein the pigment is present, based on the total weight of the part (A), in an amount from 3.0 to 17.0 wt%, such as from 5.0 to 15.0 wt%, such as from 7.0 to 13.0 wt%.

[0268] (B) a second part for providing a UV-resistant layer (B’) when cured comprising:

[0269] (i) a UV absorber, a hindered amine light stabiliser, and an anti-oxidant; wherein the UV absorber is present, based on the total weight of the second part (B), in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%, wherein the hindered amine light stabiliser is present, based on the total weight of the second part (B), in an amount from 2.0 to 10.0 wt%, such as from 3.0 to 8.0 wt%, such as from 3.5 to 5.0 wt%, and wherein the anti-oxidant is present, based on the total weight of the second part (B), in an amount from 0.01 to 0.40 wt%, such as from 0.05 to 0.20 wt%, such as from 0.05 to 0.15 wt%. wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

[0270] 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.

[0271] 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.

[0272] 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.

[0273] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0274] 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.

[0275] 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%.

[0276] Examples

[0277] Materials

[0278] EPIKOTE® 828 is a diglycidyl ether of bisphenol A (DGEBA) prepared from the reaction between bisphenol A and epichlorohydrin obtained from Westlake Epoxy.

[0279] Bisphenol A (BPA) was obtained from Tokyo Chemical Company.

[0280] Mono-monomer 1 (MM-1) is a mono(meth)acrylate monomer obtained from GEO Speciality Chemicals.

[0281] Di-monomer 1 (DM-1) is a di(meth)acrylate monomer obtained from Sartomer.

[0282] Tri-monomer 1 (TM-1) is a tri(meth)acrylate monomer obtained from Sartomer.

[0283] Di-monomer 2 (DM-2) is a di(meth)acrylate monomer obtained from Evonik.

[0284] Tri-monomer 2 (TM-2) is a tri(meth)acrylate monomer obtained from Arkema. Photoinitiator 1 is a phosphine oxide obtained from IGM Resins.

[0285] Photoinitiator 2 is a different phosphine oxide obtained from IGM Resins.

[0286] Photoinitiator 3 is a Norrish Type II photoinitiator obtained from Sartomer.

[0287] Wax 1 is a wax obtained from CliQ.

[0288] Pigment 1 is a white pigment obtained from Venator.

[0289] Pigment 2 is a black pigment having a refractive index (Rf) of 2.37 obtained from Scott Bader Company Ltd.

[0290] Pigment 3 is an orange / yellow pigment obtained from Scott Bader Company Ltd.

[0291] UV Absorber 1 is a benzotriazole UV absorber obtained from BASF.

[0292] HALS 1 is a hindered amine light stabiliser obtained from 3V Sigma.

[0293] Anti-oxidant 1 is a phenolic anti-oxidant obtained from 3V Sigma.

[0294] UV-curable filler composition was obtained from the combination of a vinyl ester oligomer, derived from EPIKOTE® 828, BPA and methacrylic acid, and a reactive diluent monomer mixture comprising (meth)acrylate monomer, a di(meth)acrylate monomer and a tri(meth)acrylate monomer.

[0295] Methods

[0296] Adhesion testing

[0297] Before conducting the adhesion pull-off strength tests, the laminate / substrate, the UV-curable coating composition of the first aspect, and the dolly were each prepared as follows. The surface of the laminate (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. The UV-curable coating composition of the first aspect was applied at a thickness of 1 mm, cured according to the curing method as described below, and allowed to cool to ambient temperature (20 to 30 °C). The resulting cured product was sanded with 40 grit sandpaper, dusted off, cleaned with isopropyl alcohol or acetone and dried. A minimum of four dollies, used to account for variation in the adhesion pull-off strength measurements, were 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 was allowed to fully cure for a minimum of 25 hours at ambient temperature (20 to 30 °C). The cured product on the laminate was abraded back using a hole saw to the point at which until the laminate is exposed around the dolly.

[0298] 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 point of failure for each dolly tested. The point of failure occurs when the cured product is separated from the laminate. The adhesion pull-off strength is cited with reference to the type of substrate.

[0299] QUV weathering tests were conducted according to ISO 4892-3:2006(E), with the following settings:

[0300] Exposure period: 1000 hours continuously dry; Filter: Daylight filter; Irradiance: 0.55 W / m2nm at 340nm;

[0301] Black standard temperature: 60°C; Chamber temp: Not controlled; Relative humidity: Not controlled.

[0302] Xenon weathering tests were conducted according to ISO 4892-2:2006(E), with the following settings: Light source: Type 1 A (UVA-340) fluorescent UV lamp; Exposure cycle: Method A; Exposure period: 2000 hours with a cycle of 4 hours dry then 4 hours condensation; Irradiance: 0.76 W m-2 nm-1 at 340 nm; Black standard temperature dry: 60°C; Black standard temperature condensation: 50°C; Relative humidity: Not controlled.

[0303] The gloss retention and colour changes (in the CIELAB colour space and dE) were checked after 250, 500, 750 and 1000 hours of exposure for both the QUV and Xenon weathering tests.

[0304] Number average molecular weight

[0305] 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;

[0306] Calibration Standards: Agilent Polystyrene High EasyVials; Elution time: 30 minutes.

[0307] General vinyl ester resin composition and multi-part UV-curable coating composition preparation

[0308] The vinyl ester resin composition, was prepared according to the following general protocol:

[0309] 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 epoxy equivalent weight (EEW) value.

[0310] 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 epoxy equivalent weight (EEW) value. 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. A gas mixture of 50:50 air:nitrogen was then continuously fed into the reactor. The reaction mixture was monitored via melt viscosity and epoxy equivalent weight (EEW) value

[0311] 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 monomers (MM-1 , DM-1 and TM-1) 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.

[0312] In the fourth stage, a second portion of 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.

[0313] The second vinyl ester resin composition was then added with pigments, photoinitiators and / or a wax, as shown in Table 1 below, together with further components of 1 .5 wt% of an air release agent, 0.26 wt% of a wetting agent, 0.15 wt% of a surface additive and 2.8 wt% of a silica, to form a first part (A) of the multipart UV-curable coating composition. Therefore, the first vinyl ester resin composition VE A comprises a vinyl ester oligomer, a mono-monomer, a di-monomer and a tri-monomer (see Table 1 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).

[0314] Separately, the second vinyl ester resin composition was then added with polyfunctional reactive diluent monomers, a UV absorber, a hindered amine light stabiliser, an anti-oxidant, and photoinitiators, as shown in Table 1 below, together with further components of 1 .5 wt% of an air release agent, 0.26 wt% of a wetting agent, 0.15 wt% of a surface additive and 2.8 wt% of a silica, to a form a second part (B) of the multi-part UV-curable coating composition.

[0315] Curing method

[0316] Each multi-part UV-curable coating 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 multi-part UV-curable coating composition was 6.5 mm. The thickness of the pigmented layer (A’), derived from the first part (A), and the UV-resistant layer (B’), derived from the second part (B), was each 400 pm. The pigmented layer (A’) imparted on the cured product was a light grey colour (RAL 7035) in each Inventive Example. Table 1 - Inventive Examples A1, A2 and B1 (Compositions)

[0317] Inventive Examples A1 and A2 is each a pigmented layer (A’) derived from the first part (A) of the multipart UV-curable coating composition according to the first aspect of the invention. Inventive Examples A1 and A2 differ by the amounts of photoinitiator and wax in the coating composition, both of which are components intended to remove surface tackiness. Inventive Example B1 is a UV-resistant layer (B ) derived from the second part (B) of the multi-part UV-curable coating composition according to the first aspect of the invention. Table 2 - Inventive Examples A 1 and B 1 (Mechanical Testing)

[0318] The mechanical testing of Inventive Examples A1 and B1 were employed with the intention of investigating the adhesion pull-off strength of the pigmented layer (A’), and the adhesion pull-off strength of the UV-resistant layer (B') of the multi-part UV-curable coating composition, respectively. In particular, the adhesion pull-off strength of the pigmented layer (A’) was investigated with different substrates (epoxy-glass fibre laminate, a UV-curable filler composition, and a polyester-glass fibre laminate) with increasing thicknesses of the pigmented layer (A’) from 400 to 1000 pm to a polyester-glass fibre laminate.

[0319] In particular, a thickness of 400 pm was employed as this corresponds to the thickness applied in a typical wind turbine repair process.

[0320] The notation in Table 2 indicates that no measurement was taken.

[0321] Table 2 shows that the pigmented layer (A’) of the first aspect (Inventive Example A1) has excellent adhesion to both epoxy-glass fibre laminates and polyester-glass fibre laminates, and to a UV-curable filler composition, as demonstrated by an adhesion pull-off strength of over 2.0 MPa for each substrate.

[0322] The adhesion pull-off strength to a polyester-laminate at a thickness of 400 pm was very similar between Inventive Example A1 and Inventive Example B1 . This indicates that the pigmented layer (A') bonds to the polyester-glass fibre laminate with a similar strength as the UV-resistant layer (B’) bonds to the polyester-glass fibre laminate. Therefore, the limitation in adhesion pull-off strength is the bond between the pigmented layer (A ) to the polyester-glass fibre laminate, rather than the bond between the pigmented layer (A’) and the UV-resistant layer (B’). Table 3 - Inventive Examples 1 to 3 (Weathering Testing)

[0323] Inventive Example 1 corresponds to a cured product of the multi-part UV-curable coating composition comprising the pigmented layer (A’) of Inventive Example A1 and the UV-resistant layer (B’) of Inventive Example B1. Inventive Example 2 corresponds to a cured product of the multi-part UV-curable coating composition comprising the pigmented layer (A’) of Inventive Example A2 and the UV-resistant layer (B’) of Inventive Example B1. Inventive Example 3 corresponds to a cured product of the multi-part UV- curable coating composition comprising a pigmented layer (A’) of Inventive Example A2 except without 0.1 wt% of Anti-oxidant 1.

[0324] The notation in Table 3 indicates that no measurement was taken.

[0325] Typically, a lower dE value corresponds to a lower change in the deviation from the original colour. Minor deviations in dE between exposure times may be due to environmental factors which are outside the control of the weathering tests.

[0326] Table 3 shows that the multi-part UV-curable coating compositions of the first aspect (Inventive Examples 1 to 3) exhibited significantly low dE values across the total of 1000 hours of exposure time, indicating excellent weathering performance. In particular, each of Inventive Examples 1 to 3 shows that the increase in dE over the significant duration of time is small, indicating excellent UV stability of the multipart UV-curable coating composition, owing to the combination of a UV absorber and free radical scavenger, such as either a hindered light amine stabiliser and an anti-oxidant (Inventive Examples 1 and 2), or a hindered light amine stabiliser alone (Inventive Example 3).

Claims

Claims:1 . A multi-part UV-curable coating composition comprising:(A) a first part for providing a pigmented layer (A’) when cured comprising:(i) a pigment;(B) a second part for providing a UV-resistant layer (B’) when cured comprising:(i) a UV absorber and a free radical scavenger; wherein the first part (A) and the second part (B) each comprise a vinyl ester resin composition comprising a vinyl ester oligomer and a reactive diluent monomer, and a photoinitiator.

2. The multi-part UV-curable coating composition of claim 1 , wherein the vinyl ester resin composition is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 70 to 94 wt%, such as from 74 to 92 wt%, such as from 76 to 90 wt%, such as from 78 to 88 wt%, such from 80 to 86 wt%, such as from 82 to 85 wt%.

3. The multi-part UV-curable coating composition of claim 1 or 2, wherein the vinyl ester resin composition is present in the second part (B), based on the total weight of the first part (B) of the multi-part UV-curable coating composition, in an amount from 60 to 95 wt%, such as from 70 to 95 wt%, such as from 80 to 95 wt%, such as from 83 to 94 wt%, such as from 85 to 93 wt%, such as from 87 to 92 wt%, such as from 89 to 91 wt%.

4. The multi-part UV-curable coating composition of any one of claims 1 to 3, wherein the pigment has a refractive index (Rf) of 2.40 or less, such as 2.35 or less, such as 2.30 or less, as measured in accordance with ASTM D542.

5. The multi-part UV-curable coating composition of any one of claims 1 to 4, wherein the pigment is present, based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 3.0 to 17.0 wt%, such as from 5.0 to 15.0 wt%, such as from 7.0 to 13.0 wt%.

6. The multi-part UV-curable coating composition of any one of claims 1 to 5, wherein the pigment is a metal sulfide pigment.

7. The multi-part UV-curable coating composition of any one of claims 1 to 6, wherein the UV absorber is present, based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 0.5 to 4.0 wt%, such as from 1 .0 to 3.0 wt%, such as from 1 .5 to 2.5 wt%.

8. The multi-part UV-curable coating composition of any one of claims 1 to 7, wherein the free radical scavenger is present, based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 2.0 to 10.0 wt%, such as from 3.0 to 8.0 wt%, such as from 3.5 to 5.0 wt%.

9. The multi-part UV-curable coating composition of claim 8, wherein the free radical scavenger is a hindered amine light stabiliser or an anti-oxidant, or a combination thereof.

10. The multi-part UV-curable coating composition of claim 9, wherein the hindered amine light stabiliser is present, based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 2.0 to 10.0 wt%, such as from 3.0 to 8.0 wt%, such as from 3.5 to 5.0 wt%.

11. The multi-part UV-curable coating composition of claim 9, wherein the anti-oxidant is present, based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount from 0.01 to 0.40 wt%, such as from 0.05 to 0.20 wt%, such as from 0.05 to 0.15 wt%.

12. The multi-part UV-curable coating composition of any one of claims 1 to 11 , 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.

13. The multi-part UV-curable coating composition of any one of claims 1 to 12, wherein the photoinitiator is present in the first part (A), based on the total weight of the first part (A) of the multi-part UV- curable coating composition, in an amount from 0.5 to 6.0 wt%, such as from 2.0 to 4.0 wt%, such as from 2.5 to 3.5 wt%.

14. The UV-curable coating composition of any one of claims 1 to 13, wherein the photoinitiator is present in the second part (B), based on the total weight of the second part (B) of the multi-part UV-curable coating composition, in an amount of less than 1 .0 wt%, such as less than 0.5 wt%.

15. The multi-part UV-curable coating composition of any one of claims 1 to 14, further comprising a wax in the first part (A) of the multi-part UV-curable coating composition.

16. The UV-curable coating composition of claim 15, wherein the wax is present, based on the total weight of the first part (A) of the multi-part UV-curable coating composition, in an amount from 0.1 to 5.0 wt%, such as from 0.2 to 4.0 wt%.

17. The multi-part UV-curable coating composition of any one of claims 1 to 16, wherein the reactive diluent monomer comprises one or more polyfunctional reactive diluent monomers.

18. A method of curing the multi-part UV-curable coating composition of any one of claims 1 to 17, the method comprising the steps of:(i) applying the first part (A) onto a substrate;(ii) exposing the first part (A) to UV light at a wavelength from 200 to 405 nm to initiate a cure via a free radical polymerisation, and obtaining a pigmented layer (A’);(iii) applying the second part (B) onto the pigmented layer (A’); and(iv) exposing the second part (B) to UV light at a wavelength from 200 to 405 nm to initiate a cure via a free radical polymerisation, and obtaining a UV-resistant layer (B’).

19. Use of the multi-part UV-curable coating composition of any one of claims 1 to 17 in a composite repair process.

20. The use of claim 19, wherein the composite repair process is a wind turbine repair process.

21. A cured product of the multi-part UV-curable coating composition of any one of claims 1 to 17, or the cured product obtained from the method of claim 18, wherein the cured product comprises a pigmented layer (A’) corresponding to the first part (A) of the multi-part UV-curable coating composition, and a UV-resistant layer (B’) corresponding to the second part (B) of the multi-part UV- curable coating composition, such that in use the pigmented layer (A’) is positioned between a substrate and the UV-resistant layer (B’).

22. The cured product of claim 21 , wherein the thickness of the pigmented layer (A’) and the UV-resistant layer (B ) is each independently from 200 to 600 mm, such as from 200 to 500 mm.

23. The cured product of claim 21 or 22, characterised in that the dE value is less than 4.0, wherein the dE is measured using QUV accelerated weathering tests at 1000 hours in accordance with the method provided in the methods section.

24. A wind turbine part comprising the cured product of any one of claims 21 to 23; optionally wherein the wind turbine part is a wind blade part, a tower part or a nacelle part.

25. A multi-part repair system comprising a plurality of parts, which in combination contains the multi-part UV-curable coating composition of any one of claims 1 to 17 together with a UV-curable coating composition, wherein the UV-curable filler composition comprises a vinyl ester oligomer and a reactive diluent monomer mixture.

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