The present invention relates to radiation-curable formulations which comprise at least one aliphatic, urethane-functional prepolymer having at least two ethylenically unsaturated double bonds and at least one monofunctional ester of an α,β-ethylenically unsaturated carboxylic acid with a monofunctional alkanol, said prepolymer having as a structural element at least one saturated 5- or 6-membered carbocycle or one 5- or 6-membered heterocycle with one or two oxygens in the ring.
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 the action of 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 if the radiation-curable composition employed comprises formulations that contain an ethylenically unsaturated, urethane-functional polymer or prepolymer. Ethylenically unsaturated urethane-functional polymers and prepolymers are known, for example, 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, pp. 73-123. Because of the high viscosity of ethylenically unsaturated, urethane-functional polymers and prepolymers, such compositions are often admixed with ethylenically unsaturated compounds of low molecular mass in order to reduce the viscosity. These compounds, like the ethylenically unsaturated polymers and prepolymers, are polymerized in the course of curing and so incorporated into the coating. They are therefore referred to as reactive diluents. Hence the properties of the resulting coatings are determined both by the ethylenically unsaturated polymer or prepolymer employed and by the reactive diluent. For optimum coating properties, furthermore, it is necessary to harmonize the ethylenically unsaturated polymers or prepolymers with the reactive diluents.
DE-A-27 260 41 discloses radiation-curable compositions comprising at least one polyetherurethane which is modified at the ends with acrylate and/or methacrylate groups, a low molecular mass polyfunctional acrylate with ether groups, and hydroxyalkyl acrylates. Radiation-curable compositions of this kind lead to coatings having increased flexibility.
EP-A-508 409 discloses radiation-curable compositions which comprise at least one ethylenically unsaturated polyesterurethane and at least one nonaromatic, low molecular mass substance having at least one, preferably at least two, (meth)acryloyl groups, as crosslinker (=reactive diluent). Radiation-curable compositions of this kind lead to coatings having improved weathering stability.
A fundamental problem with the radiation-curable compositions of the prior art is that, although it is possible by selecting and harmonizing the components (prepolymer and reactive diluent) to improve individual in-use properties such as coating hardness, flexibility and weathering resistance, this is always at the expense of other properties.
It is an object of the present invention to provide radiation-curable compositions which lead to coatings having balanced profiles of properties, with great hardness, high flexibility and high weathering resistance, and which at the same time feature low application viscosity and a high curing rate.
We have found that this object is achieved, surprisingly, by a radiation-curable composition which comprises at least one aliphatic, urethane-functional prepolymer and at least one monofunctional ester of an α,β-ethylenically unsaturated carboxylic acid with a monofunctional alkanol which has as a structural element at least one saturated 5- or 6-membered carbocycle or one corresponding heterocycle having one or two oxygens in the ring.
The present invention consequently provides radiation-curable formulations which comprise
i) at least one aliphatic, urethane-functional prepolymer which on average has at least two ethylenically unsaturated double bonds per molecule (=component A),
ii) at least one monofunctional ester of an α,β-ethylenically unsaturated carboxylic acid with a monofunctional alkanol which has as a structural element at least one saturated 5- or 6-membered carbocycle or one saturated 5- or 6-membered heterocycle with one or two oxygens in the ring (=component B), and
iii) if desired, di- or polyfunctional esters of an α,β-ethylenically unsaturated carboxylic acid with an aliphatic di- or polyol (=component C).
In accordance with the invention, the aliphatic, urethane-functional prepolymer is free from aromatic structural elements such as phenylene or naphthylene or substituted derivatives thereof. Component B contains no nitrogens.
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, from 0 to 40% by weight and, preferably, from 0 to 30% by weight, of component C and up to 20% by weight, based on the overall weight of components A, B and C, of customary auxiliaries, with the proviso that the amounts by weight of components A, B and C add up to 100% by weight. In general, the weight of components B and C is in the range from 10 to 80% by weight, preferably from 20 to 70% by weight and, in particular, from 30 to 60% by weight, based in each case on the overall weight A+B+C.
Depending on the desired profile of properties the compositions of the invention comprise component B and component C or exclusively component B. Where high coating hardness is desired the formulation of the invention preferably comprises component B and component C. If instead greater value is placed on high flexibility, component C may be omitted. In addition, as the amount of component C increases, the viscosity of the formulations of the invention is improved. In the first case the ratio of component B to component C is preferably in the range from 20:1 to 1:1 and, in particular, in the range 10:1 to 1.5:1.
In general, component A is composed essentially of one or more aliphatic structural elements, urethane groups and at least two ethylenically unsaturated structural units. Aliphatic structural elements include both alkylene groups, preferably with 4 to 10 carbons, and cycloalkylene groups, preferably with 6 to 20 carbons. Both the alkylene and cycloalkylene groups can be substituted one or more times by C1-C4-alkyl, especially by methyl, and may include one or more nonadjacent oxygens. The aliphatic structural elements may be connected to one another by way of quaternary or tertiary carbons, by way of urea groups, biuret, uretdione, allophanate, cyanurate, urethane, ester or amide groups or by way of ether oxygen or amine nitrogen. Component A is preferably free from uretdione or allophanate groups and from amine nitrogen. Furthermore, component A in accordance with the invention has at least two ethylenically unsaturated structural elements. These are preferably vinyl or allyl groups, which can also be substituted by C1-C4-alkyl, especially methyl, and which are derived in particular from α,β-ethylenically unsaturated carboxylic acids and/or their amides. Particularly preferred ethylenically unsaturated structural units are acryloyl and methacryloyl groups, such as acrylamido and methacrylamido and, in particular, acryloxy and methacryloxy. With particular preference, component A has at least three ethylenically unsaturated structural elements per molecule.
Very particular preference is given to components A in which the aliphatic structural elements are linked by way of cyanurate, biuret and/or urethane groups and whose ethylenically unsaturated structural elements are acryloxy groups.
The number-average molecular weight Mn of the urethane-functional prepolymers of component A is preferably ≦2000 and is in particular in the range from 400 to 1500. The double bond density in such prepolymers is preferably above 1.5 mol/kg of prepolymer and, in particular, is in the range from 2 to 6 mol/kg of prepolymer.
Ethylenically unsaturated, urethane-functional prepolymers of this kind are fundamentally known to the skilled worker. Preferred aliphatic urethanes that are free of urea groups are obtainable, for example, by reacting
i) at least one aliphatic compound or one aliphatic prepolymer having at least two and preferably three or 4 isocyanate groups (component a1) with
ii) at least one compound which has at least one reactive OH group and at least one ethylenically unsaturated double bond (component a2) and, if desired,
iii) one or more aliphatic compounds having at least one reactive OH group (component a3).
In this case the ratio of the OH groups of components a2 and a3 to the NCO groups of component a1, OH/NCO, is ≧1, so that the resulting prepolymer contains no NCO groups. Component a2 is preferably employed in an amount such that the OH groups it contains (OHa2) are in a ratio to the NCO groups of component a1, OHa2/NCO, which is in the range from 0.4 to 0.95 and, preferably, from 0.6 to 0.9.
Compounds suitable as component a1 are aliphatic diisocyanates, oligomeric adducts of aliphatic diisocyanates with polyfunctional alcohols having preferably 2 to 20 carbons, and the uretdiones, isocyanurates, biurets and allophanates of aliphatic diisocyanates. Examples of suitable aliphatic diisocyanates 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) and 2,4- or 2,6-diisocyanato-1-methylcyclohexane. Suitable polyfunctional alcohols include aliphatic di- or polyols having preferably 2 to 20 carbons, such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene 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, dimethylolcyclohexane, glycerol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol, ditrimethylolpropane, erythritol and sorbitol. Component a1 is preferably selected from the trimerization products of the abovementioned aliphatic diisocyanates, in other words the biurets and the isocyanurates, and the adducts of the abovementioned aliphatic diisocyanates with one of the abovementioned polyfunctional aliphatic alcohols having at least three reactive OH groups. It is particularly preferred to employ as component a1 the isocyanurate and/or the biuret of hexamethylene diisocyanate and, with very particular preference, its isocyanurate.
Examples of suitable components a2 are the esters of ethylenically unsaturated carboxylic acids with one of the abovementioned aliphatic polyols and also the vinyl, allyl and methallyl ethers of these polyols, provided they also have one isocyanate-reactive OH group. It is also possible to employ the amides of ethylenically unsaturated carboxylic acids with amino alcohols. Preference as component a2 is given to the esters of acrylic and methacrylic acid, such as 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 1,4-butanediol mono(meth)acrylate, neopentyl glycol mono(meth)acrylate, trimethylolpropane di(meth)acrylate, pentaerythritol di- and -tri(meth)acrylate. With particular preference component a2 is selected from hydroxypropyl acrylate and butanediol monoacrylate, and in particular a2 is 2-hydroxyethyl acrylate.
Examples of suitable aliphatic compounds having at least one reactive OH group (component a3) are alkanols having preferably 1 to 10 carbons, cycloalkanols having preferably 5 to 10 carbons, and monoalkyl ethers of polyalkylene glycols. Examples of suitable alkanols are methanol, ethanol, n- and isopropanol, n-, 2-, iso- and tert-butanol, amyl alcohol, isoamyl alcohol, n-hexanol, n-octanol, 2-ethylhexanol and decanol. Suitable cycloalkanols include, for example, cyclopentanol and cyclohexanol, which are unsubstituted or substituted one or more times by C1-C4-alkyl, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, 2-butyl, isobutyl or tert-butyl, especially by methyl. Examples of monoalkyl ethers of polyalkylene glycols are the mono-C1-C4-alkyl ethers and, in particular, the methyl ethers of ethylene glycol, diethylene glycol or triethylene glycol.
Component A is prepared in a known manner by reacting component a1 with components a2 and, if used, a3 at from 0 to 100° C. and, in particular, at from 20 to 70° C. It is preferred to react component a1 and a2 first of all. Component a3 is added subsequently under reaction conditions.
To accelerate the reaction it is possible to employ catalysts as are described, for example, in Houben-Weyl, Methoden der Organischen Chemie, Vol. XIV/2, Thieme-Verlag, Stuttgart 1963, p. 60f. and Ullmanns Enzyklopädie der Technischen Chemie, 4th ed., Vol. 19 (1981), p. 306. Tin-containing catalysts are preferred, such as dibutyltin dilaurate, tin(II) octoate or dibutyltin dimethoxide. Such catalysts are generally employed in an amount of from 0.001 to 2.5% by weight, preferably from 0.005 to 1.5% by weight, based on the overall amount of the reactants.
To stabilize the free-radically polymerizable compounds (component a2) it is preferred to add to the reaction from 0.001 to 2% by weight, in particular from 0.005 to 1.0% by weight, of polymerization inhibitors. These are the usual compounds suitable for hindering free-radical polymerization, examples being hydroquinones or hydroquinone monoalkyl ethers, 2,6-di-tert-butylphenols, such as 2,6-di-tert-butylcresole, nitrosamines, phenothiazines or phosphorous esters. The reaction can be carried out either without solvent or with the addition of solvents. Suitable solvents are inert solvents such as acetone, methyl ethyl ketone, tetrahydrofuran, dichloromethane, toluene, C1-C4-alkyl esters of acetic acid, such as ethyl acetate or butyl acetate. The reaction is preferably carried out without solvent.
As component B it is possible in principle to employ all monofunctional esters of α,β-ethylenically unsaturated carboxylic acids with a monofunctional alcohol which has as a structural element at least one saturated 5- or 6-membered heterocycle with one or two oxygens in the ring. Component B is preferably derived from acrylic or methacrylic acid. Examples of suitable compounds of component B embrace compounds of the formula I
R is H or CH3, especially H,
k is from 0 to 4, especially 0 or 1, and
Y is a 5- or 6-membered saturated carbocycle or a 5- or 6-membered saturated heterocycle with one or two oxygens, the heterocycle being unsubstituted or substituted by C1-C4-alkyl, such as by methyl, ethyl, n-propyl, isopropyl, n-butyl, 2-butyl, isobutyl or tert-butyl.
The 5- or 6-membered saturated heterocycle is preferably derived from tetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 1,3- or 1,4-dioxane.
With particular preference, component B is selected from trimethylolpropane monoformal acrylate, glycerol monoformal acrylate, cyclohexylmethyl acrylate, 4-tetrahydropyranyl acrylate, 2-tetrahydropyranylmethyl acrylate and tetrahydrofurfuryl acrylate. Very particular preference is given to the use as component B of trimethylolpropane monoformal acrylate.
In addition, the radiation-curable formulations may comprise, in the amounts indicated above, a di- or polyfunctional ester of an α,β-ethylenically unsaturated carboxylic acid with an aliphatic di- or polyol. Suitable examples are the esterification products of the di- or polyols set out above in connection with component a1. Preference is given to the esters of acrylic and methacrylic acid, especially the diesters of diols. Preferably, the diols and/or polyols contain no heteroatoms other than in OH functions. Examples of suitable components B include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, butylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, 1,4-cyclohexanediol di(meth)acrylate and 1,4-bis(hydroxymethyl)cyclohexane di(meth)acrylate, and also trimethylolethane tri(meth)acrylate, trimethylolpropane tri(meth)acrylate and pentaerythritol tetra(meth)acrylate. Particularly preferred components B are butanediol diacrylate, hexanediol diacrylate, 1,4-cyclohexanediol diacrylate and 1,4-bis(hydroxymethyl)cyclohexane diacrylate. Hexanediol diacrylate is especially preferred.
The radiation-curable formulations of the invention may also include, depending on their intended use, up to 20% by weight of customary auxiliaries, such as thickeners, leveling assistants, defoamers, UV stabilizers, lubricants and fillers. Suitable auxiliaries are sufficiently well known to the skilled worker from paints and coatings technology. Suitable fillers include silicates, for example silicates obtainable by hydrolysis of silicon tetrachloride such as Aerosil® from Degussa, silica, talc, aluminum silicates, magnesium silicates, calcium carbonates, etc. Suitable stabilizers include UV absorbers, such as oxanilides, triazines and benzotriazole (the latter obtainable as Tinuvin® grades from Ciba-Spezialitätenchemie) and benzophenones. These can be used alone or together with suitable free-radical scavengers, examples being sterically hindered amines, such as 2,2,6,6-tetramethylpiperidine, 2,6-di-tert-butylpiperidine or derivatives thereof, such as bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate. Stabilizers are normally employed in amounts of from 0.1 to 5.0% by weight and preferably from 0.5 to 3.5% by weight, based on the components A to C present in the formulation.
Insofar as curing takes place by means of UV radiation, the formulations of the invention comprise at least one photoinitiator which is able to initiate the polymerization of ethylenically unsaturated double bonds. Such photoinitiators include 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 benzoin methyl, ethyl and butyl ethers, benzil ketals, such as benzil dimethyl ketal, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one, 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. These photoinitiators are, where necessary, employed in amounts of 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 components A, B and C of the formulations of the invention. If the formulation of the invention is cured by means of electron beams, the use of photoinitiators can be omitted. When electron beam curing is employed, the formulations of the invention may additionally contain colored pigments.
Preferably, the formulations of the invention contain no pigments or fillers. In addition, the formulations of the invention are preferably free from inert, nonpolymerizable solvents.
The formulations of the invention are prepared by blending the components in a conventional manner. Blending may take place at room temperature or at up to 100° C. and is effected, for example, by means of customary mixing equipment such as stirring vessels or static mixers.
The formulations of the invention are found to be particularly appropriate for the coating of substrates such as wood, paper, plastic surfaces, mineral construction materials, such as shaped cement blocks and cement fiber slabs, and especially for metals or coated metals.
Accordingly, the present invention also provides a method of coating substrates, especially metals or coated metals, and the coated substrates obtainable by this method. The substrate is generally coated by applying at least one radiation-curable formulation of the invention in the desired thickness to the substrate which is to be coated, removing any solvent and then curing the coating by the action of high-energy radiation such as UV radiation or electron beams. This operation may, if desired, be repeated one or more times. The radiation-curable formulations are applied to the substrate conventionally, for example by spraying, brushing, rolling or flow-coating or by coating with a filler knife or doctor blade. The coating add-on is generally in the range from 3 to 500 g/m2 and preferably from 10 to 200 g/m2, corresponding to wet film thicknesses of from about 3 to 500 μm, preferably from 10 to 200 μm. Application can be made either at room temperature or above, but preferably not above 100° C. The coatings are subsequently cured through the action of high-energy radiation, preferably UV radiation with a wavelength of from 250 to 400 nm, or by irradiation with high-energy electrons (electron beams; from 150 to 300 keV). Examples of UV sources used are high-pressure mercury vapor lamps, for example the CK or CK1 lamps from IST. The radiation dose which is usually sufficient for crosslinking is within the range from 80 to 3000 mJ/cm2.
In one preferred procedure curing takes place continuously by passing the substrate that has been treated with the formulation of the invention at a constant speed in front of a radiation source. This requires the curing rate of the formulation of the invention to be sufficiently high.
The formulations of the invention feature low viscosity, preferably ≦10 Pas (determined at 23° C. using an ICI cone-plate viscometer) and high reactivity, represented by a value of ≧10 m/min (corresponding to the rate at which the substrate, treated with a radiation-curable formulation in a wet film thickness of 100 μm, can be passed at a distance of 10 cm in front of a UV source having an output of 120 W/cm so that full cure still takes place). It is possible at the same time to realize high hardnesses, characterized by a pendulum attenuation (in analogy to DIN 53157, see below) ≧80 sec., and high flexibilities, characterized by Erichsen indentations ≧5 mm (see below), without the systems receiving low grades for viscosity and reactivity. Moreover, the coatings obtainable from the formulations of the invention feature enhanced weathering resistance as can be determined, for example, by sun tests.
The examples below are intended to illustrate the present invention without, however, limiting it.