EP1218462B1 - Method for producing scratch-resistant coatings - Google Patents

Description

The present invention relates to a process for producing scratch-resistant coatings based on radiation-curable coating compositions.

Coating agents that cure by UV radiation are used in the art for the production of high quality coatings. Radiation-curable coating compositions are generally flowable preparations based on polymers or oligomers having crosslinking groups which, upon exposure to UV radiation, undergo a crosslinking reaction with one another. This leads to the formation of a high molecular weight network and thus to the formation of a solid, polymeric film. In contrast to the thermally curable coating materials hitherto frequently used, radiation-curable coating compositions can be used free of solvents or dispersants. In addition, they are characterized by very short curing times, which is particularly advantageous in continuous processing in painting lines.

UV-curable coating compositions generally have a high surface hardness and a good chemical resistance. For some time, there has been a desire for coatings that have a high scratch resistance, so that the coating is not damaged during cleaning, for example, and loses its shine. At the same time, the coatings should retain the properties usually achieved in radiation-cured coatings.

In the literature, the physical processes in the generation of scratches and the relationships between the scratch resistance and other physical characteristics of the coating were variously described (to scratch-resistant coatings see, eg, JL Courter, 23 ^rd Annual International Waterborne, High-Solids and Powder Coatings Symposium, New Orleans 1996).

Various test methods are described for the quantitative assessment of the scratch resistance of a coating. Examples are the examination by means of the BASF brush test (P. Betz and A. Bartelt, Progress in Organic Coatings, 22 (1993), pages 27-37), by means of the washing brush system of the company AMTEC or various test methods analog Ritz hardness measurements, as for example G. Jüttner, F. Meyer, G. Menning, Kunststoffe 1988, 88, 2038-42. Another test for determining the scratch resistance is described in European Coatings Journal 4/99, pp. 100 to 106.

According to the current state of development, three routes to scratch-resistant surfaces are discussed, which in principle can also be applied to UV-curing systems.

The first way is based on increasing the hardness of the coating material. For example, EP-A 544 465 describes coating compositions for scratch-resistant coatings containing colloidal silica and hydrolysis products of alkoxysilyl acrylates. The increase in hardness is based here on the incorporation of the silica in the polymer matrix of the coating. The high hardness, however, is at the expense of other properties, such as the penetration depth or the adhesion, which are essential for coating materials.

The second way is based on choosing the coating material so that it is stressed during scratching in the reversible deformation area. These are materials that allow a high reversible deformation. However, there are limits to the use of elastomers as coating material. Such coatings usually show poor chemical stability.

A third approach attempts to make coatings with tough, d. H. To produce plastic deformation behavior while keeping the shear stress occurring during scratching within the coating material as small as possible. This is achieved by reducing the friction coefficient, z. B. by using waxes or slip additives. Lacquer additives for UV-curing systems are described, for example, in B. Hackl, J. Dauth, M. Dreyer; Paint & Lacquer 1997, 103, 32-36.

US Pat. No. 5,700,576 describes a UV-curable, scratch-resistant coating comprising 1-30% by weight of a prepolymeric thickener with thiol groups and 20-80% by weight of one or more polyfunctional acrylates or methacrylates and thinners, in particular reactive diluents, which have a containing free-radically polymerizable group, free-radical initiator and other conventional additives for paint production included. The polymerization and thus curing of the coating is achieved by irradiation with UV light, for. B. under inert gas triggered.

JP-A-63214375 describes the preparation of coatings on metallic surfaces wherein the curing of the coating agent takes place under inert gas atmosphere with an oxygen concentration of less than 0.5% by volume.

DE-A-29 28 512 describes a method for producing a scratch-resistant coating on a plastic molding, in which applying a coating of at least one free-radically polymerizable monomer and cured by UV radiation, wherein the curing carried out in a low-oxygen or oxygen-free inert gas atmosphere becomes.

However, the proposed solutions for the production of scratch-resistant coatings can not satisfy because they are relatively expensive and the other coating properties are not satisfactory.

In another invention, which is the subject of a co-pending application, it has been found that the production of scratch-resistant coatings with a balanced property profile succeeds when curing a radiation-curable coating based on urethane acrylates under inert gas conditions. Inert gases typically contain no more than 500 ppm oxygen, which under normal conditions corresponds to an oxygen partial pressure of less than 0.05 kPa. The extensive exclusion of oxygen requires a complex technology. For the exclusion of oxygen in bodies, d. H. non-planar objects with a three-dimensional design, the hardening of the coating carried out in outwardly closed chambers, which are held consistently under an inert gas atmosphere. This would require a complicated lock technology, especially in continuous paint lines and would therefore not be economical.

The present invention has for its object to provide a simple method for the production of scratch-resistant coatings, which overcomes the disadvantages of the prior art.

It has surprisingly been found that the preparation of scratch-resistant coatings succeeds when curing a conventional radiation-curable coating agent by exposure to ultraviolet radiation in an oxygen-containing inert gas atmosphere having an oxygen partial pressure of not more than 18 kPa, without strict inert gas conditions are required.

• Aufbringen wenigstens eines durch UV-Strahlung härtbaren Beschichtungsmittels auf wenigstens eine Oberfläche eines zu beschichtenden Gegenstands, wobei das Beschichtungsmittel wenigstens ein Polymer und/oder Oligomer P1 mit im Mittel wenigstens einer ethylenisch ungesättigten Doppelbindung pro Molekül umfasst,
• Aushärten des Beschichtungsmittels durch Einwirkung von UV-Strahlung,

das dadurch gekennzeichnet ist, dass man das Aushärten des Beschichtungsmittels unter einem sauerstoffhaltigen Schutzgas durchführt, das einen Sauerstoffpartialdruck im Bereich von 0,5 bis 18 kPa aufweist.Accordingly, the present invention relates to a method for producing scratch-resistant coatings, comprising the following steps:

• Applying at least one UV-curable coating agent to at least one surface of an article to be coated, the coating composition comprising at least one polymer and / or oligomer P1 having on average at least one ethylenically unsaturated double bond per molecule,
• Curing of the coating agent by the action of UV radiation,

characterized in that the curing of the coating agent is carried out under an oxygen-containing inert gas having an oxygen partial pressure in the range of 0.5 to 18 kPa.

An oxygen partial pressure of 18 kPa corresponds to a volume fraction of the oxygen of about 20% by volume in the case of a protective gas under normal pressure. Under the same conditions, an oxygen partial pressure of 0.2 kPa corresponds to a volume fraction of the oxygen of 2200 ppm oxygen in the protective gas. (See also E.W. Bader / ed.), Handbook of the entire occupational medicine, Vol. 1 Urban and Schwarzenberg, Berlin, Munich, Vienna 1961, p. 665). An oxygen partial pressure of 9 kPa corresponds to 10 vol% oxygen in the protective gas.

For the process according to the invention, it is only necessary for the coating compositions in the areas where the curing takes place to be exposed to an oxygen concentration of less than 18 kPa at the moment of their exposure to UV radiation. The relevant areas are the surface areas of the object to be coated provided with the radiation-curable coating materials at the moment of their exposure to UV radiation. To achieve optimum scratch resistance, the oxygen partial pressure is preferably not more than 17 kPa (≈19 vol%), in particular not more than 15.3 kPa (≈17 vol%) and more preferably not more than 13.5 kPa (≈15) vol%). Optimum curing results are generally achieved at oxygen partial pressures in the range from 0.5 kPa to 10 kPa (≈5,500 ppm-11% by volume), in particular in the range from 0.5 to 6.3 kPa (≈5,500 ppm-7% by volume). %). The oxygen partial pressure may not be less than 0.5 kPa, preferably 0.9 kPa (≈ 1 vol%), 1.8 kPa (≈ 2 vol%) or 2.5 kPa (≈ 3 vol%).

As protective gases are inert gases such as nitrogen, carbon monoxide, carbon dioxide and noble gases, eg. As argon, and mixtures thereof with air or oxygen into consideration, being preferred as inert gases argon and nitrogen and nitrogen in particular.

In principle, all polymers and / or oligomers which on average have at least one ethylenically unsaturated double bond per polymer or oligomer molecule which are suitable as polymers P1 for the radiation-curable preparations according to the invention are suitable can be radically polymerized under the influence of electromagnetic radiation, such as UV radiation.

In general, the content of ethylenically unsaturated double bonds in P1 in the range of 0.01 to 1.0 mol / 100 g P1, preferably in the range of 0.05 to 0.8 mol / 100 g P1, and most preferably 0.1 to 0.6 mol / 100 g P1, lie. The terms polymer and oligomer here and below include polymers, polycondensates and polyaddition products, chemically modified polymers and prepolymers. Suitable prepolymers are, for. B. obtainable by reacting polyfunctional compounds having at least two reactive groups, with monofunctional or polyfunctional compounds having at least one ethylenically unsaturated double bond and at least one reactive group which can react with the reactive groups of the aforementioned polyfunctional compounds to form bonds.

The polymers or oligomers generally have a number average molecular weight M _N of at least 400 g / mol. Preferably, M _{N is} a maximum of 50,000 and is in particular in the range of 500 to 5,000.

Coating agents whose polymers or oligomers P1 per molecule have on average at least 2 and more preferably 3 to 6 double bonds are preferably used in the process according to the invention.

The polymers or oligomers P1 preferably have a double bond equivalent weight of from 400 to 2,000, particularly preferably from 500 to 900.

In addition, the radiation-curable coating compositions preferably have a viscosity of 250 to 11,000 mPas (determined by means of rotary viscometer according to DIN EN ISO 3319).

Such radiation-curable polymers and / or oligomers P1 are well known to the person skilled in the art. An overview of such coating compositions can be found for example in P.K.T. Oldring (Editor) Chemistry and Technology of UV- and EB-Formulations for Coatings and Paints, Vol. II, SITA Technology, London 1991. As far as radiation-curable coating compositions are concerned, the content of this work is referred to in its entirety.

In the polymers or oligomers P1, the double bonds usually have a vinylidene structure (CH ₂ = CR structure with R = H or CH ₃ ), which of vinyl, allyl, methallyl esters, ethers or -amines or derived from α, β-ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid or their amides. In the process according to the invention, preference is given to those polymers and / or oligomers P1 whose double bonds are present in the form of acrylate, methacrylate, acrylamide or methacrylamide groups. Examples of these are polyether acrylates, polyester acrylates, unsaturated polyesters, epoxy acrylates, urethane acrylates, amino acrylates, melamine acrylates, silicone acrylates and the corresponding methacrylates. Particularly preferred polymers and / or oligomers P1 are selected from urethane (meth) acrylates, polyester (meth) acrylates, oligoether (meth) acrylates and epoxide (meth) acrylates, wherein, with regard to the weathering stability of the coatings, urethane (meth) acrylates and polyesters (meth) acrylates, especially aliphatic urethane acrylates are particularly preferred.

The silicone (meth) acrylates are generally linear or cyclic polydimethylsiloxanes which have acrylic and / or methacrylic groups which are bonded via an oxygen atom or via an alkylene group to the silicon atoms of the polydimethylsiloxane backbone. Silicone acrylates are described, for example, in P.K.T. Oldring (see above), p 135 to p 152 described. The disclosure made there is hereby fully incorporated by reference.

Suitable ethylenically unsaturated epoxy acrylates are, in particular, the reaction products of epoxy group-containing compounds or oligomers with acrylic acid or methacrylic acid. Typical epoxy group-containing compounds are the polyglycidyl ethers of polyhydric alcohols. These include the diglycidyl ethers of bisphenol A and its derivatives, furthermore the diglycidyl ethers of oligomers of bisphenol A, as obtainable by reacting bisphenol A with the diglycidyl ether of bisphenol A, furthermore the polyglycidyl ethers of novolaks. The reaction products of acrylic acid or methacrylic acid with the abovementioned epoxides can additionally be modified with primary or secondary amines. Furthermore, by reacting OH groups in epoxy resins with suitable derivatives of ethylenically unsaturated carboxylic acids, eg. As the acid chlorides, other ethylenically unsaturated groups in the epoxy (meth) acrylates are introduced. Epoxy (meth) acrylates are well known to those skilled in the art and are commercially available. For further details reference is made to PKT Oldring, p. 37 to p. 68 and the literature cited therein.

Melamine acrylates are understood to mean the reaction products of melamine / formaldehyde condensation products with hydroxyalkyl esters of acrylic acid or of methacrylic acid, and also with acrylic acid, methacrylic acid or with their ester-forming derivatives. Suitable melamine / formaldehyde condensation products are, for example, hexamethylolmelamine (HMM) and hexamethoxymethylolmelamine (HMMM). Furthermore, both HMM and HMMM can be reacted with the amides of ethylenically unsaturated carboxylic acids, e.g. As acrylamide or methacrylamide, are modified to ethylenically unsaturated melamine (meth) acrylates. Melamine (meth) acrylates are known to the person skilled in the art and are described, for example, in P.K.T. Oldring, p. 208 to p. 214 as well as in EP-A 464 466 and DE-A 25 50 740, to which reference is hereby made for further details.

Polyester (meth) acrylates are also known to the person skilled in the art. They are available by various methods. For example, acrylic acid and / or methacrylic acid can be used directly as an acid component in the construction of the polyesters. In addition, it is possible to use hydroxyalkyl esters of (meth) acrylic acid as alcohol component directly in the construction of the polyester.

The polyester (meth) acrylates are preferably prepared by reacting hydroxyl-containing polyesters with acrylic or methacrylic acid or their ester-forming derivatives. It is also possible to start from carboxyl-containing polyesters, which are then reacted with a hydroxyalkyl ester of acrylic or methacrylic acid. Unreacted (meth) acrylic acid may be removed by scrubbing, distillation or, preferably, by reacting with an equivalent amount of a mono- or di-epoxide compound using suitable catalysts, e.g. B. triphenylphosphine, are removed from the reaction mixture. The products of this reaction usually remain in the radiation-curable coating composition and are incorporated into the polymer network during curing. For further details, please refer to P.K.T. Oldring, p. 123 to p. 135, referenced. Their number average molecular weight is usually in the range of 500 to 10,000, and preferably in the range of 800 to 3,000.

Suitable hydroxyl-containing polyesters for the preparation of the polyester (meth) acrylates can be prepared in the usual way by polycondensation of di- or polybasic carboxylic acids with diols and or polyols, wherein the OH-bearing component is used in excess. Accordingly, polyesters containing carboxyl groups are prepared by excessively using the carboxyl group-containing component. As the carboxylic acid component come in this case aliphatic and / or aromatic C ₃ -C ₃₆ carboxylic acids, their esters and anhydrides in question. These include maleic acid, maleic anhydride, succinic acid, succinic anhydride, glutaric, glutaric, adipic, pimelic, succinic, sebacic, phthalic, phthalic, isophthalic, terephthalic, tetrahydrophthalic, tetrahydrophthalic, trimellitic, trimellitic, pyromellitic and pyromellitic anhydrides. As diol component comes z. Ethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol, dimethylolcyclohexane, diethylene glycol, Triethylene glycol, mixtures thereof and also polyaddition of cyclic ethers, such as polytetrahydrofuran, polyethylene glycol and polypropylene glycol in question. Particularly suitable alcohols of higher value are trihydric to hexahydric alcohols, such as glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, dipentaerythritol, ditrimethylolpropane, sorbitol, erythritol and 1,3,5-trihydroxybenzene, and also the alkoxylated derivatives of the aforementioned higher-value alcohols.

Polyether (meth) acrylates are also known in principle to the person skilled in the art. Polyether (meth) acrylates are composed of a polyether base body which has acrylate and / or methacrylate groups at its ends. The polyether base body is obtainable for example by specific polymerization of epoxides such as ethylene oxide or propylene oxide or by reacting a polyhydric alcohol, for example an alcohol which has been mentioned above as a polyol component for the production of polyesters, with epoxides such as ethylene oxide and / or propylene oxide. This polyether base body still contains free OH groups, which can be esterified by known processes with acrylic acid and / or methacrylic acid, or ester-forming derivatives such as acid chlorides, C ₁ -C ₄ -alkyl esters or anhydrides (cf., for example, Houben-Weyl, Vol XIV, 2, Macromolecular Substances II, (1963)). Also suitable as polyethers are polymerization products of tetrahydrofuran and of oxetane.

A flexibilization of the polyether (meth) acrylates and the polyester (meth) acrylates is possible, for example, that corresponding OH-functional prepolymers or oligomers (polyether or polyester base) with longer-chain aliphatic dicarboxylic acids, especially aliphatic dicarboxylic acids with at least 6 C. Atoms, such as adipic acid, sebacic acid, dodecanedioic acid and / or dimer fatty acids. This flexibilization reaction can be carried out before or after the addition of acrylic or methacrylic acid to the oligomers or prepolymers.

The preferred urethane (meth) acrylates according to the invention are as a rule oligomeric compounds which have urethane groups and acryloyloxyalkyl or methacryloxyalkyl groups or (meth) acrylamidoalkyl groups. Urethane (meth) acrylates usually have a number average molecular weight M _N in the range of 500 to 5,000, preferably in the range of 500 to 2,000 daltons (determined by means of GPC on the basis of authentic comparative samples). Preference according to the invention is given to urethane (meth) acrylates having on average at least two double bonds, in particular having on average three to six double bonds per molecule. The aliphatic urethane (meth) acrylate prepolymers PU which are particularly preferred according to the invention are substantially free of aromatic structural elements, such as phenylene or naphthylene or substituted phenylene or naphthylene groups.

The urethane (meth) acrylates used according to the invention or their mixtures with a reactive diluent preferably have a viscosity (determined using a rotational viscometer according to DIN EN ISO 3319) in the range from 250 to 11,000 mPa.s, in particular in the range from 2,000 to 7,000 mPa.s on.

The aliphatic urethane (meth) acrylates are known in principle to the person skilled in the art and can be prepared, for example, as described in EP-A-203 161. As far as the urethane (meth) acrylates and their preparation are concerned, this document is fully incorporated by reference.

Preferred urethane (meth) acrylates according to the invention are obtainable by reacting at least 25% of the isocyanate groups of an isocyanate group-containing compound (component A) with at least one hydroxyalkyl ester of acrylic acid and / or methacrylic acid (component B) optionally with at least one further compound containing at least one isocyanate-reactive functional group (component C), for example chain extender C1.

• 1. das Äquivalentverhältnis der Isocyanatgruppen in Komponente A zu den reaktiven Gruppen in Komponente C zwischen 3:1 und 1:2, bevorzugt zwischen 3:1 und 1,1:1 und insbesondere bei etwa 2:1 liegt und
• 2. die Hydroxygruppen der Komponente B der stöchiometrischen Menge der freien Isocayanatgruppen der Komponente A, d. h. der Differenz aus der Gesamtzahl der Isocyanatgruppen der Komponente A abzüglich der reaktiven Gruppen der Komponente C, (oder abzüglich der zur Reaktion gebrachten, reaktiven Gruppen der Komponente C, sofern nur ein Teilumsatz der reaktiven Gruppen beabsichtigt ist) entsprechen.

The relative amounts of component A, B and C are preferably chosen so that

• 1. the equivalent ratio of the isocyanate groups in component A to the reactive groups in component C is between 3: 1 and 1: 2, preferably between 3: 1 and 1.1: 1 and in particular about 2: 1, and
• 2. the hydroxy groups of component B of the stoichiometric amount of the free isocyanate groups of component A, ie the difference between the total number of isocyanate groups of Component A minus the reactive groups of component C, (or minus the reacted, reactive groups of component C, if only a partial conversion of the reactive groups is intended) correspond.

The urethane (meth) acrylate preferably contains no free isocyanate groups. In an advantageous embodiment, therefore, the component B is reacted in stoichiometric ratio with the free isocyanate groups of the reaction product of component A and component C.

The urethane (meth) acrylates can also be prepared by first reacting part of the isocyanate groups of a low molecular weight diisocyanate or polyisocyanate as component A with at least one hydroxyalkyl ester of an ethylenically unsaturated carboxylic acid as component B and then reacting the remaining isocyanate groups with the component C, z. B. a chain extender C1, converts. It is also possible to use mixtures of chain extenders.

Also in this case the relative amounts of component A, B and C are chosen such that the equivalent ratio of the isocyanate groups to the reactive groups of the chain extender is between 3: 1 and 1: 2, preferably 2: 1 and the equivalent ratio of the remaining isocyanate groups to the hydroxy groups of the hydroxyalkyl ester is 1: 1.

Isocyanate group-containing compounds A are understood here and below to mean low molecular weight, aliphatic or aromatic di- or polyisocyanates and aliphatic or aromatic, isocyanate group-containing polymers or oligomers (prepolymers) having at least two and preferably three to six free isocyanate groups per molecule. The boundary between the low molecular weight di- or polyisocyanates or the prepolymers containing isocyanate groups is fluid. Typical prepolymers containing isocyanate groups generally have a number average molecular weight M _n in the range from 500 to 5,000 daltons, preferably in the range from 500 to 2,000 daltons. The low molecular weight di- or polyisocyanates preferably have a molecular weight below 500 daltons, in particular below 300 daltons.

Typical low molecular weight aliphatic di- or polyisocyanates are tetramethylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, tetradecamethylene diisocyanate, 1,6-diisocyanato-2,2,4-trimethylhexane, 1,6-diisocyanato-2,2,4,4-tetramethylhexane, 1,2-, 1,3- or 1,4-diisocyanatocyclohexane, 4,4'-di (isocyanatocyclohexyl) methane, 1-isocyanato-3,3,5-trimethyl-5- (isocyanatomethyl) cyclohexane (= isophorone diisocyanate), 2,4- or 2,6-diisocyanato-1-methylcyclohexane, and the uretdiones, biurets, cyanurates and allophanates of the aforementioned diisocyanates. Examples of aromatic di- or polyisocyanates are diisocyanates, such as 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, tetramethylxylylene diisocyanate, 1,4-diisocyanatobenzene, 4,4'- and 2,4-diisocyanatodiphenylmethane, p-xylylene diisocyanate, and also isopropenyldiisocyanate. methyltoluylene diisocyanate and the uretdiones, biurets, cyanurates and allophanates of the aforementioned aromatic diisocyanates.

The polyisocyanates containing isocyanurate groups are, in particular, simple trisisocyanato-isocyanurates, which are cyclic trimers of the diisocyanates, or mixtures with their higher homologs having more than one isocyanurate ring. By way of example, the isocyanurate of hexamethylene diisocyanate and the cyanurate of toluene diisocyanate, which are commercially available, may be mentioned here. Cyanurates are preferably used in the preparation of urethane (meth) acrylates.

Uretdione diisocyanates are cyclic dimerization products of diisocyanates. The Uretdiondiisocyanate can z. B. as the sole component or in admixture with other polyisocyanates, in particular the polyisocyanates containing isocyanurate groups for the preparation of urethane (meth) acrylates. Suitable biuret polyisocyanates preferably have an NCO content of 18 to 22 wt .-% and an average NCO functionality of 3 to 4.5.

Allophanates of the diisocyanates, for example, by reacting excess amounts of diisocyanates with simple, polyhydric alcohols, such as. As trimethylolpropane, glycerol, 1,2-dihydroxypropane or mixtures thereof. For the preparation of urethane (meth) acrylates suitable allophanate polyisocyanates generally have an NCO content of 12 to 20 wt .-% and an average NCO functionality of 2.5 to 3.

Suitable hydroxyalkyl esters of acrylic acid and methacrylic acid (component B) are the half-esters of acrylic acid or methacrylic acid with C ₂ -C ₁₀ -alkanediols, such as 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 4- Hydroxybutyl acrylate and 4-hydroxybutyl methacrylate. In addition to or instead of the hydroxyalkyl esters of acrylic acid and / or methacrylic acid, others may also be used to introduce double bonds into the urethane (meth) acrylate prepolymer hydroxyl-containing esters of acrylic acid or methacrylic acid, such as trimethylolpropane diacrylate or dimethacrylate and hydroxyl-bearing amides of acrylic acid and methacrylic acid, such as 2-hydroxy-ethylacrylamide and 2-hydroxyethylmethacrylamide are used.

Suitable chain extenders (component C1) are aliphatic di- or polyols having up to 20 carbon atoms, such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-methyl -1,5-pentanediol, 2-ethyl-1,4-butanediol, 2,2-bis (4'-hydroxycyclohexyl) propane, dimethylolcyclohexane, glycerin, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, bistrimethylolpropane, erythritol and sorbitol; Di- or polyamines having up to 20 carbon atoms, such as ethylenediamine, 1,3-propanediamine, 1,2-propanediamine, neopentanediamine, hexamethylenediamine, octamethylenediamine, isophoronediamine, 4,4'-diaminodicyclohexylmethane, 3,3'-dimethyl-4,4 '-diaminodicyclohexylmethane, 4,7-dioxadecane-1,10-diamine, 4,9-dioxadodecane-1,12-diamine, 4,7,10-trioxatridecane-1,13-diamine, 2- (ethylamino) -ethylamine, 3- (methylamino) propylamine, diethylenetriamine, N3-amine (3- (2-aminoethyl) aminopropylamine), dipropylenetriamine or N4-amine (N, N'-bis (3-aminopropyl) ethylenediamine); Alkanolamines of up to 20 carbon atoms, such as monoethanolamine, 2-amino-1-propanol, 3-amino-1-propanol, 2-amino-1-butanol, isopropanolamine, 2-amino-2-methyl-1-propanol, 5-amino 1-pentanol, 2-amino-1-pentanol, 6-aminohexanol, methylaminoethanol, 2- (2-aminoethoxy) ethanol, N- (2-aminoethyl) ethanolamine, N-methylethanolamine, N-ethyl-ethanolamine, N-butylethanolamine , Diethanolamine, 3- (2-hydroxyethylamino) -1-propanol or diisopropanolamine. Di- or polymercaptans having up to 20 carbon atoms, such as 1,2-ethanedithiol, 1,3-propanedithiol, 1,4-butanedithiol, 2,3-butanedithiol, 1,5-pentanedithiol, 1,6-hexanedithiol, 1.8 Octanedithiol, 1,9-nonanedithiol, 2,3-dimercapto-1-propanol, dithiothreitol, dithioerythritol, 2-mercaptoethyl ether or 2-mercaptoethyl sulfide. Also suitable as chain extenders are oligomeric compounds having two or more of the abovementioned reactive functional groups, for example hydroxyl-containing oligomers, such as polyethers, polyesters or acrylate / methacrylate copolymers containing hydroxyl groups. Oligomeric chain extenders are extensively described in the literature and typically have molecular weights in the range of 200 to 2,000 daltons. Preferred chain extenders are the di- or polyols having up to 20 carbon atoms, especially the aliphatic diols having 2 to 20 carbon atoms, e.g. For example, ethylene glycol, diethylene glycol, neopentyl glycol and 1,6-hexanediol.

Urethane (meth) acrylates which are obtainable by reacting component B with at least one prepolymer containing isocyanate groups with at least two isocyanate groups per molecule as component A are preferably used in the process according to the invention. Preference is given to prepolymers containing such isocyanate groups which are obtainable by reacting one of the abovementioned low molecular weight di- or polyisocyanates with at least one of the compounds of component C1, the equivalent ratio of the isocyanate groups to the reactive groups of component C1 being in particular about 2: 1 , Also preferred are compounds containing isocyanate groups selected from isocyanurates and biurets of aliphatic or aromatic diisocyanates.

Component C further includes compounds C2, which cause a flexibilization of the UV-cured coating. A flexibilization can also be achieved by reacting at least a portion of the free isocyanate groups of the binder with hydroxyalkyl esters and / or alkylaminamides of relatively long-chain dicarboxylic acids, preferably aliphatic dicarboxylic acids having at least 6 carbon atoms. Examples of suitable dicarboxylic acids are adipic acid, sebacic acid, dodecanedioic acid and / or dimer fatty acids. The flexibilization reactions can each be carried out before or after the addition of component B to the prepolymers containing isocyanate groups. Flexibilization is also achieved when using as chain extender C1 longer-chain aliphatic diols and / or diamines, in particular aliphatic diols and / or diamines having at least 6 carbon atoms.

In addition to the polymers and / or oligomers P1, the coating compositions may contain one or more reactive diluents. Reactive diluents are low molecular weight, liquid compounds which have at least one, polymerizable, ethylenically unsaturated double bond. An overview of Reactiwerdünner can be found z. In J.P. Fouassier (ed.), Radiation Curing in Polymer Science and Technology, Elsevier Science Publisher Ltd., 1993, Vol. 1, p 237-240. They usually serve to influence the viscosity and the coating properties, such as the crosslinking density.

The coating compositions used according to the invention preferably contain reactive diluents in an amount of up to 70% by weight, particularly preferably from 15 to 65% by weight, based on the total weight of P1 and reactive diluent in the coating composition.

Examples of reactive diluent classes include (meth) acrylic acid and its esters with diols, polyols and aminoalcohols, maleic acid and its esters or half esters, vinyl esters of saturated and unsaturated carboxylic acids, vinyl ethers and vinylureas. Examples which may be mentioned are C ₂ -C ₁₂ -alkylene glycol di (meth) acrylates such as 1,4-butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate and 1,12-dodecyl diacrylate, esters of acrylic acid or methacrylic acid with (poly ) ether diols such as di- or Tripropylenglykoldi (meth) acrylate, triethylene glycol di (meth) acrylate and polyethylene glycol di (meth) acrylate, esters of acrylic acid or methacrylic acid with olefinically unsaturated alcohols such as vinyl (meth) acrylate, allyl (meth) acrylate and dicyclopentadienyl, esters acrylic acid or methacrylic acid with higher-valued alcohols such as glycerol tri (meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, furthermore monounsaturated compounds such as vinyl acetate, styrene, vinyltoluene, Ethoxy (ethoxy) ethyl acrylate, N-vinylpyrrolidone, phenoxyethyl acrylate, dimethylaminoethyl acrylate, hydroxyethyl (meth) acrylate, butoxyethyl (meth) acrylate and isobornyl (meth) acrylate, etc. two or more unsaturated compounds such as divinylbenzene and dimethylacrylamide. It is also possible to use the reaction product of two moles of acrylic acid with one mole of a dimer fatty alcohol, which generally has 36 carbon atoms. Also suitable are mixtures of said reactive diluents.

Preference is given to reactive diluents based on esters of acrylic acid or of methacrylic acid and, among these, preferably mono- and diacrylates and also mono- and dimethacrylates, in particular isobornyl acrylate, hexanediol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate and Laromer® 8887 from BASF AG. Very particular preference is given to isobornyl acrylate, hexanediol diacrylate, dipropylene glycol diacrylate and tripropylene glycol diacrylate.

The coating compositions according to the invention contain photoinitiators or photoinitiator combinations, as are customarily used in radiation-curable coating compositions, and which can initiate the polymerization of ethylenically unsaturated double bonds on exposure to UV radiation. Radiation-curable coating compositions generally contain, based on the total weight of P1 and optionally the reactive diluents, at least 0.1% by weight, preferably at least 0.5% by weight and up to 10% by weight, preferably 0.5 to 6 wt .-%, in particular 1 to 4 wt .-%, of at least one photoinitiator. Examples of suitable photoinitiators are benzophenone and benzophenone derivatives, such as 4-phenylbenzophenone and 4-chlorobenzophenone, Michler's ketone, anthrone, acetophenone derivatives, such as 1-benzoylcyclohexan-1-ol, 2-hydroxy-2,2-dimethylacetophenone and 2,2-dimethoxy-2-phenylacetophenone, benzoin and benzoin ethers such as methyl, ethyl and butyl benzoin ethers, benzil ketals such as benzil dimethyl ketal, 2-methyl-1- (4-methylthiophenyl) -2-morpholinopropane-1 -on, anthraquinone and its derivatives such as β-methylanthraquinone and tert-butylanthraquinone, acylphosphine oxides such as 2,4,6-trimethylbenzoyldiphenylphosphine oxide, ethyl 2,4,6-trimethylbenzoylphenylphosphinate and bisacylphosphine oxides. Such initiators are, for example, the products available commercially under the trademarks Irgacure® 184, Darocure® 1173 from Ciba Geigy, Genocure® from Rahn or Lucirin® TPO from BASF AG. Preferred photoinitiators are also phenylglyoxalic acid, its esters and their salts, which can also be used in combination with one of the abovementioned photoinitiators. For further details, reference is hereby made in full to the German patent application P 198 267 12.6.

The coating compositions may also contain customary auxiliaries and / or additives, for example light stabilizers (for example HALS compounds, benzotriazoles, oxalanilides and the like), slip additives, polymerization inhibitors, matting agents, defoamers, leveling agents and film-forming auxiliaries, e.g. Cellulose derivatives, or other commonly used in topcoats additives. These customary auxiliaries and / or additives are usually used in an amount of up to 15% by weight, preferably 2 to 9% by weight, based on the total weight of P1 and, if appropriate, the reactive diluents.

Flowable or liquid coating compositions are preferably used in the process according to the invention. These can be sprayed or sprayed or applied to the surfaces of the article to be coated by conventional methods, for example by dip coating, spraying or doctoring.

Optionally, the still moist coating may be subjected to a drying step prior to curing with UV radiation. If desired, the still moist coating can also be first cross-linked and then cured with UV radiation.

As a rule, the coating composition of the invention is applied in an amount of 3 to 200 g / m ² , preferably 5 to 150 g / m ² . As a result, coating thicknesses in the cured state of 3 to 200 microns, preferably 5 to 150 microns are generated.

In the process of the invention, the coating compositions are often used in the form of clearcoats, so that they usually have no or only transparent fillers and opaque Contain pigments. However, it is also possible to use them in the form of pigmented coating compositions. In this case, the coating compositions contain from 2 to 40% by weight, based on the total weight of the coating agent, of one or more pigments. Further, in this case, the coating agents may contain from 1 to 30% by weight, based on the total weight of the coating agent, of one or more fillers.

In addition, it is also possible to use the UV-curable coating compositions in the process according to the invention in the form of aqueous preparations. Such binder dispersion or emulsions are virtually free of environmentally harmful volatiles, such as monomers or cosolvents. The crosslinking according to the process described here under a protective gas atmosphere is carried out after complete evaporation of the water or in spray application additionally after complete escape of the trapped air. With regard to the production and processing of radiation-curable aqueous binder dispersions or emulsions, reference is made at this point by way of example to EP-A 12 339.

By means of the method according to the invention, a wide variety of substrates can be coated, for example glass, metal substrates, such as aluminum, steel and other iron alloys, furthermore wood, paper, plastics and mineral substrates, for. B. concrete roof tiles and fiber cement boards. The method according to the invention is also suitable for coating packaging containers and for coating films for the furniture industry. The inventive method is characterized in particular by the fact that except planar or largely planar objects and body, d. H. Objects with a three-dimensional design, can be provided with scratch-resistant coatings.

For the production of coatings on metal substrates, the coating compositions according to the invention are preferably applied to primed or basecoat-coated metal surfaces, e.g. As metal sheets or metal bands, three-dimensionally designed metal objects, eg. As molded metal sheets, such as body parts, housing, frame profiles for windows o. Ä., Applied. As primers, the commonly used basecoats can be used. The basecoat used is both conventional and aqueous basecoats. Furthermore, it is also possible to apply the coating compositions of the invention to metal substrates which are first coated with an electrodeposition coating and then with a functional layer and wet-on-wet with a basecoat. In the mentioned In general, it is necessary for the process that the basecoat and the filler or the functional layer are baked before application of the coating composition according to the invention.

Equipment for the curing of radiation-curable coatings under normal atmospheric conditions as well as under strict exclusion of oxygen are known to the person skilled in the art (cf., for example, BR Holmes, UV and EB Curing Formulations for Printing Inks, Coatings and Paints, SITA Technology, Academic Press, London, United Kingdom 1984). The process according to the invention can basically be carried out in both plant types. The equipment for curing under normal atmospheric conditions are then provided with additional means by which the areas of the system in which the coating is cured, for example the curing unit in a painting line, with an inert gas or a mixture of inert gas and oxygen or air Reaching the desired oxygen concentration rinse at the cure site. For example, one can provide one or more nozzles or nozzle bars for the inert gas supply in the openings of the plant, through which the substrate provided with the wet coating of the UV source, for example, a high-pressure mercury lamp is supplied. In addition, it is recommended to provide further possibilities of the protective gas supply in the area of the UV source. In conventional UV curing equipment, which includes a UV curing unit having an entrance and an exit opening and a conveyor which passes the still wet coated article through the entrance opening into the curing unit, past the W source, and then through the exit opening transported out of the curing unit, you can see at least one device for purging with inert gas, for. As a nozzle bar, in the inlet and the outlet opening and optionally other devices for purging with inert gas in the interior of the curing unit, z. B. in close proximity to the UV source before. The surfaces of uniformly shaped body, z. As bodies and body panels, one can pass similar to the drying zone of a car wash, through an enriched with inert gas region of a UV source. It is also possible to extend the contour of a body which is in the enriched with inert gas region, with a movably arranged UV source. Systems for UV curing of bodies, in particular bodies with a complex three-dimensional shape are known for example from US 4,208,587 and WO 98/53008. The plant types described there can be used in the manner described above be retrofitted in the process according to the invention with suitable flushing devices for inert gas.

The UV source used for curing can be provided with nozzles or slots through which, during curing, i. H. of exposing the object provided with the moist coating agent, protective gas constantly flows, so that at the location of the radiation curing, the oxygen concentration is reduced to the value according to the invention. The nozzles or slots are preferably arranged as a ring or ring around the UV source. To harden the entire surface of a body, it is also possible to guide such a fitted UV source over the body by means of suitable devices, for example by means of a robotic arm (cf. also WO 98/53008).

The curing of the coated surfaces by means of UV radiation can of course also take place in externally sealed rooms or chambers with reduced oxygen content in the atmosphere.

An advantage of the method according to the invention is that the desired oxygen concentrations can be realized without great technical effort. Also, the amount of inert gas used is less than the amount usually required to achieve a strict exclusion of oxygen, since rinsing with an inert gas sufficient for the establishment of the oxygen concentrations of the invention, which does not lead to complete displacement of the oxygen from the atmosphere located in the curing zone. In that regard, the process according to the invention can also be described as a process for UV curing of UV-curable coatings under reduced or restricted inert gas atmosphere.

These advantages are particularly important in elaborately designed bodies to fruition. In the case of such bodies, there is basically the problem that flushing with inert gas does not allow a complete exclusion of oxygen in the surface region of the body. UV curing of bodies provided with UV-curable coatings has therefore hitherto been considered possible only in externally closed curing units and thus uneconomical. In contrast, the inventive method allows for arbitrarily shaped objects due to its tolerance to residual amounts of oxygen in the surface regions of the coated article, a simple hardening of the provided with a radiation-curable coating surfaces. Another advantage is that the ambient air of the actual curing unit, such as in a paint line, still sufficient Contains oxygen and so does not, for sufficiency of spaces with inert gas atmosphere, Erstickungsgefahr.

The coatings obtained by the process according to the invention have a significantly improved scratch resistance. High scratch resistance is to be understood as a good performance in the Scotch-Brite test. Thus, according to the Scotch-Brite test, the coatings obtainable according to the invention often have delta-gloss values of not more than 30, whereby values of not more than 20 or not more than 10 are achieved without a complete exclusion of oxygen being required.

• Aus den in den Ausführungsbeispielen angebebenen Komponenten wurden, sofern nicht ausdrücklich etwas anders angegeben wird, unter intensivem Rühren mittels eines Disolvers oder eines Rührers, die Beschichtungsmittel hergestellt.
• Zur Herstellung der kratzfesten Beschichtungen wurden die in den Ausführungsbeispielen beschriebenen Beschichtungsmittel mit einem Kastenrakel, Spaltgröße 200 µm, als Film auf gereinigten, schwarz eingefärbten Glasplatten aufgetragen. Die Aushärtung der Filme erfolgte in einer IST Beschichtungsanlage M 40 2x1-R-IR-SLC-So inert mit Vorrichtungen für eine Schutzgaszufuhr im Bereich der Eingangs- und Ausgangsöffnung mit 2 UV-Strahlern (Wellenlängenbereich, Quecksilber-Hochdrucklampen Typ M 400 U2H und M 400 U2HC) und einer Förderband-Laufgeschwindigkeit von 10 m/min. Die Strahlendosis betrug ca. 1.800 mJ/cm². Durch Drosselung der Stickstoff-Zufuhr wurde der Sauerstoffgehalt in der Härtungszone eingestellt. Die Messung des Sauerstoffgehalts im Härtungsbereich erfolgte zwischen den beiden UV-Strahlern, mit Hilfe einer Galvanoflux-Sonde (elektrochemische Zelle auf Basis eines Blei/Bleioxid-Redoxpaares mit drei Messbereichen: 0-1.000 ppm, 0-5 % und 0-25 %). Vor jeder Härtung wurde die Sauerstoffkonzentration eingestellt und zur Equilibrierung der Atmosphäre 20 min. gewartet.
• Die mechanischen Beständigkeit der unter verschiedenen Sauerstoffgehalten gehärteten Beschichtungen wurde durch Bestimmung der Pendelhärte nach König, DIN 53157, ISO 1522 und durch Bestimmung der Kratzfestigkeit mit dem Scotch-Brite-Test nach Lagerung für 24 Stunden im Klimaraum untersucht.
• Im Scotch-Brite Test wird als Prüfkörper ein 3 x 3 cm großer Siliciumcarbid modifizierter Faservlies (Scotch Brite SUFN, 3M Deutschland, 41453 Neuss) an einem Zylinder befestigt.
  Dieser Zylinder drückt das Faservlies mit 750 g an die Beschichtung und wird pneumatisch über die Beschichtung bewegt. Die Wegstrecke der Auslenkung beträgt 7 cm. Nach 10 bzw. 50 Doppelhüben (DH) wird im mittleren Bereich der Beanspruchung der Glanz (Sechsfachbestimmung) analog DIN 67530, ISO 2813 bei einem Einfallwinkel von 60° gemessen. Aus den Glanzwerten der Beschichtungen vor und nach den mechanischen Beanspruchungen wird die Differenz (Delta-Glanz-Wert) gebildet. Der Glanzverlust, d. h. die Delta-Glanz-Werte, sind umgekehrt proportional zur Kratzfestigkeit.

The invention will be explained in more detail by means of exemplary embodiments. All parts mean parts by weight, unless expressly stated otherwise.

• Unless expressly stated otherwise, the coating compositions were prepared from the components indicated in the exemplary embodiments with vigorous stirring by means of a disolver or a stirrer.
• To prepare the scratch-resistant coatings, the coating compositions described in the exemplary embodiments were applied as a film on cleaned, black-colored glass plates using a box doctor blade, gap size 200 μm. The films were cured in an IST coating system M 40 2x1-R-IR-SLC-So inert with devices for an inert gas supply in the area of the inlet and outlet opening with 2 UV lamps (wavelength range, high-pressure mercury lamps type M 400 U2H and M 400 U2HC) and a conveyor belt running speed of 10 m / min. The radiation dose was about 1,800 mJ / cm ² . By throttling the nitrogen supply, the oxygen content in the curing zone was adjusted. The oxygen content in the curing area was measured between the two UV lamps, using a Galvanoflux probe (electrochemical cell based on a lead / lead oxide redox pair with three measuring ranges: 0-1,000 ppm, 0-5% and 0-25%). , Before each cure, the oxygen concentration was adjusted and for equilibration of the atmosphere for 20 min. maintained.
• The mechanical resistance of the coatings cured under different oxygen contents was investigated by determining König pendulum hardness, DIN 53157, ISO 1522 and by determining the scratch resistance with the Scotch Brite test after storage for 24 hours in an air-conditioned room.
• In the Scotch-Brite test, a 3 × 3 cm silicon carbide-modified fiber fleece (Scotch Brite SUFN, 3M Germany, 41453 Neuss) is attached to a cylinder as test specimen.
  This cylinder presses the nonwoven fabric at 750 g onto the coating and is pneumatically moved across the coating. The distance of the deflection is 7 cm. After 10 or 50 double strokes (DH), the gloss (sixfold determination) is measured in the middle region of the stress analogously to DIN 67530, ISO 2813 at an angle of incidence of 60 °. From the gloss values of the coatings before and after the mechanical stresses, the difference (delta-gloss value) is formed. The loss of gloss, ie the delta-gloss values, are inversely proportional to the scratch resistance.

Example 1: (coating based on a urethane acrylate)

100 parts of Laromer® LR 8987: commercially available mixture of an aliphatic urethane acrylate with 30% by weight of hexanediol diacrylate from BASF AG.
Molecular weight about 650 g / mol,
Functionality about 2.8 double bonds / mol (about 4.5 mol / kg),
Viscosity 2-6 Pa.s (DIN EN ISO 3219).

2 parts Irgacure I 184: commercially available photoinitiator from Ciba-Geigy.

Table 1: Test results of the coating Example 1 when cured under different oxygen levels oxygen content Scratch resistance (loss of gloss) sway 10 DH 50 DH (S) 21% (air) * 50.0 56.4 175 15% 9.5 15.8 183 10% 6.5 11.8 185 7% 6.7 9.3 181 5% 6.7 8.7 183 3% 4.4 8.4 182 1.3% 4.2 9.1 182 0.5% 3.9 8.0 188 340 ppm (inert) * 4.2 9.2 189 * Verglechsbeispiel

Example 2: (Coating Based on a Polyester Acrylate)

100 parts of Laromer® LR 8800: commercial mixture of a polyester acrylate modified with an aromatic epoxy acrylate from BASF AG. Polyester acrylate based on trimethylolpropane and maleic acid.
Molecular weight about 900 g / mol,
Functionality about 3.5 (about 3.9 mol double bond / kg).
Viscosity 4-8 Pa.s (DIN EN ISO 3219).
2 parts Irgacure 1 184: commercially available photoinitiator from Ciba-Geigy. Table 2: Test results of the coating Example 2 when cured under different oxygen levels oxygen content Scratch resistance (loss of gloss) sway 10 DH 50 DH (S) 21% (air) * 77.0 78.5 99 11% 59.7 74.2 111 7% 4.9 12.1 122 5% 3.5 5.4 120 3% 5.9 10.5 113 1.3% 2.2 4.5 127 0.5% 3.7 6.3 120 340 ppm (inert) * 3.0 5.2 116 * Comparative Example

Example 3: (coating based on an oligoether acrylate)

100 parts of Laromer® LR 8863, commercially available oligoether acrylate from BASF AG.
Molecular weight about 500 g / mol,
Functionality about 3 (about 6.0 mol double bonds / kg),
Viscosity about 0.1 Pa.s (DIN EN ISO 3219).
2 parts Irgacure I 184: commercially available photoinitiator from Ciba-Geigy. Table 3: Test results of the coating Example 3 when cured under different oxygen levels oxygen content Scratch resistance (loss of gloss) sway 10 DH 50 DH (S) 21% (air) * nB nB nB 15% nB nB nB 11% 60.3 67.9 164 7% 29.0 51.7 160 5% 2, 3 5.1 175 3% 2.6 6.7 174 1.4% 1.4 3.4 175 0.5% 1.7 4.5 173 340 ppm (inert) * 1.0 3,3 174 * Comparative Example nB:
not measurable, because the surface is too soft.

Example 4: (coating based on an amine-modified oligoether acrylate)

100 parts of Laromer® LR 8869: commercially available, amine-modified oligoether acrylate from BASF AG.
Molecular weight about 550 g / mol,
Functionality approx. 3.
Viscosity 0.08-0.12 Pa.s (DIN EN ISO 3219).
2 parts Irgacure I 184: commercially available photoinitiator from Ciba-Geigy. Table 4: Test results of the coating Example 4 when cured under different oxygen levels oxygen content Scratch resistance (loss of gloss) sway 10 DH 50 DH (S) 21% (air) * 79.2 80.8 76 17% 17.7 40.0 70 15% 22.0 37.1 115 11% 9.5 17.7 115 5% 5.1 12.8 118 3% 6.0 12.2 127 1.4% 2.8 5.3 126 0.5% 1.9 5.6 112 340 ppm (inert) * 1.0 3.7 122 * Comparative Example

Claims (9)

1. A process for producing scratch-resistant coatings, comprising the following steps:

   - applying at least one UV-curable coating composition to at least one surface of an article to be coated, said coating composition comprising at least one polymer and/or oligomer P1 comprising on average at least one ethylenically unsaturated double bond per molecule, and

   - curing the coating composition by exposure to UV radiation,

   which comprises conducting the curing of the coating composition under an oxygen-containing protective gas which has an oxygen partial pressure in the range from 0.5 to 18 kPa.
2. The process according to claim 1, wherein the polymer and/or oligomer P1 has a double bond content in the range from 0.01 to 1 mol/100 g of P1.
3. The process according to either of the preceding claims, wherein the number-average molecular weight of P1 is within the range from 400 to 10,000 daltons.
4. The process according to any of the preceding claims, wherein the ethylenic double bonds in P1 are in the form of acrylate, methacrylate, acrylamido or methacrylamido groups.
5. The process according to claim 4, wherein P1 is selected from urethane (meth)acrylates, polyester (meth)acrylates, oligoether (meth)acrylates, and epoxy (meth)acrylates.
6. The process according to any of the preceding claims, wherein the UV-curable coating compositions comprise one or more reactive diluents in addition to P1.
7. The process according to claim 6, wherein the reactive diluent is selected from compounds having one or two acrylate and/or methacrylate groups.
8. The process according to any of the preceding claims, wherein the article to be coated is a three-dimensional structure.
9. The process according to any of the preceding claims, wherein that region of an installation in which the coating is cured by exposure to UV radiation is flushed with a protective gas.