Process for coating a substrate and coated substrate

Abstract

A process for coating a substrate with a coating having a gloss value at an angle of 60° of at most 40 gloss units, comprising applying a layer of liquid, solvent-free, radiation-curable coating composition at a dry film thickness and curing by electron-beam radiation, the coating composition comprising a radiation-curable resin system, a first matting agent, and optionally a second matting agent in a weight ratio of first to second matting agent of at least 1.0 and a combined amount of first and second matting agents of 8-25 wt.%, based on the weight of the radiation-curable resin system, wherein the first matting agent consists of spherical particles of non-reactive polymer with an average particle size Dv50 of 1.15-1.90 times the dry film thickness, wherein the second matting agent consists of particles with an average particle size Dv50 smaller than the dry film thickness.

Description

[0001] PROCESS FOR COATING A SUBSTRATE AND COATED SUBSTRATE

[0002] Field of the Invention

[0003] The present invention relates to a process for coating a substrate with a coating having a gloss value at an angle of 60° of at most 40 gloss units (GU), in particular a coil coating process for coating a metal substate, and to a coated substrate obtainable by such process.

[0004] Background of the Invention

[0005] In conventional coil coating of steel or other metal substrates, a solvent borne coating composition is applied on the metal substate (unwound from a coiled metal substrate) and the coated substrate is then heated by passing it at high speed through a curing oven to evaporate the solvent and cure the resin to form a coating film. This process leads to emissions of volatile organic compounds (VOC) and is very energy intensive.

[0006] Energy intensity and emissions of VOC can significantly be reduced by curing a solvent-free radiation curable coating composition applied on a substrate by means of electron beam radiation. Electron beam curing takes place at ambient temperature and therefore does not require heating the coated substrate.

[0007] It is however challenging to provide a matt or low gloss coating by means of electron-beam curing. The conventional way of providing a matt or low gloss coating is by using particulate matting agents, such as silica, silica-coated wax, or other particulate matting agents, in a solvent borne coating composition. During evaporation of the solvent, the wet applied coating layer shrinks and the particulate matting agents provide an uneven surface that results in a matt or low gloss appearance. In electron beam curing, however, curing proceeds very fast and due to the absence of solvent, shrinkage of the wet applied coating composition is very minor. As a result, producing matt surfaces by using conventional matting agents in the coating composition is very difficult.

[0008] In DE 10 2008 029 580 A1 is disclosed a process for producing low gloss coatings wherein a water-containing coating composition comprising a matting agent is applied on a metal coil, heated to a surface temperature of from 40 to 120 °C by means of infrared irradiation during 1 to 5 seconds, and then cured by electron beam irradiation. A disadvantage of the process of DE 10 2008 029 580 A1 is that two radiation curing steps are needed (infrared and electron beam). In US 2014 / 371384 a method for producing a matt and scratch-resistant coating under actinic radiation is disclosed, wherein a radiation-curable coating composition is applied to a substrate, irradiated with UV-radiation in a first radiation step, irradiated with monochromatic UV light using an excimer UV lamp in a second radiation step, and then finish cured by means of actinic radiation such as UV radiation, electron beam radiation, X-ray radiation, or gamma radiation.

[0009] There is a need for providing matt or low gloss coatings that are cured by electron beam radiation, preferably without additional irradiation step(s).

[0010] Summary of the Invention

[0011] It has now been found that a matt or low gloss electron beam cured coating can be obtained from a liquid, solvent-free, radiation-curable coating composition that comprises a radiation- curable resin system and a first matting agent that consists of spherical particles of non- reactive polymer with an average particle size Dv50 larger than the dry film thickness of an applied layer of the coating composition, optionally in combination with a second matting agent that has an average particle size Dv50 that is lower than the dry film thickness in a weight ratio of the first matting agent to the second matting agent of at least 1 .0, if the combined amount of the first and second matting agents is in the range of from 8 to 25 wt.%, based on the weight of the radiation-curable resin system.

[0012] Accordingly, in a first aspect, the invention provides a process for coating a substrate with a coating having a gloss value at an angle of 60° of at most 40 gloss units (GU), the process comprising: a) applying a layer of liquid, solvent-free, radiation-curable coating composition on the substrate at a dry film thickness; and b) curing the applied layer of coating composition by electron-beam radiation, wherein the liquid, solvent-free, radiation-curable coating composition comprises: a liquid, solvent-free, radiation-curable resin system; a first matting agent; and optionally a second matting agent in a weight ratio of the first matting agent to the second matting agent of at least 1 .0, wherein the combined amount of first and second matting agents is in the range of from 8 wt.% to 25 wt.%, based on the weight of the radiation-curable resin system, wherein the first matting agent consists of spherical particles of non-reactive polymer with an average particle size Dv50 such that the ratio of the average particle size Dv50 and the dry film thickness is in the range of from 1.15 to 1.90, wherein the second matting agent consists of particles of a material different from the non- reactive polymer of the first matting agent with an average particle size Dv50 that is smaller than the dry film thickness.

[0013] An advantage of the process according to the invention is that matt or low gloss electron beam cured coatings can be obtained without additional irradiation step(s). The process is particularly suitable for coil coating of a metal substrate.

[0014] In a second aspect, the invention provides a coated substrate obtainable by a process according to the first aspect of the invention.

[0015] Detailed Description of the Invention

[0016] The process according to the invention is a process for coating a substrate with a coating have a gloss value at an angle of 60° of at most 40 gloss units (Gil). The process comprises: a) applying a layer of a liquid radiation-curable coating composition on the substrate at a dry film thickness; and b) curing the applied coating composition by electron-beam radiation.

[0017] The substrate may be any suitable substrate. Preferably, the substate is a metal, wood, or polymeric substrate, more preferably a metal substate. Reference herein to a wood substrate is a substrate of wood or of engineered wood. Polymeric substrates include substrates of fibre- reinforced polymer, often referred to as composite substrates.

[0018] The layer of liquid coating composition may be applied on the substrate in any suitable way, such as spraying, rolling, brushing, or dip coating. Preferably, the coating composition is applied in a coil coating process on a metal substrate.

[0019] The layer of liquid coating composition is applied on the substrate at a dry film thickness. Reference herein to dry film thickness is to the film thickness of the coating composition as cured, i.e. after the curing of (b). The dry film thickness is measured according to ISO 2808:2019, using method 6B. The coating composition is liquid, solvent-free, and radiation-curable and comprises a liquid, solvent-free, radiation-curable resin system.

[0020] Reference herein to a liquid coating composition is to a coating composition that is liquid at a temperature of 25 °C.

[0021] Reference herein to a solvent-free coating composition or resin system is to a coating composition or resin system that does not comprise an organic solvent or water as a liquid medium for any resin solids and is essentially free of water or organic solvent. Essentially free means that no water or organic solvent is intentionally added. Minor amounts of water or organic solvent present in any additives or raw materials (such as monomers) may be present, preferably in an amount below 2 wt.%, more preferably below 1 wt.%, based on the total weight of the coating composition.

[0022] Radiation curable resin systems are known in the art. Such resin system comprises polymers and, oligomers, and / or monomers, all with one or more functional groups that can be activated by radiation to achieve radical polymerization. Such functional group has a double bond that can be activated by radiation to achieve radical polymerization. A solvent-free, radiation- curable resin system consists of 100% resin solids, wherein all resin solids participate in the radiation curing.

[0023] Reference herein to an oligomer with one or more functional groups that can be activated by radiation to achieve radical polymerization is to a compound with a polymer backbone made up of in the range of from 2 to 10 monomers, for example a polyester, a polyurethane, a poly(meth)acrylate, a polyether, or an epoxy polymer backbone, with one or more functional groups having a double bond as terminal or side groups.

[0024] Reference herein to a monomer with one or more functional groups that can be activated by radiation to achieve radical polymerization is to a compound without a polymeric backbone that has one or more of such functional groups.

[0025] The coating composition applied in step a) may comprise any suitable liquid, solvent-free, radiation-curable resin system. The resin system preferably consists of a mixture of polymers and / or oligomers, and monomers, all with one or more functional group that has a double bond that can be activated by radiation to achieve radical polymerization, more preferably a (meth)acryloyl or vinyl functional group. The resin system preferably consists of a mixture of one or more oligomers and / or monomers with two or more of such functional groups, and one or more oligomers and / or monomers with one such functional group.

[0026] Reference herein to (meth)acryloyl is to methacryloyl, acryloyl, or a combination thereof.

[0027] Preferably, the resin system consists of a mixture of ethylenically unsaturated polymers and / or oligomers, and monomers, more preferably a mixture of oligomers and monomers, all comprising one or more (meth)acryloyl-functional groups or vinyl functional groups, even more preferably all comprising one or more (meth)acryloyl-functional groups.

[0028] The monomers preferably comprise 1 to 6 functional groups with a double bond that can be activated by radiation to achieve radical polymerization, more preferably 1 to 6 (meth)acryloyl groups. Preferably the monomers have a molecular weight of at most 600 g / mol, more preferably at most 500 g / mol, even more preferably at most 400 g / mol.

[0029] Examples of suitable monomers include, but are not limited to, (meth) acrylic acid, alkyl esters of (meth) acrylic acid, esters of a mono-alcohol or a polyol and (meth) acrylic acid.

[0030] Examples of suitable monomers with one functional group are cyclohexyl acrylate, benzyl acrylate, tert-butyl cyclohexyl acrylate, trimethylcyclohexyl acrylate, tricyclodecane methanol acrylate.

[0031] Examples of suitable monomers with two to six functional groups are esters of a polyol with 2 to 6 hydroxyl groups and (meth)acryl acid, such as ethyleneglycol di(meth)acrylate, propyleneglycol di(meth)acrylate, butanediol di(meth)acrylate, hexanediol di(meth)acrylate, decanediol di(meth)acrylate, 3-methyl-1 ,5-pentanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tricyclodecane dimethanol di(meth)acrylate, glycerol di(meth)acrylate, glycerol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, di(trimethylolpropane) tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, di pentaerythritol tetra(meth)acrylate dipentaerythritol, penta(meth)acrylate dipentaerythritol hexa(meth)acrylate.

[0032] The polymers or oligomers with functional groups that can be activated by radiation to achieve radical polymerization may be any suitable polymers or oligomers. The type and length of the backbone of the polymer or oligomer will be selected in view of the desired properties of the final coating such as flexibility, chemical resistance, and weatherability.

[0033] Preferred polymers or oligomers include polymers or oligomers functionalised with (meth)acryloyl groups selected from the groups consisting of polyester, polyurethane, poly(meth)acrylate, polyether, epoxy polymers, polycarbonate, polyamide, polyolefin, and mixtures thereof. Preferably the polymers or oligomers are oligomers, more preferably oligomers with a number average molecular weight in the range of from 600 to 15,000 g / mol, more preferably of from 1 ,000 to 5,000 g / mol. The number average molecular weight can be measured by means of gel permeation chromatography using polystyrene standards.

[0034] To obtain a cured polymer with sufficient crosslinking density, at least part of the polymers or oligomers, have more than one functional group that can be activated by radiation, preferably in the range of from 2 to 6 of such functional groups.

[0035] The monomers described hereinabove typically act as reactive diluent and control the viscosity of the resin system so that a solvent-free, liquid resin system can be obtained. The resin system may comprise any suitable amount of monomer, depending on the desired viscosity. The resin system may for example comprise in the range of from 10 to 90 wt.% monomer and 90 to 10 wt.% polymer or oligomer, preferably in the range of 10 to 70 wt.% monomer and 90 to 30 wt.% polymer or oligomer.

[0036] The coating composition comprises a first matting agent and optionally a second matting agent, wherein the combined amount of first and second matting agents is in the range of from 8 wt.% to 25 wt.%, based on the weight of the radiation-curable resin system. If the optional second matting agent is present, it is present in a weight ratio of the first matting agent to the second matting agent of at least 1.0. Preferably, the coating composition does not comprise any further matting agents.

[0037] Reference herein to a matting agent is to chemically inert particles that are dispersible in a coating composition, have little influence on the rheological properties of the coating composition, keep the resulting coating highly transparent for visible light, and provide matting power. Matting agents are to be contrasted to non-transparent opacifying or color pigments or to rheology modifying agents. Matting agents for coatings are well known in the art. The first matting agent consists of spherical particles of non-reactive polymer with an average particle size Dv50 such that the ratio of the average particle size Dv50 and the dry film thickness is in the range of from 1.15 to 1.90.

[0038] Reference herein to average particle size Dv50 is to the particle size value at which 50% of the total volume of particles has a particle size below that value. Likewise, Dv98 is the particle size value at which 98% of the total volume of particles has a particle size below that value. Dv50 or Dv98 is determined by laser diffraction according to ISO 13320 using the Mie model.

[0039] Reference herein to non-reactive polymer is to polymer that does not chemically react with the resin system under electron beam radiation conditions. The polymers therefore do not contain any activatable double bonds. Any activatable double bond present in any raw material used for the manufacture of such polymer particles have been fully crosslinked or polymerized during the manufacturing of the spherical polymer particles.

[0040] Reference herein to spherical particles is to particles with a spherical or spheroidal shape and having a largest diameter and a smallest diameter with a ratio of the largest diameter to the smallest diameter in the range of from 1.0 to 1.5. The largest and smallest diameter of the spherical particles and the ratio thereof can suitably be measured by means of scanning electron microscopy.

[0041] Preferably, the spherical particles of non-reactive polymer of the first mating agent have a ratio of the largest diameter to the smallest diameter in the range of from 1.0 to 1.1. It has been found that spherical particles with a ratio of the largest diameter to the smallest diameter in the range of from 1.0 to 1.1 result in coatings with very low gloss (30 Gil or below) over a larger range of Dv50 to dry film thickness ratio than spherical particles with a ratio of the largest diameter to the smallest diameter above 1.1 or above 1 .2.

[0042] In some embodiments, the coating composition comprises first matting agent consisting of spherical non-reactive polymer particles with a ratio of the largest diameter to the smallest diameter in the range of from 1.1 to 1.5 or from 1.2 to 1.5 and the ratio of the average particle size Dv50 of the spherical non-reactive polymer particles and the dry film thickness of the coating layer applied is in the range of from 1.35 to 1.90, preferably of from 1.4 to 1.8.

[0043] Preferably, the non-reactive polymer of the first matting agent particles is a thermoplastic polymer, more preferably polystyrene, poly(methyl methacrylate), poly(ethyl methacrylate), poly(butyl methacrylate), poly(methyl acrylonitrile), or polyamide. Even more preferably, the non-reactive polymer is polystyrene, poly(methyl methacrylate), poly(ethyl methacrylate), poly(butyl methacrylate), or poly(methyl acrylonitrile).

[0044] Preferably, the non-reactive polymer of the first matting agent particles is a non-swellable polymer. Reference herein to a non-swellable polymer is to a polymer that takes up less than 2 wt.% of solvent when immersed in ortho-xylene at a temperature of 130 °C for 60 minutes.

[0045] A preferred group of non-reactive, non-swellable polymer is the group of crosslinked non- reactive polymers. In its manufacture, crosslinking agents such as for example dibutyl vinyl may be used to create a crosslinked polymer network. In this way, a non-swellable polymer is formed. Particularly preferred crosslinked, non-reactive polymers are polymers selected from the group consisting of crosslinked polystyrene, crosslinked poly(methyl methacrylate), crosslinked poly(ethyl methacrylate), crosslinked poly(butyl methacrylate), and crosslinked poly(methyl acrylonitrile). Crosslinked poly(methyl methacrylate) is a particularly preferred non- swellable, non-reactive polymer for the particles of the first matting agent.

[0046] Preferably, the first matting agent is a monodisperse matting agent, wherein the spherical particles of non-reactive polymer have a ratio of Dv98 to Dv50 in the range of from 1 .0 to 2.0, more preferably of from 1 .0 to 1.5, even more preferably of from 1 .0 to 1.2.

[0047] Examples of commercially available monodisperse, crosslinked poly(methyl methacrylate) particles are Dynoadd P-510, Dynoadd P-520, and Dynoadd P-530. An example of commercially available monodisperse polyamide particles is Orgasol® (ex. Arkema) polyamide powder.

[0048] It has been found that by using, in addition to the first matting agent, a second matting agent that consists of particles of a material different from the non-reactive polymer of the first matting agent, the appearance of the coating surface can be improved if the particles of the second matting agent have an average particle size Dv50 that is smaller than the dry film thickness.

[0049] The second matting agent may be any known matting agent for coating compositions such as ultrafine silica, micronized wax, talc, aluminum stearate, and calcium stearate. Preferably, the second matting agent is silica or micronized wax. More preferably, the particles of the second matting agent comprise silica, even more preferably, the second matting agent is silica or is a silica-coated micronized wax.

[0050] The wax may be any type of wax, such as polyethylene wax, polypropylene wax, amide wax, Canauba wax, Fischer-Tropsch wax, polytetrafluoroethylene wax, or a hybrid wax.

[0051] A particularly suitable second matting agent is micronized wax, spot-coated with silica (i.e. , all micro-sized wax particles are individually coated with silica). Such micronized wax, spot- coated with silica is commercially available, for example as Deurex® S 3319M.

[0052] The second matting agent consists of particles that have an average particle size Dv50 that is smaller than the dry film thickness. Preferably, the ratio of the average particle size Dv50 of the particles of the second matting agent and the dry film thickness is in the range of from 0.20 to 0.98.

[0053] The combined amount of first and second matting agents is in the range of from 8 wt.% to 25 wt.%, based on the weight of the resin system, preferably in the range of from 9 wt.% to 20 wt.%, even more preferably of from 10 wt.% to 18 wt.%.

[0054] In case the coating composition does not comprise the second matting agent, the coating composition comprises in the range of from 8 to 25 wt.% of the first matting agent, based on the weight of the resin system, preferably in the range of from 9 wt.% to 20 wt.%, even more preferably of from 10 wt.% to 18 wt.%.

[0055] In case the coating composition comprises the second matting agent, the weight ratio of the first matting agent to the second matting agent is at least 1 .0, preferably in the range of from 1 .0 to 4.0, more preferably of from 1 .0 to 3.0.

[0056] In addition to the resin system, the first matting agent, and the optional second matting agent, the coating composition may comprise further compounds such as color pigments, extenders, and coating additives. Any additives known to be suitable for coating compositions may be used, for example UV stabilizers, leveling agents, dispersing agents, initiator, defoaming agents, or biocides. The amount of additives is preferably in the range of from 0.1 to 5.0 wt.%, more preferably of from 0.2 to 3.0 wt.%, based on the total weight of the coating composition. The coating composition may be a clear coating composition or an opaque coating composition. If the coating composition is an opaque coating composition, it will typically comprise one or more color pigments and / or extenders.

[0057] The coating composition may comprise any suitable amount of the radiation-curable resin system. Preferably, the coating composition comprises in the range of from 40 wt.% to 93 wt.%, more preferably in the range of from 50 wt.% to 90 wt.%, based on the total weight of the coating composition.

[0058] The coating composition applied in step a) of the process according to the invention is cured by electron-beam radiation in b). Electron-beam radiation and process conditions for electronbeam radiation are known in the art. Any suitable process conditions may be applied.

[0059] Typically, electron beam curing of coatings comprises exposing the applied coating composition to accelerated electrons in an electron beam accelerator, in an inert gas atmosphere, at a dose in the range of from 15-40 kGy.

[0060] The electron beam cured coating obtained by the process according to the invention may be a topcoat or a coating over which a further coat (e.g., a clear coat) is applied. Preferably a topcoat. The electron beam cured coating may be applied directly on a substrate, on a pretreated substrate, or on a primer coating that is already applied on a bare or pre-treated substrate. Preferably, the electron beam cured coating obtained by the process according to the invention is applied over a primer coating that is already applied on the substrate.

[0061] Preferably, the substrate is a metal substrate, and the process is a coil coating process wherein in a) a layer of the liquid, solvent-free, radiation-curable coating composition is applied on the substrate at a dry film thickness is in the range of from 8 pm to 30 pm, more preferably of from 10 pm to 25 pm. In such coil coating process, the metal substrate is roll coated at high speed, in a continuous process. At a typical coil coating dry film thickness of 15 pm for example, the particles of the first matting agent have a Dv50 in the range of from 17.3 to 28.5 pm.

[0062] More than one layers of the coating composition may be applied in a).

[0063] The invention further provides a coated substrate obtainable by the process according to the first aspect of the invention. The coated substrate has a gloss value at an angle of 60° of at most 40 gloss units (Gil), preferably at most 30 Gil.

[0064] Reference herein to a gloss value in gloss units is a gloss value in in gloss units as measured according to ISO 2813:2014 by light reflection at an angle of 60°.

[0065] Any preferred ranges, features, or embodiments described above for process according to the invention are also preferred ranges, features, or embodiments for the coated substrate according to the invention.

[0066] The process according to the invention will be further illustrated by means of the following nonlimiting examples.

[0067] Examples

[0068] Test methods

[0069] The gloss of the cured coatings was measured according to ISO 2813:2014 by light reflection at an angle of 60° using a glossmeter from Rhopoint Instruments. The gloss is expressed in gloss units (Gil).

[0070] The smoothness of the cured surface was visually inspected and rated from 1 (clearly textured surface - less desired) to 5 (smooth surface - most desired).

[0071] EXAMPLE 1

[0072] A solvent-free radiation-curable resin system (100% resin solids) was prepared by mixing a mono-acryloyl functional monomer (reactive diluent) and mono, di, and tri-acryloyl functional oligomers in the amounts as indicated in Table 1.

[0073] Coating compositions were prepared by adding one or more matting agents to the resin system. The type, ratio of largest to smallest diameter, and the particle size values Dv50 and Dv98 of the matting agents used are given in Table 2. In Table 3, the amounts of matting agent(s) in wt.% based on the weight of the resin system is given.

[0074] The coating compositions were applied in a single layer at a dry film thickness as indicated in Table 3 on a pretreated and primer-coated steel panel of 20 cm x 30 cm. The steel panel was pre-treated with Gardobond® X 4802 / 2 RFU and then primer-coated using a two-component, corrosion-resistant, solvent-borne polyurethane primer composition comprising ion-exchanged silicate as corrosion inhibitor. The dry film thickness of the primer coating was 5 pm. Table 1 Resin system - monomer / oligomer composition

[0075] Table 2 Matting agents

[0076] 1poly(methyl methacrylate) Table 3 Coating compositions - matting agents and properties

[0077] Comparison example;awt.% based on resin solids;2Dry film thickness The coating composition applied on the pretreated and primer-coated steel panel was then cured by electron beam radiation using a EBLab200 laboratory electron beam machine, using an accelerating voltage of 150 kV and an energy dose of 40 kGy.

[0078] The gloss and surface smoothness of the cured coatings were measured as indicated above. The results are given in Table 3.

[0079] EXAMPLE 2

[0080] Clear and opaque coating compositions were prepared by adding first and second matting agents and optionally color pigment and additives to the model resin system of Example 1 (see Table 1). The coating composition is given in Table 4. The combined amount of first and second matting agents based on the weight of the resin system was between 14.1 and 14.7 wt.% for experiment 26 (clear coat) and experiments 27 and 28 (opaque coats).

[0081] The coating compositions were applied in a single layer at a dry film thickness of 15 pm on a pretreated and primer-coated steel panel of 20 cm x 30 cm. The steel panel was pretreated and primer-coated as described in EXAMPLE 1.

[0082] The applied coating composition was then cured by electron beam radiation as described in EXAMPLE 1.

[0083] The gloss of the cured coatings was measured as indicated above. The results are given in

[0084] Table 4.

[0085] Table 4 Coating compositions

[0086] Comparison example

Claims

CLAIMS1. A process for coating a substrate with a coating having a gloss value at an angle of 60° of at most 40 gloss units (GU), the process comprising: a) applying a layer of liquid, solvent-free, radiation-curable coating composition on the substrate at a dry film thickness; and b) curing the applied layer of coating composition by electron-beam radiation, wherein the liquid, solvent-free, radiation-curable coating composition comprises: a liquid, solvent-free, radiation-curable resin system; a first matting agent; and optionally a second matting agent in a weight ratio of the first matting agent to the second matting agent of at least 1.0, wherein the combined amount of first and second matting agents is in the range of from 8 wt.% to 25 wt.%, based on the weight of the radiation-curable resin system, wherein the first matting agent consists of spherical particles of non-reactive polymer with an average particle size Dv50 such that the ratio of the average particle size Dv50 and the dry film thickness is in the range of from 1.15 to 1.90, wherein the second matting agent consists of particles of a material different from the non-reactive polymer of the first matting agent with an average particle size Dv50 that is smaller than the dry film thickness.

2. A process according to claim 1, wherein the spherical particles of non-reactive polymer of the first mating agent have a largest diameter and a smallest diameter and a ratio of the largest diameter to the smallest diameter in the range of from 1.0 to 1.1.

3. A process according to claim 1 or 2, wherein the non-reactive polymer of the first matting agent is selected from the group consisting of polystyrene, poly(methyl methacrylate), poly(ethyl methacrylate), poly(butyl methacrylate), poly(methyl acrylonitrile), and polyamide, preferably polystyrene, poly(methyl methacrylate), poly(ethyl methacrylate), poly(butyl methacrylate), more preferably poly(methyl methacrylate).

4. A process according to any one of the preceding claims, wherein the non-reactive polymer of the first mating agent is a non-swellable polymer, preferably a non- swellable polymer selected from the group consisting of crosslinked polystyrene, crosslinked poly(methyl methacrylate), crosslinked poly(ethyl methacrylate), crosslinked poly(butyl methacrylate), and crosslinked poly(methyl acrylonitrile), more preferably is crosslinked poly(methyl methacrylate).

5. A process according to any one of the preceding claims, wherein the spherical particles of non-reactive polymer of the first matting agent have a ratio of Dv98 to Dv50 in the range of from 1.0 to 2.0, preferably of from 1.0 to 1.5.

6. A process according to any one of the preceding claims, wherein the coating composition does not comprise any matting agent other than the first matting agent.

7. A process according to any one of claims 1 to 5, wherein the radiation-curable coating composition comprises the second matting agent, and wherein the weight ratio of the first matting agent to the second matting agent is in the range of from 1.0 to 4.0, preferably in the range of from 1.0 to 3.0.

8. A process according to claim 7, wherein the particles of the second matting agent have an average particle size Dv50 such that the ratio of the average particle size Dv50 of the particles of the second matting agent and the dry film thickness is in the range of from 0.20 to 0.98.

9. A process according to claim 7 or 8, wherein the second matting agent comprises silica, preferably is silica or a silica-coated micronized wax.

10. A process according to any one of claims 7 to 9, wherein the coating composition does not comprise any matting agent other than the first and the second matting agent.

11. A process according to any one of the preceding claims, wherein the radiation curable resin system consists of a mixture of (meth)acryloyl-functional oligomers and (meth)acryloyl-functional monomers.

12. A process according to any one of the preceding claims, wherein the substrate is a metal substrate.

13. A process according to claim 12, wherein the process is a coil coating process, and the dry film thickness is in the range of from 8 pm to 30 pm, preferably of from 10 pm to 25 pm.

14. A process according to any one of the preceding claims, wherein the metal substrate is a pretreated and / or primed metal substrate.

15. A coated substrate obtainable by a process according to any one of the preceding claims.

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