Radiation-curable formulations based on ethylenically unsaturated prepolymers and difunctional esters of ethylenically unsaturated carboxylic acids with diols having a linear alkylene chain of 7 to 14 carbon atoms, a process for coating substrates using these radiation-curable formulations, and their use as automotive topcoats.

RADIATION-CURABLE FORMULATIONS
 The present invention relates to radiation-curable formulations which
 comprise at least one prepolymer having at least two olefinic double bonds
 per molecule and at least one difunctional ester of
 .alpha.,.beta.-ethylenically unsaturated carboxylic acids with diols
 having a linear alkylene chain which has 7 to 14 carbon atoms.
 Radiation-curable compositions have acquired widespread importance in the
 art, especially as high-grade surface coating materials. By
 radiation-curable compositions are meant formulations which include
 ethylenically unsaturated polymers or prepolymers and which, directly or
 after a physical drying step, are cured by exposure to high-energy
 radiation; for example, by irradiation with UV light or by irradiation
 with high-energy electrons (electron beams).
 Particularly high-grade coatings are obtained on the basis of
 polyfunctional .alpha.,.beta.-ethylenically unsaturated polymers or
 prepolymers. Such ethylenically unsaturated prepolymers are known from P.
 K. T. Oldring (ed.), Chemistry and Technology of UV- and EB-Formulations
 for Coatings, Inks and Paints, Vol. II, SITA Technology, London 1991, on
 the basis, for example, of epoxy acrylates (pp. 31-68), urethane acrylates
 (pp. 73-123) and melamine acrylates (pp. 208-214).
 Ethylenically unsaturated compounds of low molecular mass are often added
 to such compositions in order to reduce their viscosity. Like the
 ethylenically unsaturated polymers and prepolymers, said compounds are
 polymerized in the course of curing and so are incorporated into the
 coating. They are therefore referred to as reactive diluents. The
 properties of the resulting coatings are therefore determined both by the
 ethylenically unsaturated polymer or prepolymer employed and by the
 reactive diluent. Furthermore, for optimum coating properties, it is
 necessary to harmonize the ethylenically unsaturated polymers or
 prepolymers with the reactive diluents.
 Because of their advantageous processing properties, radiation-curable
 formulations have become established for the coating of wood, paper and
 plastics in the interior applications segment. Exterior application, on
 the other hand, is still tied up with problems because of the high caliber
 of requirements it imposes on the weathering stability of such
 radiation-curable formulations.
 The prior art discloses a large number of radiation-curable formulations
 which comprise, as monomers, oligomers or reactive diluents,
 multifunctional acrylates based on diols or polyols. For instance, GB-B-1
 138 117 describes radiation-curable coating materials based on unsaturated
 polyester resins, with styrene and methyl methacrylate as reactive
 diluent. JP 62 110 779 and JP 62 132 568 likewise disclose
 radiation-curable coating materials, based on urethane oligomers and
 trifunctional reactive diluents. A series of further patents describes the
 use of such radiation-curable formulations for optical applications,
 examples being U.S. Pat. No. 5,250,391 for holograms, EP-A-324 480 for
 refractive index imaging and JP 62 047 842 for optical disks. These
 formulations, however, are not intended for exterior applications, or
 comprise radiation-curable prepolymers or resins of high molecular mass.
 The use of 1,6-hexanediol diacrylate as a reactive diluent is known from WO
 92/17337 and DE 25 37 783 A. In formulations, it produces good gloss
 stability under weathering conditions (N. Round et al., Radiation Curing
 Conference Proceedings (1986)), and yet Radiat. Curing (1984), 11 (3),
 24-30, 32/3 describes how the physical properties of hydrocarbon-based
 diol diacrylates are poorer than those of glycol ether-based diol
 diacrylates, especially as regards hardness and abrasion characteristics.
 In addition, radiation-curable formulations based on aliphatic urethane
 acrylates and 1,6-hexanediol diacrylate show signs of incompatibility with
 substrates, for example, comprising plastics such as PMMA, polycarbonate
 and acrylonitrile-styrene polymers, said incompatibility being manifested
 in incipient dissolution of the substrate and an associated deterioration
 in the properties.
 A fundamental problem with the prior art radiation-curable compositions is
 that, although individual application properties such as coating hardness,
 flexibility and weathering stability can be improved by selecting and
 harmonizing the components (prepolymer and reactive diluent), this is
 always at the expense of other application properties. It is an object of
 the present invention, therefore, to provide radiation-curable
 compositions which, without the addition of inert organic solvents,
 combine good processing properties with good properties of the coating. In
 particular, it is intended that the formation should have good reactivity
 and high substrate compatibility in conjunction with good mechanical
 strength and high weathering stability and chemical resistance.
 We have found that this object can, surprisingly, be achieved by a
 radiation-curable formulation which comprises at least one prepolymer
 having at least two olefinic double bonds per molecule and at least one
 difunctional ester of .alpha.,.beta.-ethylenically unsaturated carboxylic
 acids with diols having a linear alkylene chain which has 7 to 14 carbon
 atoms.
 The present invention therefore provides radiation-curable formulations
 which comprise
 i) at least one prepolymer which comprises at least two olefinic double
 bonds per molecule (component A), and
 ii) at least one diester of .alpha.,.beta.-ethylenically unsaturated
 carboxylic acid with diols having a linear alkylene chain which has 7 to
 14 carbon atoms (component B).
 In general, the compositions of the invention contain from 20 to 90% by
 weight, preferably from 30 to 80% by weight and, in particular, from 40 to
 70% by weight of component A, from 10 to 80% by weight, preferably from 20
 to 60% by weight and, in particular, from 30 to 50% by weight of component
 B, and up to 20% by weight, based on the overall weight of components A
 and B, of customary additives.
 In general, component A is composed of one or more structural elements
 which function as carriers for the structural units comprising olefinic
 double bonds. Suitable structural elements include aliphatic urethanes
 based on diisocyanates and suitable monools, diols and polyols, dimers and
 trimers of diisocyanates, melamine-formaldehyde adducts, polyetherpolyols,
 cooligomers of unsubstituted or substituted vinylaromatic compounds with
 .alpha.,.beta.-ethylenically unsaturated carboxylic acids and/or their
 derivatives, and other groups suitable for synthesizing a prepolymer.
 A prepolymer (component A) having at least two unsaturated double bonds is
 obtained by combining the abovementioned structural elements with suitable
 structural units carrying .alpha.,.beta.-ethylenically unsaturated double
 bonds. Such structural units include, in particular, vinyl- or
 allyl-functional hydroxyalkanes, vinyl or allyl esters of aliphtic,
 functionalized carboxylic acids, and esters of
 .alpha.,.beta.-ethylenically unsaturated carboxylic acids with diols and
 polyols.
 Accordingly, examples of suitable prepolymers (components A) are urethane
 oligomers, preferably aliphatic urethane oligomers having at least two
 olefinic double bonds, with aliphatic structural elements comprising not
 only alkylene groups, preferably those having 4 to 10 carbon atoms, but
 also cycloalkylene groups having preferably 6 to 20 carbon atoms, it being
 possible for both the alkylene and the cycloalkylene units to be
 substituted one or more times by C.sub.l -C.sub.4 -alkyl, especially
 methyl, or to include one or more nonadjacent oxygen atoms.
 Such prepolymers with urethane and/or urea groups that are suitable as
 components A are obtainable by reacting di-, tri- and polyfunctional
 isocyanate compounds with .alpha.,.beta.-ethylenically unsaturated
 compounds which in addition have at least one isocyanate-reactive group,
 examples being OH groups or NH groups.
 The difunctional isocyanate compounds are usually selected from aliphatic,
 aromatic and cycloaliphatic diisocyanates. In general, they have 4 to 22
 carbon atoms. In particular, they comprise tetramethylene diisocyanate,
 hexamethylene diisocyanate, (1,6-diisocyanatohexane), octamethylene
 diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate,
 tetradecamethylene diisocyanate, trimehylhexane diisocyanate or
 tetramethylhexane diisocyanate, 1,4-, 1,3- or
 1,2-diisocyanato-cyclohexane, 4,4'-di(isocyanatocyclohexyl)methane,
 1-isocyanato-3,3,5-trimethyl-5-(isocyanatomethyl)cyclohexane (isophorone
 diisocyanate), 2,4- or 2,6-diisocyanato-1-methyl-cyclohexane,
 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, tetramethylxylylene
 diisocyanate, 1,4-diisocyanatobenzene, 4,4'- and
 2,4-diisocyanatodiphenylmethane, p-xylylene diisocyanate, and
 isopropenyldimethyltolylene diisocyanate. Preference is given, however, to
 the aliphatic and cycloaliphatic diisocyanates. Suitable trifunctional
 isocyanate compounds A include both compounds having a defined empirical
 formula and having 3 NCO groups per molecule and oligomers of low
 molecular mass, having a number-average molecular weight M.sub.n of &lt;1000,
 which contain on average about 3.0 isocyanate groups per oligomer
 molecule. Particularly suitable trifunctional isocyanate compounds are the
 isocyanurates and biurets of said diisocyanates, and also the adducts of
 said diisocyanates with trifunctional alcohols such as glycerol,
 trimethylolethane, trimethylolpropane etc., or triamines. By reacting
 diisocyanates with polyols or polyamines it is possible in the same way to
 obtain polyfunctional isocyanate compounds. Preferred isocyanate compounds
 are the isocyanurates and biurets, especially the isocyanurates.
 Further examples of polyisocyanates are isocyanurate-functional
 polyisocyanates, biuret-functional polyisocyanates, urethane- and
 allophanate-functional polyisocyanates, oxadiazinetrione-functional
 polyisocyanates, uretonimine-modified polyisocyanates, or mixtures
 thereof. The isocyanurate-functional polyisocyanates are, in particular,
 simple trisisocyanato isocyanurates - cyclic trimers of the
 diisocyanates--or mixtures of their higher homologs, having more than one
 isocyanurate ring.
 In general, .alpha.,.beta.-ethylenically unsaturated structural units for
 reaction with the di-, tri- and polyfunctional isocyanate compounds to
 form .alpha.,.beta.-ethylenically unsaturated prepolymers as components A
 are selected from the esters of ethylenically unsaturated carboxylic acids
 such as acrylic, methacrylic, crotonic, acrylamidoglycolic,
 methacrylamidoglycolic and vinylacetic acid with a diol or polyol having
 preferably 2 to 20 carbon atoms, such as ethylene glycol, diethylene
 glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol,
 diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene
 glycol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol,
 2-methyl-1,5-pentanediol, 2-ethyl-1,4-butanediol,
 1,4-dimethylolcyclohexane, glycerol, trimethylolethane,
 trimethylolpropane, trimethylolbutane, pentaerythritol,
 ditrimethylolpropane, erythritol and sorbitol, with the proviso that the
 ester has at least one isocyanate-reactive OH group. Further candidates
 for use are the amides of the abovementioned ethylenically unsaturated
 carboxylic acids with amino alcohols, such as 2-aminoethanol,
 3-amino-1-propanol, 1-amino-2-propanol or 2-(2-aminoethoxy)ethanol, and
 also the vinyl, allyl and methallyl ethers of the abovementioned diols and
 polyols, provided they still have one free OH group. Preference is given
 to the esters of acrylic or methacrylic acid, such as 2-hydroxyethyl and
 hydroxypropyl (meth)acrylates, 1,4-butanediol and neopentyl glycol
 mono(meth)acrylates, trimethylolpropane mono- and di(meth)acrylate and
 pentaerythritol di- and tri(meth)acrylate. With particular preference, the
 esters are selected from 2-hydroxyethyl acrylate, hydroxypropyl acrylate,
 and 1,4-butanediol monoacrylate. Examples of amides of ethylenically
 unsaturated carboxylic acids with amino alcohols are
 2-hydroxyethylacrylamide and -methacrylamide, 2- and 3-hydroxypropyl
 (meth)acrylamide, and 5-hydroxy-3-oxopentyl (meth)acrylamide. Particular
 preference as component A is given, accordingly, to urethane acrylates and
 urethane methacrylates of isocyanurates and biurets, especially urethane
 acrylates formed from isocyanurates or biurets and hydroxyethyl acrylate,
 hydroxypropyl acrylate and 1,4-butanediol monoacrylate. Very particular
 preference as components A is given to said urethane acrylates and
 urethane methacrylates when they have on average from two to three
 olefinic double bonds per molecule.
 In general, the components A based on urethane derivatives or urea
 derivatives have no free isocyanate groups. This is customarily achieved
 by reacting the NCO groups of the isocyanate compounds with an at least
 equimolar amount of OH and/or NH groups, with at least 0.3 mol of OH
 and/or NH groups per mole of isocyanate groups being contributed by the
 .alpha.,.beta.-ethylenically unsaturated structural units. Customarily,
 the proportion of OH and/or NH groups of the .alpha.,.beta.-ethylenically
 unsaturated structural units among the total of OH and/or NH groups
 reacted lies within the range from 30% to 100%, in particular from 50% to
 90% and, with very particular preference, in the range from 60% to 80%.
 Accordingly, from 0 to 70%, preferably from 10 to 50% and, with particular
 preference, from 20 to 40% of the total number of OH and/or NH groups
 reacted with isocyanate groups are contributed by saturated amine or
 alcohol components. Examples of suitable saturated alcohol components are
 linear monools, diols and polyols of 1 to 14 carbon atoms, branched
 monools, diols and polyols of 3 to 20 carbon atoms, and cyclic monools,
 diols and polyols of 3 to 14 carbon atoms, with preference being given to
 methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol and
 tert-butanol. Examples of suitable amines include primary monoamines,
 exemplified by C.sub.1 -C.sub.20 -alkylamines such as n-butylamine,
 n-hexylamine, 2-ethylhexyl-1-amine and 1-octadecylamine, and amines having
 cycloaliphatic, heterocyclic or (hetero)aromatic structural elements, such
 as benzylamine, 1-(3-aminopropyl)imidazole and tetrahydrofurfurylamine.
 Mention should also be made of compounds having two primary amino groups,
 examples being C.sub.1 -C.sub.20 -alkylenediamines such as
 ethylenediamine, butylenediamine, 1,5-diamino-3-oxopentane etc. Preference
 is given to secondary amines, examples being di-C.sub.1 -C.sub.20
 -alkylamines, such as diethylamine, di-n-propylamine, di-n-butylamine,
 diethanolamine, dicyclohexylamine, bis-2-ethylhexylamine, diallylamine and
 N-ethylethanolamine. Likewise preferred are heterocyclic secondary amines
 in which the NH group is located on the heterocycle, such as pyrrolidine,
 piperidine, piperazine, N-methylpiperazine, morpholine and
 2,2,6,6-tetramethylpiperidine. Further suitable prepolymers (components A)
 are ethylenically unsaturated melamine resins, such as the reaction
 products of melamine/formaldehyde condensates with OH-containing
 unsaturated compounds, ethylenically unsaturated dicarboxylic anhydrides,
 or the amides of ethylenically unsaturated monocarboxylic acids. Suitable
 melamine-formaldehyde condensates are, in particular, hexamethylolmelamine
 (HMM) and hexamethoxymethylolmelamine (HMMM). Suitable OH-containing
 unsaturated compounds include, for example, the hydroxy-C.sub.2 -C.sub.4
 -alkyl esters of ethylenically unsaturated carboxylic acids, especially
 those of acrylic and methacrylic acid. Other suitable candidates for
 reaction with HMM are ethylenically unsaturated alcohols, such as allyl
 alcohol or crotyl alcohol, or ethylenically unsaturated dicarboxylic
 anhydrides, such as maleic anhydride. In addition, both HMM and HMMM can
 be modified with amides of ethylenically unsaturated carboxylic acids,
 such as acrylamide or methacrylamide, to give ethylenically unsaturated
 melamine resins. Preference is given to melamine derivatives whose
 ethylenically unsaturated structural units comprise acryloyloxy or
 methacryloyloxy groups.
 Other suitable prepolymers (components A) include unsaturated epoxy resins
 and epoxy resin derivatives. The latter include, in particular, the
 products of reaction of epoxy-functional compounds or oligomers with
 ethylenically unsaturated monocarboxylic acids such as acrylic,
 methacrylic, crotonic, and cinnamic acid. Instead of or together with the
 esters of monocarboxylic acids it is also possible to employ the
 monoesters of ethylenically unsaturated dicarboxylic acids with
 monoalcohols such as methanol, ethanol, n-propanol, isopropanol,
 n-butanol, tert-butanol, n-hexanol and 2-ethylhexanol. Suitable
 epoxy-functional substrates include in particular the polyglycidyl ethers
 of polyhydric alcohols. These include the diglycidyl ethers of bisphenol A
 and its derivatives, the diglycidyl ethers of oligomers of bisphenol A as
 are obtainable by reacting bisphenol A with the diglycidyl ether of
 bisphenol A, and the polyglycidyl ethers of novolaks. The reaction
 products of the ethylenically unsaturated carboxylic acids with the
 glycidyl ethers that are present can be modified with primary or secondary
 amines. Furthermore, by reacting OH groups in epoxy resins with suitable
 derivatives of ethylenically unsaturated carboxylic acids, such as the
 acid chlorides, further ethylenically unsaturated groups can be introduced
 into the epoxy resin. Ethylenically unsaturated epoxy resins are
 sufficiently well known to the skilled worker and are obtainable
 commercially. For further details reference is made to P. K. T. Oldring,
 pp. 37 to 68 and the literature cited therein. Prepolymers (components A)
 which are likewise suitable are unsaturated polyester prepolymers or
 polyester resins, and also the ethylenically modified polyesters that are
 synthesized by derivatizing the free OH groups in conventional polyesters
 with .alpha.,.beta.-ethylenically unsaturated structural units such as
 vinyl- or allyl-functional hydroxyalkanes, vinyl or allyl esters of
 aliphatic, functionalized carboxylic acids, and esters of
 .alpha.,.beta.-ethylenically unsaturated carboxylic acids with diols and
 polyols, and also the so-called ethylenically unsaturated polyesters that
 are obtainable by copolycondensation of conventional dicarboxylic acids
 together with ethylenically unsaturated dicarboxylic acids and/or their
 anhydrides and low molecular mass diols.
 The so-called ethylenically unsaturated polyesters include in particular
 the copolycondensates of maleic anhydride with at least one further
 dicarboxylic acid and/or its anhydride, and a low molecular mass diol. In
 this case, the dicarboxylic acids and their anhydrides are preferably
 selected from succinic acid, succinic anhydride, glutaric acid, glutaric
 anhydride, adipic acid, phthalic acid, terephthalic acid, isophthalic
 acid, and especially phthalic anhydride. Suitable diols are preferably
 selected from ethylene glycol, 1,2-propylene glycol, 1,4-butanediol,
 1,5-pentanediol, neopentyl glycol and 1,6-hexanediol, especially
 1,2-propylene glycol.
 Suitable hydroxyl-containing polyesters for modification to ethylenically
 modified polyesters can be prepared in a conventional manner by
 polycondensation of dibasic or polybasic carboxylic acids with dihydric
 alcohols and/or with at least one further polyfunctional alcohol
 component. Suitable carboxylic acid components in this case are aliphatic
 and/or aromatic C.sub.3 -C.sub.36 -carboxylic acids, their esters and
 anhydrides. They include succinic acid, succinic anhydride, glutaric acid,
 glutaric anhydride, adipic acid, pimelic acid, suberic acid, azelaic acid,
 sebacic acid, phthalic acid, phthalic anhydride, isophthalic acid,
 terephthalic acid, tetrahydrophthalic acid, tetrahydrophthalic anhydride,
 trimellitic acid, trimellitic anhydride, pyromellitic acid, and
 pyromellitic anhydride. Examples of suitable diol components are 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 polymers of cyclic ethers, such as
 polytetrahydrofuran, polyethylene glycol and polypropylene glycol.
 Suitable polyhydric alcohols are, in particular, trihydric to hexahydric
 alcohols, such as glycerol, trimethylolethane, trimethylolpropane,
 trimethylolbutane, pentaerythritol, ditrimethylolpropane, sorbitol,
 erythritol, and 1,3,5-trihydroxybenzene. In the polycondensation step, if
 the total number of moles of the OH groups in the alcohol component is
 greater than the total number of moles of the carboxyl groups in the acid
 component, the product is a polyester which contains OH groups. These OH
 groups can be esterified conventionally with the abovementioned
 ethylenically unsaturated carboxylic acids, especially with acrylic acid
 and methacrylic acid, in accordance with conventional techniques. An
 alternative embodiment relates to polyesters formed from diols,
 dicarboxylic acids and at least one polybasic carboxylic acid. In this
 case, the hydroxyl groups are introduced into the polyester subsequently,
 by reacting the carboxylic acid groups with alkylene oxides, such as
 ethylene oxide or propylene oxide. These alcohol functions can then be
 etherified or esterified in the manner described above. For further
 details reference may be made to P. K. T. Oldring, pp. 123 to 135. The
 abovementioned products are sufficiently well known to the skilled worker
 and are obtainable commercially. Their number-average molecular weight
 lies in general within the range from 500 to 10,000 and, preferably, in
 the range from 800 to 3000.
 Other suitable ethylenically modified polyesters are those obtainable by
 cocondensation of conventional di- or polycarboxylic acids with
 conventional alcohol components and ethylenically unsaturated
 monocarboxylic acids, preferably acrylic and/or methacrylic acid. Such
 polyesters are known, for example, from EP-A 279 303, to which reference
 is made for further details. In this case, introduction of the
 ethylenically unsaturated groups into the polyester takes place as early
 as during the synthesis of the polyester from the low molecular mass
 components.
 Further suitable prepolymers (components A) are (meth)acrylate-based
 copolymers containing ethylenically unsaturated structural elements. Such
 ethylenically unsaturated polymers are obtainable in general by means of
 polymer-analogous reactions of functionalized polymers (polymers FP)
 having free hydroxyl, carbonyl, carboxyl, isocyanate, amino and/or epoxy
 groups. In general, the ethylenic double bonds are introduced by reaction
 with suitable ethylenically unsaturated compounds of low molecular mass
 that have a functional group which is able to react with the reactive
 group in the polymer and which, in doing so, gives rise to formation of a
 bond.
 The functionalized polymers P used as starting materials are generally
 obtainable by free-radical addition polymerization of at least one
 ethylenically unsaturated monomer having such a functional group and, if
 desired, other ethylenically unsaturated monomers. In general, the
 ethylenically unsaturated monomers having a functional group make up from
 5 to 50 mol %, preferably from 15 to 40 mol % and, in particular, from 20
 to 35 mol % of the total monomers to be polymerized. Examples of monomers
 having a functional group are the hydroxyalkyl acrylates and
 methacrylates, such as 2-hydroxyethyl (meth)acrylate, hydroxypropyl
 (meth)acrylate and 4-hydroxybutyl (meth)acrylate, aminoalkyl acrylates and
 methacrylates, such as 2-aminoethyl (meth)acrylate, carbonyl compounds,
 such as acrolein, methacrolein, vinyl ethyl ketone,
 N,N-diacetonylacrylamide and -methacrylamide, vinyl isocyanate,
 dimethyl-3-isopropenylbenzyl isocyanate, 4-isocyanatostyrene, and
 isocyanates of ethylenically unsaturated carboxylic acids, e.g.,
 methacryloyl isocyanate, .omega.-isocyanatoalkyl (meth)acrylates, glycidyl
 compounds, such as glycidyl allyl ether and glycidyl methallyl ether, the
 glycidyl esters of ethylenically unsaturated carboxylic acids, such as
 glycidyl (meth)acrylate, ethylenically unsaturated anhydrides, such as
 maleic anhydride and methacrylic anhydride, and the amides of
 ethylenically unsaturated carboxylic acids, such as acrylamide and
 methacrylamide. Suitable comonomers are generally selected from the esters
 of acrylic and methacrylic acid and, if desired, from vinylaromatic
 compounds. Examples of suitable comonomers are the C.sub.1 -C.sub.4
 acrylates and methacrylates, such as methyl, ethyl, n-propyl, isopropyl,
 n-butyl, isobutyl and tert-butyl (meth)acrylates. Further suitable
 comonomers are styrene, 1-methylstyrene, 4-tert-butylstyrene, and
 2-chlorostyrene. To a minor extent it is also possible to use monomers
 such as vinyl acetate, vinyl propionate, vinyl chloride, vinylidene
 chloride, conjugated dienes, such as butadiene and isoprene, vinyl ethers
 of C.sub.1 -C.sub.20 -alkanols, such as vinyl isobutyl ether,
 acrylonitrile, methacrylonitrile, and heterocyclic vinyl compounds, such
 as 2-vinylpyridine and N-vinylpyrrolidone. In one preferred embodiment the
 comonomers comprise at least one monomer selected from the esters of
 methacrylic acid, especially methyl methacrylate, and at least one further
 comonomer selected from the alkyl esters of acrylic acid and/or styrene.
 The ethylenically unsaturated compounds having a functional group which are
 suitable for the polymer-analogous reaction are preferably selected from
 the abovementioned ethylenically unsaturated monomers having a functional
 group. A precondition is that the functionality of the ethylenically
 unsaturated compound is able to react with the functionalities on the
 polymer and, in doing so, to give rise to formation of a bond with the
 polymer. Suitable types of reaction in this context are condensation and
 addition reactions. Examples of suitable pairs of functionalities are
 those such as isocyanate/hydroxyl, isocyanate/amino, anhydride/hydroxyl,
 anhydride/amino, carbonyl/amino, carboxylic chloride/hydroxyl,
 glycidyl/hydroxyl, glycidyl/amino or amido, and glycidyl/carboxyl. In one
 preferred embodiment the ethylenically unsaturated polymer is obtainable
 by reacting a functionalized polymer which has glycidyl groups with
 ethylenically unsaturated compounds that contain hydroxyl groups,
 especially the hydroxyalkyl esters of the abovementioned ethylenically
 unsaturated carboxylic acids, an example being 2-hydroxyethyl acrylate.
 Examples of such ethylenically unsaturated polymers are given in EP-A 650
 979, the full extent of the disclosure of which is incorporated herein by
 way of reference.
 Component B of the radiation-curable formulation is at least one
 difunctional ester of .alpha.,.beta.-ethylenically unsaturated carboxylic
 acids with diols having a linear alkylene chain which has 7 to 14 carbon
 atoms. In general, such difunctional olefinically unsaturated esters of
 low molecular mass are referred to as reactive diluents. Their low
 viscosity makes it possible to obtain low-solvent and, in particular,
 solvent-free formulations, based on prepolymers, which nevertheless have
 good applications properties. In the course of radiation-induced curing,
 the reactive diluents are incorporated into the coating, where their
 difunctionality gives them an additional crosslinking action.
 The reactive diluents (component B) are of the formula I:
 ##STR1##
 where R.sup.1 independently at each occurrence is H, CH.sub.3 or CH.sub.2
 -COOX,
 R.sup.2 independently at each occurrence is H, CH.sub.3 or COOX,
 X is C.sub.1 -C.sub.12 -alkyl, and
 m is an integer from 7 to 14.
 Accordingly, the .alpha.,.beta.-ethylenically unsaturated carboxylic acids
 of component B can be selected, independently of one another, from
 monocarboxylic acids, such as acrylic, methacrylic and crotonic acid, and
 from dicarboxylic acids, such as maleic and fumaric acid. Also suitable
 are vinylacetic, mesaconic and citraconic acids, although particular
 preference is given to acrylic and methacrylic acid. In accordance with
 the formula I, the ester-forming diols are preferably selected from linear
 .alpha.,.omega.-diols having alkylene groups which contain 7 to 14 carbon
 atoms, such as 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol,
 1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol, 1,13-tridecanediol,
 and 1,14-tetradecanediol. Preference is given to 1,8-octanediol,
 1,9-nonanediol, 1,10-decanediol, and 1,12-dodecanediol, and particular
 preference to 1,10-decanediol. Particularly preferred components B,
 accordingly, are 1,8-octanediol diacrylate, 1,9-nonanediol diacrylate,
 1,10-decanediol diacrylate and 1,12-dodecanediol diacrylate, and also
 1,8-octanediol dimethacrylate, 1,9-nonanediol dimethacrylate,
 1,10-decanediol dimethacrylate, and 1,12-dodecanediol dimethacrylate.
 The radiation-curable formulations of the invention can be present in the
 form of liquid or flowable formulations of the components A and B, as
 solutions or dispersions, for example, and, provided the components are
 liquid, can also be present in pure form.
 In addition, it can also be present in the form of pulverulent formulations
 as used, for example, for the powder coating of metallic surfaces. Also
 suitable are hotmelt formulations, which acquire the ability to flow only
 at elevated temperature. Depending on the type of recipe, the
 radiation-curable formulations comprise customary auxiliaries, such as
 thickeners, leveling assistants, defoamers, UV stabilizers, emulsifiers
 and/or protective colloids, and fillers. Suitable auxiliaries are known to
 the skilled worker from paint and coating technology. Suitable fillers,
 especially for aqueous dispersions of components A and B, include
 silicates obtainable by hydrolysis of silicon tetrachloride (Aerosil.RTM.
 from Degussa), siliceous earth, talc, aluminum silicates, magnesium
 silicates, calcium carbonates, etc. Suitable stabilizers embrace typical
 UV absorbers, such as oxanilides, triazines, benzotriazoles (obtainable as
 Tinuvin.RTM. grades from Ciba Geigy), and benzophenones. These can be
 employed in combination with customary free-radical scavengers, examples
 being sterically hindered amines, such as 2,2,6,6-tetramethylpiperidine
 and 2,6-di-tert-butylpiperidine (HALS compounds). Stabilizers are employed
 customarily in amounts from 0.1 to 5.0 and, preferably, from 0.5 to 2.5%
 by weight, based on the polymerizable components of the formulation.
 Insofar as curing takes place by means of UV radiation, the formulations
 for use in accordance with the invention include at least one
 photoinitiator. A distinction can be made here between photoinitiators for
 free-radical curing mechanisms (addition polymerization of ethylenically
 unsaturated double bonds) and photoinitiators for cationic curing
 mechanisms (cationic addition polymerization of ethylenically unsaturated
 double bonds or of compounds that include epoxy groups). When curing is
 carried out by means of irradiation with high-energy electrons (electron
 beam curing) the use of photoinitiators can be omitted. When electron beam
 curing is employed, the formulations of the invention may also comprise
 colored pigments.
 Suitable photoinitiators for cationic photopolymerizations, i.e., the
 polymerization of vinyl compounds or compounds that include epoxy groups,
 are, for example, aryldiazonium salts, such as 4-methoxybenzenediazonium
 hexafluorophosphate, benzenediazonium tetrafluoroborate and
 toluenediazonium tetrafluoroarsenate, aryliodonium salts, such as
 diphenyliodonium hexafluoroarsenate, arylsulfonium salts, such as
 triphenylsulfonium hexafluorosphosphate, benzene- and toluenesulfonium
 hexafluorophosphate, and bis[4-diphenylsulfoniophenyl] sulfide
 bishexafluorophosphate, disulfones, such as diphenyl disulfone and phenyl
 4-tolyl disulfone, diazodisulfones, imidotriflates, benzoin tosylates,
 isoquinolinium salts, such as N-ethoxyisoquinolinium hexafluorophosphate,
 phenylpyridinium salts, such as N-ethoxy-4-phenylpyridinium
 hexafluorophosphate, picolinium salts, such as N-ethoxy-2-picolinium
 hexafluorophosphate, ferrocenium salts, titanocenes, and titanocenium
 salts.
 The photoinitiators are employed in amounts from 0.05 to 20% by weight,
 preferably from 0.1 to 10% by weight and, in particular, from 0.1 to 5% by
 weight, based on the polymerizable componets of the formulations of the
 invention.
 Formulations of the invention are found to be particularly suitable for
 coating substrates such as wood, paper, plastic surfaces, mineral building
 materials such as molded cement slabs and fiber cement slabs, and, in
 particular, coated and uncoated metals.
 The present invention, accordingly, also provides a process for coating
 substrates, especially coated or uncoated metals, and the coated
 substrates obtainable by this process. The substrates are generally coated
 by applying the desired thickness of at least one radiation-curable
 formulation of the invention to the substrate that is to be coated,
 removing any solvent present, and then curing the applied formulation by
 exposure to high-energy radiation such as UV radiation or electron beams.
 If desired, this procedure can be repeated one or more times. The
 radiation-curable formulations are applied to the substrate in a known
 manner, for example, by spraying, troweling, knife coating, brushing,
 rolling or pouring. The coating thickness is generally in the range from 3
 to 500 g/m.sup.2 and, preferably from 10 to 200 g/m.sup.2, corresponding
 to wet film thicknesses from about 3 to 500 .mu.m, preferably from 10 to
 200 .mu.m. Application can take place either at room temperature or at
 elevated temperature, but preferably not above 100.degree. C.
 Subsequently, the coatings are cured by exposure to high-energy radiation,
 preferably UV radiation with a wavelength from 250 to 400 nm, or
 high-energy electrons (electron beams; from 150 to 300 keV). Examples of
 UV sources used are high-pressure mercury vapor lamps, such as CK or CK1
 lamps from IST. The radiation dose usually sufficient for crosslinking in
 the case of UV curing lies within the range from 80 to 3000 mJ/cm.sup.2.
 In one preferred process, curing takes place continuously by conveying the
 substrate, treated with the formulation of the invention, past a radiation
 source at a constant rate. This requires the curing rate of the
 formulation of the invention to be sufficiently high.
 Because of their high reactivity, the formulations of the invention are
 easy to process. They are particularly suitable for coating the
 abovementioned substrates, and give rise to coatings having high
 mechanical strength, high weathering stability and high chemical
 resistance.
 Accordingly, the present invention additionally provides for the use of the
 radiation-curable formulations of the invention for coating substrates,
 especially their use as topcoats, or for preparing topcoats, for multicoat
 automotive finishing systems.
 The examples given below are intended to illustrate the present invention
 but without restricting it.

EXAMPLES
 I. Preparing the Radiation-curable Formulations
 1.) Preparing the formulations B1, B2 and B3 of the invention
 4% by weight of the photoinitiator Darocure 1173 (Ciba-Geigy) and 30 parts
 of the reactive diluents of the invention are added in each case to
 portions of 70 parts of a urethane acrylate prepared by reacting 75% of
 the isocyanate groups of the isocyanurate of hexamethylene diisocyanate
 with hydroxyethyl acrylate and 25% of NCO groups with methanol. In this
 way, Example B1 is obtained with 1,10-decanediol diacrylate, Example B2
 with 1,8-octanediol diacrylate and Example B3 with 1,9-nonanediol
 diacrylate.
 2.) Preparing the formulation B4 of the invention
 50 parts of 1,10-decanediol diacrylate and 4% by weight of the
 photoinitiator Darocure 1173 (Ciba-Geigy) are added to 50 parts of
 urethane acrylate prepared by reacting 75% of the isocyanate groups of the
 isocyanurate of hexamethylene diisocyanate with hydroxyethyl acrylate and
 25% of the isocyanate groups with methanol.
 3.) Preparing Comparative Example CB1
 The preparation is as described under 1.), but using 1,6-hexanediol
 diacrylate as reactive diluent.
 4.) Preparing Comparative Example CB2
 The preparation is as described under 2.), but using 1,6-hexanediol
 diacrylate as reactive diluent.
 The radiation-curable formulations described under 1.) to 4.) are flowable
 at room temperature and have viscosities &lt;10 Pas.
 5.) Preparing Comparative Example CB3
 The formulation is prepared as under 1.), but using 4-tert-butylcyclohexyl
 acrylate as reactive diluent. The resultant formulation, CB3, is turbid
 owing to the incompatibility of the components and is therefore unsuitable
 as a radiation-curable formulation, in particular as a clearcoat.
 II. Determining the Reactivity and Mechanical Properties of Cured Coatings
 of the Formulations B1, B2, B3 and CB1
 1.) Determining the reactivity
 The reactivity is stated in m/min and corresponds to the rate at which a
 substrate treated with a radiation-curable formulation in a wet film
 thickness of 100 .mu.m can be conveyed at a distance of 10 cm past a UV
 source with an output of 120 W/cm such that complete curing still takes
 place. Complete curing is tested with the fingernail test. The rate
 reported is the fastest rate at which no impressions are left in the
 coating when scratched with the fingernail. The results are summarized in
 Table 1.
 2.) Determining the coating hardness
 The coating hardness was characterized by determining the pendulum
 attenuation in accordance with DIN 53157. For this purpose, the
 radiation-curable compositions of the inventive and comparative examples
 were applied with a wet film thickness of 100 .mu.m to glass. The
 resultant sample was cured by passing it twice at a distance of 10 cm in
 front of a high-pressure mercury vapor lamp (120 W/cm) with a belt speed
 of 10 m/min. Subsequently, the pendulum attenuation is determined using a
 pendulum instrument in accordance with DIN 53157 (Konig). The result is
 given in seconds. The results are summarized in Table 1.
 3.) Determining the flexibility
 The flexibility of the coating was determined by measuring the Erichsen
 indentation in accordance with DIN 53156. This was done by applying the
 respective formulation in a wet film thickness of 50 .mu.m to BONDER panel
 132 using a spiral-wound coating bar. Curing was carried out in the manner
 described above by exposure with a high-pressure mercury vapor lamp (120
 W/cm). Subsequently, the Erichsen indentation was determined by pressing a
 metal ball into the uncoated side of the metal panel (DIN 53156). The
 results are summarized in Table 1.
 TABLE 1
 Pendulum Erichsen
 Reactivity attenuation indentation
 Example [m/min] [sec] [mm]
 B1 15 145 3.4
 B2 15 154 2.4
 B3 15 153 2.0
 CB1 15 149 1.5
 III. Compatibility Testing of the Radiation-curable Formulations B1 and CB1
 on plastics surfaces
 An important field of application of radiation-curable formulations is the
 protection of plastics surfaces against weathering. In this field it is an
 advantage for the plastics surface to be attacked as little as possible by
 the formulations. To determine the compatibility, the formulations B1 and
 CB1 are gripped onto surfaces of polymethyl methacrylate (PMMA) and
 acrylonitrile-polystyrene-acrylate copolymer (Luran S) and the alteration
 (surface damage) of the surfaces is assessed as a function of time (see
 Table 2).
 TABLE 2
 Exposure Assessment of surface damage
 Surface time B1 CB1
 PMMA 1 h none slight incipient
 swelling
 2 h none formation of an
 annular margin
 3 h none incipient swelling
 Luran S 1 h slight incipient severe incipient
 dissolution dissolution
 2 h slight incipient severe incipient
 dissolution dissolution
 3 h slight incipient fully corroded
 dissolution
 IV. Determining the Chemical Resistance of the Cured Coatings of the
 Formulations B1 and CB1 and B4 and CB2
 In order to assess the chemical resistance, coatings of the formulations
 B1, CB1, B4 and CB2 were prepared as described below and then various
 chemicals were applied to the surfaces of the coatings and left to act for
 24 hours in each case at different temperatures. The temperature stated is
 the upper temperature starting from which damage to the coating surface is
 observed.
 TABLE 3
 B4.sup.1) CB2.sup.1)
 Chemical [.degree. C.] [.degree. C.]
 Sulfurous acid (6%) 70 66
 Pancreatin + H.sub.2 O 77 77
 Sodium hydroxide solution (5%) 77 49
 Sulfuric acid (10%) 63 56
 Water 77 77
 .sup.1) Formulations B4 and CB2 were applied with a wet film thickness of
 100 .mu.m to a substrate, exposed with a
 # high-pressure mercury lamp (120 W/cm) at a rate of 10 m/min, then
 heated to 100.degree. C. and again exposed at a rate of 10 m/min.
 Formulations B4 and CB2 were applied with a wet film thickness of 100 .mu.m
 to a substrate, exposed with a high-pressure mercury lamp (120 W/cm) at a
 rate of 10 m/min, then heated to 100.degree. C. and again exposed at a
 rate of 10 m/min.