Process for the preparation of UV-hardenable pigmented coatings.

The invention relates to a process for the production of UV-cured, opaque pigmented coatings with high scratch resistance and solvent resistance based on unsaturated polyesters which are free from copolymerizable monomers (such as styrene). The process is particularly suitable for coating wood, wood-like materials and plastic films.

The production of UV-cured, opaque pigmented coatings has always posed serious problems that have not yet been solved because the opaque pigment does not allow UV rays to penetrate into deeper layers. Compromises have therefore been sought, but these have not been able to establish themselves on the market. It has already been proposed to add UV-curable unsaturated polyester resins to incompatible polymers in order to simulate a covering coating due to the opacity resulting from the different refractive indices (DE-OS 24 26 602), to achieve a covering effect by including microbubbles in the coating ( J. Paint Technol. <u style="single">45</u>, No. 584 (1973), 73) and instead of using pigments fillers with very low hiding power and thereby allowing UV rays to penetrate into deeper layers (DE-AS 16 21 820).

The coatings obtainable in this way do not have sufficient opacity or insufficient elasticity. Another problem is the often inadequate storage stability of the coating compositions.

Surprisingly, it has now been found that coating compositions based on a combination of 1) so-called air-drying polyesters which are free of copolymerizable monomers and 2) cellulose nitrate, optionally dissolved in an inert organic solvent, can be used to produce UV-cured, opaque pigmented coatings , if hydroperoxides are used in addition to UV initiators. Such formulations can have a service life of approx. 5 hours at room temperature and, after an evaporation time of, for example, 2 to 3 minutes at 50 to 70 ° C., can be cured directly under UV radiation, for example using high-pressure mercury lamps.

The invention thus relates to a process for producing UV-cured, opaque pigmented coatings, according to which a polyester lacquer is made 100 parts by weight of air-drying polyester, 0.1 to 10, preferably 0.2 to 4, parts by weight of cellulose nitrate, 0 to 30% by weight, based on cellulose nitrate, plasticizer for cellulose nitrate, 10 to 150 parts by weight of pigment, 1 to 10 parts by weight of hydroperoxide as initiator, 0.005 to 1 part by weight of desiccant, 0.5 to 5 parts by weight of photoinitiator and any customary additives, applied to the substrate in an amount of 60 to 150 g / m 2, the gel coat 1 to 5, preferably 2 to 3 minutes at a temperature of 50 to 80, preferably 60 to 75 ° C., pre-gelled and then cured under UV radiation .

Preferred air-drying polyesters are polycondensation products of at least one α, β-ethylenically unsaturated dicarboxylic acid with generally 4 or 5 carbon atoms or their ester-forming derivatives (for example their anhydrides), optionally in a mixture with up to 200 mol%, based on the unsaturated acid component, of one or more aliphatic saturated dicarboxylic acids having 4 to 10 carbon atoms or cycloaliphatic or aromatic dicarboxylic acids having 8 to 10 carbon atoms or their ester-forming derivatives (e.g. their anhydrides), with at least one 3- or higher-valent hydroxy compound with 3 to 8 C atoms, the OH groups not used for polycondensation are at least partially etherified with β, γ-ethylenically unsaturated alcohols, optionally with one or more divalent hydroxy compounds 2 to 8 carbon atoms and optionally one or more 1-valent hydroxy compounds with 1 to 18 carbon atoms - that is, polyester, such as, for example in DE-AS 10 24 654, DE-PS 22 21 335 and in Wagner / Sarx, "Lackkunstharze", 5th edition, Carl Hanser Verlag, Munich 1971, p. 140 are described.

Examples of preferred unsaturated dicarboxylic acids or their derivatives are maleic acid or maleic anhydride and fumaric acid. However, mesaconic acid, citraconic acid, itaconic acid or chloromaleic acid can also be used, for example. Examples of the aliphatic saturated and cycloaliphatic and aromatic dicarboxylic acids to be used or their derivatives are phthalic acid and phthalic anhydride, isophthalic acid, terephthalic acid, hexa- and tetrahydrophthalic acid or their anhydrides, endomethylene tetrahydrophthalic acid, and their anhydride, succinic acid or succinic anhydride and succinic acid esters and chlorides, adipic acid, sebacic acid. To produce flame-retardant polyesters, for example, hexachlorodomethylene tetrahydrophthalic acid, tetrachlorophthalic acid or tetrabromophthalic acid can be used. Ethylene glycol, 1,2-propanediol, 1,3-propanediol, diethylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 2,2-bis (4- hydroxycyclohexyl) propane, bis-oxyalkylated bisphenol A can be used; Hydroxy compounds preferred for introducing the β, γ-ethylenically unsaturated ether groups are trimethylolpropane mono- and diallyl ethers, glycerol mono- and diallyl ethers and pentaerythritol mono-, di- and triallyl ethers. The polyesters to be used according to the invention preferably contain at least 10% by weight, in particular at least 20% by weight, based on polyester, of residues β, γ-ethylenically unsaturated ether groups.

Monohydric hydroxy compounds preferred for the preparation of the polyester are alkanols, cycloalkanols and cycloalkanols, for example n-hexanol, n-octanol, n-decanol, laurol, stearyl alcohol, hexahydrobenzyl alcohol, preferably branched alkanols, such as 2-ethylhexanol-1 and isononanols.

The acid numbers of the polyesters should be 1-40, preferably 10-30, the OH numbers 10-100, preferably 20-50 and the number average molecular weights 300-5000, preferably 500-2000.

Preferred cellulose nitrates are common qualities of collodion wool, ie cellulose nitric acid esters with a nitrogen content of 10.2 to 12.4% by weight.

It is not essential for the invention whether the collodion wool used is a butanol, isopropanol, ethanol, methanol or water-moist form or whether celluloid-like masses (generally called chips), i.e. with gelatinizing plasticizers, be used.

Preferred plasticizers for cellulose nitrates are, in particular, phthalic and adipic esters of alcohols having 4 to 8 carbon atoms, for example dibutyl phthalate, diisobutyl carbinyl phthalate, dicyclohexyl phthalate, dioctyl phthalate, dibutyl phthalate, dioctyl adipate, phosphoric acid esters of alcohols with 6 to 10 carbon atoms, for example Tricresyl phosphate, triphenyl phosphate, trioctyl phosphate, cresylphenyl phosphate, dioctylphenyl phosphate, but also diethylene glycol monolaurate, dipentaerythritol hexapropionate, 2-methoxyethyl acetyl tributylaconitate, butoxy ethyl diglycol carbonyl, ethylitlyl ethoxylate ethyl acetate, acetityl ethoxylate di-2-ethyl butyrate, tributyl carballylate, dibutyl sebacate and benzenesulfonic acid N-methyl amide.

Pigments which can be used are the opaque, inorganic or organic pigments customary for unsaturated polyester paints, such as titanium dioxide, ultramarine blue, iron oxides and phthalocyanine blue.

Preferred hydroperoxides are, for example, tert-butyl hydroperoxide, cumene hydroperoxide, methyl ethyl ketone hydroperoxide, diisopropylbenzene monohydroperoxide and hydrogen peroxide.

Desiccants are, for example, iron, lead, cobalt and manganese salts of acids such as linseed oil fatty acids, tall oil fatty acids, soy fatty acids, of resin acids such as abietic acid and naphthenic acid or of acetic acid or isooctanoic acid. Cobalt octoate, cobalt naphthenate and cobalt acetate are preferred. Desiccants are preferably used in the form of organic solutions.

Compounds which are well known for radiation curing, such as ketones, diketones, α-keto alcohols and their derivatives, are suitable as photoinitiators. Preferred photoinitiators are benzoin and its derivatives, such as, for example, benzoin ether (DE-PS 16 94 149), benzil ketals and hydroxyalkylphenones, for example benzoin isopropyl ether, benzil dimethyl ketal, 2-hydroxy-2-methyl-1-phenyl-propan-1-one and 1-hydroxycyclohexyl -phenyl ketone.

Common additives which may be used are, for example, fillers, thixotropic, smoothing, matting and leveling agents and organic solvents, such as toluene, xylene, isopropanol, in particular butyl acetate. The amount of organic solvents is generally chosen so that the paints have a suitable processing viscosity.

The paints are ideally suited for processing on conventional painting lines equipped with casting machines. They can also be applied by rolling and spraying.

The percentages in the following examples mean percentages by weight, parts are parts by weight.

Examples

Polyester used

Unsaturated polyester with an acid number of 25, an OH number of 75 and a viscosity of 6000 mPa.s, measured on an 80% solution in butyl acetate at 20 ° C, made from 1 mol of maleic anhydride, 0.85 mol of ethylene glycol, 0 , 15 moles of propylene glycol and 0.3 moles of trimethylolpropane diallyl ether.

The formulations *) listed in the table were poured in an amount of 90 g / m² onto a wooden surface preheated to 40 ° C. The solvent was evaporated and heated to 70 ° C. for 3 minutes. The paint film was then cured under UV radiation (2 high-pressure mercury lamps with 80 watts / cm each, lamp spacing from paint 20 cm, processing speed 3.5 m / minute). Examples 1-3 gave colorless, perfectly water- and chemical-resistant, hard and scratch-resistant coatings, while the comparative example resulted in a wrinkled, non-scratch-resistant coating. <tables id="tabl0001" num="0001"><img file="EP0245639A2_D0001.tif" /></tables>*) Viscosity according to DIN 53 211 (DIN cup No. 4 at 20 ° C) corresponding to a run-down time of 60 sec