Air-drying coatings harden under the influence of atmospheric oxygen. Without appropriate precautions, these coatings, when they come into contact with oxygen-e. g. from the air-unstirred, will form a skin on their surfaces Since air-drying is an irreversible cross-linking process, this skinning generally cannot be cancelled even by renewed thorough mixing of the coating composition and this cross-linked top paint film will be useless for further processing.

In order to enable coating compositions to be stored for a considerable period of time in packings with air present above or to enable opened up barrels-in which the air present above will inevitably increase as a result of the removal of coating material-to remain storage stable for a certain period of time, substances are added to the coating compositions which eliminate this skinning.

These substances are known as anti-skinning agents.

Similar phenomena are also known from primers and coatings which are exposed and subject to prolonged atmospheric action, e. g. dipping primers with a long turn over. Also in such covering or coating compositions use is made of anti-skinning agents. They are further used in fillers.

The properties profile of anti-skinning agents requires that skinning be avoided, yet they must not have a negative effect on the actual hardening of the coating following application.

A widely used anti-skinning agent is butanonoxim, which is also marketed under the trade designations Aconox M, Metoxim 100, Ascinin R, Borchinox M2, Exkin 2, etc..

This substance was assigned a new value in the MAK Values List for 1997.

While in the EC Safety Specification Sheet butanonoxim is classified as Xi (irritating) as before and not yet identified as carcinogenic, nevertheless, in the MAK and BAT Values Lists for 1997 of the German Society for the Advancement of Scientific Research it appears in Group III A2, which lists substances which are unmistakably carcinogenic in animal experiments, more particularly under test conditions which are infectious to humans. In the 1997 supplement on butanonoxim the following statements with regard to the carcinogenicity of ketoxims are found: 5.6.2 Genotoxicity in vivo "After intraperitoneal administration of butanonoxim (2,24 mmoles/kg KG) to male F344-rats the formation in the liver was proved of DNA-modification DX1, RNA-modifications RX1 and RX2 as. well as an increased formation of 8- oxoguanine in DNA and RNA." "The formation of the nucleic acid modifications DX9 and 8-aminoguanosine and an increased formation of 8-oxoguanine in DNA a RNA were also demonstrated after application of the ketoxims acetonoxim, 3-pentanonoxim, 4- heptanonoxim, cyclopentanonoxim, and cyclohexanonoxim"

2 Working mechanism "The hepatocarcinogenicity of the butanonoxim observed in two studies is probably a general feature of the ketoxims, since acetonoxim and cyclo- pentanonoxim also led to an increased incidence of liver tumors in male rats." Moreover, the carcinogenic effect of butanonoxim presumably should not be attributed to the individual substance but to a metabolic process, which shows the entire group of ketoxims to be critical. From the patent literature some few examples of other anti-skinning agents are known. In US 3,346,523 of Esso Research and Engineering Co. and US 3,701,781 of Cincinnati Miiacron Chemicals, Inc., however, again compounds with oxim groups are described.

DE 3418988 of BASF AG describes the use of a-hydroxyketones such as hydroxyacetone.

Given these facts, the use of ketoxims as anti-skinning agent appears to be critical. For that reason a search was made for a suitable alternative which is not a ketoxim, which is unobjectionable from a toxicological point of view, and which in terms of properties profile is comparable with butanonoxim.

Surprisingly, the above-mentioned objective was met by means of a coating composition comprising one or more compounds of the general formula wherein Ri and R2 may be the same or different and represent a Ci-Ce alkyl group and/or one or more compounds of the general formula wherein X and Y represent either a carbon atom and a nitrogen atom or a nitrogen atom and a carbon atom, R3 is hydrogen or an alkyl group, R4, or when n is greater than 1, each R4 group, which may be the same or different, is an alkyl, alkenyl, aralkyl, aryl, alkylaryl, amine or

group, wherein R5 represents a Ci-C4 alkyl group and n is greater than or equal to 0 and less than or equal to 3. Preferably, air-drying coating compositions are concerned.

It was established that compounds of formula (I) and/or (II) used according to the invention showed a good anti-skinning effect comparable to that of butanonoxim, without the air drying of the film being negatively affected.

Surprisingly, as a side effect there is a clear reduction of the yellowish tinge otherwise common in air-drying coatings-both in the wet and the dry phase.

So far as is apparent from toxicological data collections such as the MAK and BAT Values Lists, the compounds of formulae (I) and (II) show no objectionable toxicological effects as compared with butanonoxim.

The term coating composition according to the invention is to be interpreted very broadly. For instance, it concerns varnishes, priming coats, filling pastes, glazes, mulsions. The compounds of formula (I) and/or (II) used according to the invention are especially suitable for use in all air-drying systems.

The coating compositions according to the invention preferably are solvent- based air-drying coatings and/or house paints. However, they may also be formulated as a water-dilutable or water-borne system. By house paints is meant the group of pigmented and unpigmented surface coating compositions which is used in priming or reconditioning coats for coating doors, windows, floors, equipment objects, and the like. They appear in the form of glazes, clear lacquers, priming coats-including fillers-precoats, and finishes and are used for coating substrates common in construction such as wood, metal, synthetic material, but also stone and mineral surfaces (e. g. concrete, ceramic material).

Preferably, the coating compositions according to the invention are alkyd based.

A preferred coating composition is a house paint, e. g. an alkyd paint, which contains acetylacetone as the compound of the general formula (I) and no compound of the general formula (II). Furthermore, the use of acetylacetone by itself as the compound of general formula I without a compound of formula 11 as anti-skinning agent in coating compositions is an advantageous embodiment of the present invention.

Especially good results, however, can be obtained by using at least one compound of general formula (I), more particularly acetylacetone, together with at least one compound of general formula (II).

The compound of the general formula

concerns 1,3-diketones, wherein Ri and R2 may be the same or different and a Ci-Ce alkyl group, preferably a Ci-C4 alkyl group. Ri and R2 may be, e. g., a methyl, ethyl, n-propyl, i-propyl, n-butyl, sec.-butyl, tert.-butyl, pentyl or hexyl group. Preferably, R1 and R2 each are a methyl group, i. e. acetylacetone.

The compound of general formula

concerns a heterocycle in which X and Y represent either a carbon atom and a nitrogen atom or a nitrogen atom and a carbon atom, R3 is hydrogen or an alkyl group, preferably a Ci-C4 alkyl group, R4, or when n is greater than 1, each R4 group, which may be the same or different, is an alkyl, alkenyl, aralkyl, aryl, alkylaryl, amine or

group, wherein R5 is a Ci-C4 alkyl group and n is greater than or equal to 0 and less than or equal to 3.

The alkyl group for R3 may be an alkyl group with Ci-Ci5 carbon atoms, preferably with Ci-C4 carbon atoms. For the alkyls with Ci-C4 carbon atoms suitable for use for R3 reference may be had to the groups mentioned earlier for Ri and R2. R3 being hydrogen or methyl is especially preferred.

The alkyl group for R4 may be an alkyl group with 1-15 carbon atoms, preferably with 1-4 carbon atoms. Examples of alkyl groups suitable for use are decyl, octyl, heptyl, hexyl, pentyl, and the Ci-C4 alkyl groups mentioned earlier for Ri and R2. The alkenyl groups may be alkenyl groups with 2-15 carbon atoms, preferably 2-4 carbon atoms. Examples of alkenyl groups suitable for use are ethenyl, propenyl (e. g. 1-propenyl or 2-propenyl), and butenyl. The aralkyl groups may be aralkyl groups with 7-15 carbon atoms. One example is phenylethyl. The aryl groups may be aryl groups with 6-14 carbon atoms such as phenyl, naphthyl, and anthryl. The alkylaryl groups, e. g. with 7-15 carbon atoms, may be, e. g., ethylphenyl. The amine groups employed may be primary, secondary, and tertiary amines. As substituents on the amine are considered, e. g., alkyl, aryl, aralkyl or alkylaryl groups. For specific examples of these groups reference may be had to the aforementioned examples for R4. For examples of the Ci-C4 alkyl groups for R5 reference may be had to the groups mentioned for Ri and R2.

The number n substituents R4 present on heterocycle (II) is 3 at the most, but can also be 0. The present substituents R4 may all be the same or different.

A preferably used compound of formula (I) is acetylacetone. As compound of formula (II) preferably compounds of the formula

are used, wherein R4 and n are as defined above. The use of 3,5-dimethyl- pyrazole as compound of formula (II) or (III) is quite especially preferred.

The compounds of formula (I) and formula (II), respectively, can be included in the coating composition as anti-skinning agent by themselves. Moreover, in each case only one compound of the formula in question or several compounds of the formula in question may be used. Preferably, however, in the coating compso according to the invention use is made of at least one compound of general formula (I) and at least one compound of formula (II).

Further examples of compounds of general formula (II) are pyrazole, 3- methylpyrazole, 1,4-dimethylpyrazole, imidazole, 1-methylimidazole, 2-methyl- imidazole, 4-methylimidazole, 2-ethylimidazole, 1,2-dimethylimidazole, 2-ethyl- 4-methylimidazole, N- (3-aminopropyl) imidazole, and ethyl- (3-aminopyrazole-4- carboxylate). In addition to the especially preferred compound 3,5-dimethyl- pyrazole, the compounds imidazole and 1,2-dimethylimidazole also have preference.

It was found that a combination of acetylacetone and 3,5-dimethylpyrazole is especially advantageous. Slight colour changes in the wet film, which may occur when using single components, are avoided by the combination, as in that case the effects offset each other. However, also in the case of single components the shift in colour disappears after 1-2 days when the applied film has hardened. Surprisingly, it is especially through this combination that also the yellowish tinge common with air-drying coatings is clearly reduced, both in the wet and the dry phase.

Preferably, all things considered, the compounds of formula (I) and formula (II) are used in the coating composition together in an amount of 0,1 to 5 wt. %, more preferably 0,5 to 3 wt. %, calculated on the coating composition.

In so far as acetylacetone is present, the proportion thereof, calculated on the overall quantity of coating material, preferably is 0,05 to 2 wt. %, especially preferably 0,25 to 1 wt. %.

The coating composition according to the invention may contain siccatives, for instance the naphthanate salt or octoate salt of cobalt, manganese, nickel, vanadium, zirconium or molybdenum.

The following examples elucidate the invention. While in these embodiments solid components are always used as a solution, it is possible-when a coating material is dispersed through a sand mill, ball mill or three-roll mill-to add the solid to the material to be ground and also grind it. In this way it is possible to economise on optional additional solvent.

Examples Unless indicated otherwise, all % and parts data refers to weight percentages and parts by weight.

High-gloss white test coating composition without anti-skinning agent 68 parts of a long oil soya alkyd (AN 8-13, OH no. about 60-70, SC 65% in white spirit) were dissolve together with 25 parts of titanium dioxide (rutile type), 1 part of bentonite paste (25% in white spirit), and 1 part of soya lecithin until a grain size of less than 10 pm was achieved. Next, the mixture was supplemented with 0,5 part of a 6% cobalt octoate solution and set to a processing viscosity of 5,5-6,0 dPa-s with at most 4,5 parts of white spirit.

Test coating compositions with anti-skinning agent For the determination the above-described high-gloss white test coating composition was mixed with the different anti-skinning agents in different variations of quantities and mixed thoroughly with an impeller. To evaluate the surface drying a 100 um application of each of the thus obtained test compositions was made on lined paper and the surface drying was assigned a value after 4 and 8 hours in accordance with DIN 53230. About 200 ml of the different test coating compositions were stored in a PE beaker at room temperature, exposed, and the skinning after 24 hours was evaluated in accordance with DIN 53230. The results are listed in the Table below. Each time the above-described test coating composition without further additives was also assigned a value as a blank test sample.

Results: anti-skinning agent Amount Surface Surface Surface drying Discoloura- caic. on 100 drying drying 200 ml beaker tion parts test 100 um 100 um after 24 h. in wet/dry coating application application see note 2 film comp. after 4 h. after 8 h. see note 1 see note 1 Comparative examples: A. none (blank test-1-2 0 5 slightly sample) yellowish/ none B. butanonoxim (100%) 0,5 part 2+ 0 0 slightly yellowish/ none C. hydroxyacetone 2,0 parts 1-2 0 5 slightly (100%) yellowish/ none Examples according to the invention: part 2-0-1 2+ none/none dimethylpyrazole- solution. 2. (25% 3,5- 2,0 parts 2-0-1 1-none/none dimethylpyrazole) 3. imidazole-solution 2,0 parts 2-1-2 1+ none/none 4. (25% imidazole) 4,0 parts 2-3 2 0-1 none/none parts 2+ 1-2 2 none/none dimethylimidazole- solution 6. (25% parts 2-3 2 1-none/none dimethylimidazole) 7. acetylacetone 0, 5 part 2-0 0-1 slightly (100%) greenish/ none 8. acetylacetone 2,0 parts 2-3 0 0-slightly (100%) greenish/ none 9. acetylacetone/0,5 part/ part 3+ 1+ 0 none/none solution (25% 3,5- dimethylpyrazole)

Notes: 1) Surface drying grades of 0-5; 0 stand for dust-free dry, 5 stands for wet trends are evaluated with + (better,- 0) or- (poorer, 5) 2) Surface drying grades of 0-5; 0 stands for no skin, 1 stands for incipient skinning at the edges, 2 stands for incipient skinning, 3 stands for fine continuous skin, 4 stands for skin, 5 stands for strong skin trends are evaluated with + (better,- 0) or- (poorer, 5) 3) The solid compounds of formula 11 were dissolved in a mixture of Solvesso 150 and methoxypropanol. The solvent was not taken into account in the

determination of the amount calculated on 100 parts of test coating composition.

It was found that by using the compounds of formula I and/or 11, more particularly by using acetylacetone and 3,5-dimethylpyrazole, an anti-skinning effect comparable with that of butanonoxim can be achieved, without the drying properties of the thus prepared coating compositions being adversely affected.

It was found that a combination of acetylacetone and 3,5-dimethylpyrazole was particularly advantageous. Slight colour changes in the wet film, which may occur when using single components, are avoided by the combination, since the effects in that case offset each other. However, also in the case of single components the shift in colour disappears after 1-2 days, when the applied film has hardened. Surprisingly, it is through this combination that the yellowish tinge common with air-drying coatings is also clearly reduced-both in the wet and the dry phase.

Compared with the hydroxyacetone known from DE patent specification 3418988 a clearly improved effectiveness is achieved. In addition to the aforesaid examples, three high-gloss white finishes were formulated. Raw material Comp. Ex. D Example 10 Example 11 long oil soya alkyd (AN 8-13, OH 18,00 parts 18,00 parts 18,00 parts no. ca. 60-70, SC 65% in white spirit) thixotropic, long oil sunflower 15,00 parts 15,00 parts 15,00 parts alkyd (AN 8-13, OH no. ca. 55-65, SC 40% in white spirit) bentonite 0,40 part 0,40 part 0,40 part thixogel VP-Paste (25% in white 1,50 parts 1,50 parts 1,50 parts spirit) titanium dioxide (rutile type) 30,00 parts 30,00 parts 30,00 parts parts 1,50 parts The aforesaid recipe items were dissolved for 15 minutes and then subjected to 1 passage through an agitator mill. The grain size subsequently was less than < 10 pm Next, there was supplementing with the following items. long oil soya alkyd (AN 8-13, OH 30,00 parts 30,00 parts 30,00 parts no. ca. 60-70, SC 65% in white spirit) antifoaming agent 0,50 part 0,50 part 0,50 part silicone oil solution (1% silicone oil 0,10 part 0,10 part 0,10 part in white spirit) cobalt-octoate (6% Co) 0,60 part zirconium-octoate (18% Zr) 1,00 part 1,00 part 1,00 part manganese-iso-carboxylate (12%-0,01 part 0,01 part Mn) butanonoxim 0,40 part acetylacetone-1,00 part white spirit 2,50 parts 0,49 part 1,99 parts Using white spirit the coating compositions were set fo a processing viscosity of 5,0 to 5, 5 dPas.

The coating compositions were tested on the following points: Test Comp. ex. D Example 10 Example 11 drying after 16 hours 100 pm application by knife p, K. O. K. O. K. coater immediately after preparation recoatability after 16 hours 100 um application by knife O. K. O. K. O. K. coater immediately after preparation drying after 16 hours 100 pm application by knife O. K. O. K O. K. coater (4 weeks storage at 40°C) recoatability after 16 hours 100 pm application by knife O. K. O. K. O. K. coater (4 weeks storage at 40°C) viscosity (requirement: 5,0-5,5 dPas) 5,0 dPas 5,1 dPas 5,0 dPas after application 5,0 dPas 5,3 dPas 5,1 dPas after 28 days storage at RT 5,2 dPas 5,3 dPas 5, 2 dPas after 28 days storage at 40°C change in hue (DIN 6174) RT at 4 weeks storage at 40°C none None none storage test 375 mi container, 300 ml filling, lid after 4 weeks after 4 weeks after 3 weeks with hole (about 3 mm), skinning storage test 375 micontainer , 300 ml filling, container closed normally, not tumed over 4 weeks O.K. O.K. O.K. 8 weeks O. K. O. K. O. K. 3 months O. K. O. K. O. K.