EP2208544B1 - Combination coating for wood - Google Patents

Abstract

A compound coating comprises an alkyd resin undercoat formed by applying alkyd resin coating material (a1) to wood and allowing it to dry; and siliceous coating formed by applying siliceous coating material (b1) on top of the resin undercoat. The material (a1) comprises long oil alkyd resin having oil length of at least 60% and oil iodine number of >= 100; mineral component (a2) and solvent. The material (b1) comprises water glass or mixture of silica sol and water glass, where molar ratio of silica to alkali oxide is 5-30 mol of silica/mol of alkali oxide; and mineral component (b2). Independent claims are included for the following: (1) preparing the compound coating involving priming the wood by applying and allowing to dry at least one layer of the alkyd resin coating material (a1), and applying and allowing to dry at least one layer of the siliceous coating material (b1) on top of the undercoat; and (2) a compound coating kit comprising the alkyd resin coating material (a1) and the siliceous coating material (b1), where the coating materials are spatially separated.

Description

The present invention relates to a combination coating for wood comprising an alkyd resin primer and a silicate coating applied thereto, and a kit consisting of an alkyd resin coating composition and a silicate coating composition and a method of applying the coating.

Technical background

Alkyd resins are generally polyester compounds of polybasic acids (e.g., phthalic acid, isophthalic acid, maleic acid) and polyhydric alcohols (especially glycerol or pentaerythritol), with excess OH groups esterified with unsaturated fatty acids. Numerous methods of preparing alkyd resins are known in the art. Alkyd resins, for example, are obtainable by transesterification of a drying oil with an excess of polyol and subsequent reaction with acid anhydride. Other methods are based on acid, the alcohol and the corresponding fatty acid mixture.

The fatty acid content is expressed in terms of the weight fraction of the oil and referred to as "oil length". A short oil alkyd resin has an oil length of typically 20-40%, a middle oil of 40-60%; Over 60% is generally called long-oil alkyd resin. The greater the oil length, the lower the degree of polymerization of the polyester component, and the higher the proportion of fatty acids per polyester molecule.

Alkyd resin based paints are often used to coat wood. Wood is a relatively soft, working substrate. For a good adhesion to wood must have a Coating therefore have a certain elasticity, which is the case with alkyd paints.

Alkyd resins, however, are not UV-resistant. Therefore, alkyd paint coatings on wood outdoors must be renewed regularly.

Silica coatings, such as those used outdoors for coating mineral substrates or metallic surfaces in particular, are UV-resistant and very durable, but relatively mechanically rigid, and therefore generally do not adhere permanently to wood. One possible solution to this problem is the use of a higher elasticity primer which serves as an adhesion promoter between the soft wood and the hard silicate paint.

However, alkyd paint coatings are generally unsuitable as a primer for silicate paints since the silicate paint catalyzes the saponification of the alkyd resin. This may cause the alkyd resin to decompose before it hardens. For this reason, such combinations do not adhere permanently to wood.

task

In view of the problem described above, the present invention has for its object to provide a coating for wood, which is UV-stable and permanently adherent.

Disclosure of the invention

Surprisingly, it has been found that special combinations of alkyd resin primer and silicate paint on wood are stable and durable.

Accordingly, the present invention provides a combination coating of alkyd primer and silicate coating, as well as the appropriate combination of suitable alkyd resin coating composition and silicate coating composition. In addition, the invention also relates to a method for applying the combination coating according to the invention.

Advantages of the combination coating according to the invention lie in the longer life compared with normal alkyd coating (renovation interval 7-15 years or more instead of 3-6 years with pure alkyd coating). In addition, when renovating the Alkydbeschichtung no longer be pickled and the solvent input is lower. Finally, the coating of the invention also opens up novel surface design options.

Alkyd resin coating composition

• (A-1) ein langöliges Alkydharz
• (A-2) eine mineralische Komponente; und
• (A-3) Lösungsmittel.

The alkyd primer is obtainable by applying an alkyd resin coating composition which comprises the following components:

• (A-1) A long oil alkyd resin
• (A-2) a mineral component; and
• (A-3) Solvent.

The alkyd resin coating composition which can be used according to the invention can preferably also comprise drying accelerators (A-4).

(A-1) Long-oil alkyd resin

Component (A-1) is a long oil alkyd resin having a fatty acid content (oil length) of at least 60%, for example in the range between 60-90%, in particular 70-80%, based on the weight fraction of the oil. The oil is here Semi-drying or drying oil with an iodine value according to DIN 53241-1 of more than 100, preferably more than 130 and especially preferably more than 160. Examples of oils with suitable fatty acid composition are thistle oil, linseed oil, dehydrated castor oil, sunflower oil, soybean oil, tung oil, or fish Lead oils. Tall oil fatty acid based alkyd resins are also contemplated. The component (A-1) is contained in the primer preferably in a proportion of 40-60% by weight.

With an oil length below 60% and / or an iodine number of the oil below 100, the alkyd resin dries too slowly and the alkyd resin primer can saponify upon exposure to the silicate paint, thereby rendering the durability of the combination coating insufficient.

Suitable alkyd resins are commercially available, for example, as Setalin or Uralac Jägalyd.

(A-2) Mineral component

The component (A-2) consists of mineral particles and acts as an adhesion promoter. It is required for the adhesion of the silicate coating on the undercoat layer, and is preferably contained in a proportion of 5-25% by weight in the alkyd resin coating composition. Lower levels may result in degraded adhesion of the silicate coating to the primer. Particularly preferred are proportions of 10-15 wt.%. The particle size may be for example 1-80 microns, preferably 4-10 microns.

As component (A-2), mineral particles come into consideration, such as are typically used as filler, matting agents or pigments. As examples of this can be led particles of silica or silicate material, such as precipitated SiO ₂ , diatomaceous earth (diatomaceous earth) or silicate minerals such as mica, kaolinite, muscovite or chlorite.

Carbonate minerals such as calcites or oxides such as alumina, calcia, titania, magnesia, zirconia and / or boria are also suitable.

Furthermore, oxidic pigments, for example based on iron oxide, are also suitable. Furthermore, also barite or zinc sulfide can be used.

Component (A-2) can also consist of a mixture of different minerals, for example a mixture of different phyllosilicates. Preferably, precipitated SiO ₂ or kieselguhr are used as component (A-2).

(A-3) Solvent

As the component (A-3), any solvent suitable for alkyd resin paints can be used in principle. As a typical example, hydrocarbon-based solvents such as e.g. Lead white spirit.

(A-4) Drying accelerant

For good stability of the alkyd primer against degradation by ester hydrolysis, the alkyd resin should dry quickly. For accelerating the drying, the alkyd resin coating composition preferably contains an agent (A-4) which catalyzes the radical crosslinking of the unsaturated fatty acids. Suitable for this purpose are all siccatives customary in the prior art for alkyd paints.

Examples which may be mentioned are metal salts which are soluble in the coating composition, in particular transition metal salts of organic acids, for example cobalt 2-ethylhexanoate.

Usually, the agent (A-4) is used in an amount of 0.005 to 0.3% by weight, preferably 0.01 to 0.1% by weight.

Other ingredients

The alkyd resin coating composition may also contain other ingredients conventional in the art, as long as they are not detrimental to the adhesion of the silicate coating. As examples, anti-skinning agents such as 2-butanone oxime or thickening agents (e.g., bentonites) can be cited.

Preparation of the alkyd resin coating composition

The alkyd resin coating composition can be obtained by the conventional method by diluting the component (A-1) with the solvent (A-3) until the desired viscosity is reached and then stirring in the mineral component (A-2).

Silicate coating

• (B-1) Wasserglas oder eine Mischung aus Kieselsol und Wasserglas, wobei das molare Verhältnis von SiO₂ zu Alkalioxid 5 bis 30 Mol SiO₂ pro Mol Alkalioxid beträgt; sowie
• (B-2) eine mineralische Komponente

umfasst.The silicate coating is obtainable by applying a silicate coating to the alkyd primer layer. According to the invention, a silicate coating composition is used which

• (B-1) water glass or a mixture of silica sol and water glass, wherein the molar ratio of SiO ₂ to alkali oxide is 5 to 30 moles of SiO ₂ per mole of alkali oxide; such as
• (B-2) a mineral component

includes.

Optionally, a polymer (B-3) and / or an organic ammonium compound (B-4) may also be contained. Other optional ingredients include pigments, thickening and dispersing agents, and water as a solvent. Silicate paints which can be used according to the invention are described, for example, in US Pat DE 100 01 831 respectively EP 1 222 234 described.

Component (B-1)

In component (B-1), the molar ratio of SiO ₂ to alkali oxide in the water glass or in the mixture of water glass and silica sol is 5-30 moles of SiO ₂ per mole of alkali oxide, preferably 15-25 moles of SiO ₂ per mole of alkali oxide, and particularly preferable 20 moles of SiO ₂ per mole of alkali oxide. In this case, the constituents, as usual in silicate analysis, are defined in relation to the oxides, even if compounds such as silicates or the like are actually present.

At a molar ratio below 5 moles of SiO ₂ per mole of alkali oxide, the early water resistance and adhesion to the primer deteriorate. In addition, such high levels of alkali oxide promote the saponification of the alkyd resin, which reduces the stability of the primer.

By glass is meant glassy solidified melts of alkali metal silicates and their aqueous solutions, which are obtainable from alkali metal carbonates and SiO ₂ . In the silicate coating material, the water glass or the mixture of water glass and silica sol acts as binder and brings about the special properties of silicate coating materials.

By water evaporation and reaction with carbon dioxide, the coating composition is silicated, ie, the binder is converted into a water-insoluble silicate network, which optionally includes the components of the substrate. This results in a very hard coating with a high gas permeability. The water glass used is generally made by fusing quartz sand with alkali carbonate.

The alkali oxide of the water glass is, for example, lithium, sodium or potassium oxide. Potassium oxide is preferred because it is less prone to blooming and cheaper than lithium oxide compared to sodium oxide. The alkali metal oxide is preferably 0.5 to 3% by weight, more preferably 0.5 to 0.8% by weight, based on the total weight of the silicatic coating composition. Above all, a content of alkali oxide of 0.5% by weight is preferable.

Silica sol is understood as meaning aqueous solutions of colloidal silica. Preferably, an alkaline silica sol is used. Furthermore, a solids content of 10 to 50% is preferred. In addition, the silica sol advantageously has an average particle size of <10 nm. The silica sols used are also preferably characterized by a very uniform and fine distribution spectrum.

The water glass or the mixture of water glass and silica sol is preferably 3 to 40 wt.%, Particularly preferably 15 to 35 wt.%, Based on the total weight of the siliceous coating composition. If a mixture of water glass and silica sol is used, the silica sol may be present in a proportion of from 3 to 30% by weight, based on the total weight of the siliceous coating composition.

Component (B-2)

The silicatic coating composition which can be used according to the invention comprises a mineral component (B-2). In principle, the same for component (B-2) Materials suitable, which also come for the mineral component (A-2) of the alkyd resin coating material in question.

Preferably, the mineral component (B-2) comprises one or more of the usual fillers in the art, preferably in an amount of 10 to 45 wt.% Of filler (s), based on the total weight of the siliceous coating.

A particularly smooth and shiny surface of the silicate composition is obtained when the component (B-2) contains a crystalline filler of the class of calcite.

To increase the crack resistance of the coating obtainable with the silicate coating composition, it is advantageous if the component (B-2) contains platelet-shaped substances such as phyllosilicates (mica, kaolinite, muscovite, chlorite) as filler. If component (B-2) contains such platelet-shaped substances, their proportion is preferably 5 to 25% by weight, particularly preferably 10 to 20% by weight, based on the total weight of the silicatic coating composition.

Further, the component (B-2) may contain baryte as a filler. If component (B-2) contains barite, the proportion is preferably 3 to 15% by weight, particularly preferably 9 to 11% by weight, based on the total weight of the siliceous coating composition.

Other possible mineral constituents which may be used as mineral component (B-2) alone or in admixture with each other and / or in admixture with the fillers listed above include oxides from the group of alumina, calcia, titania, magnesia, zirconia and / or Boron oxide, as well as mineral pigments to match a desired color. Advantageously, the pigments used are mineral oxides, in particular oxides having a rutile or spinel structure, for example iron oxides. If the component (B-2) contains pigments, their proportion is preferably 5 to 20% by weight with respect to the weight of the coating composition.

Polymer (B-3)

The silicatic coating composition which can be used according to the invention can furthermore contain a polymer. Polymer-containing silicate coating materials are used in particular as dispersion silicate paints. The addition of a polymer provides greater flexibility of the coating obtained after silification, which is advantageous for adhesion to the alkyd primer.

According to DIN specification 18 363, para. 2.4.1, dispersion silicate paints can contain a maximum of 5% organic components. Regardless of this DIN guideline, however, a polymer content of up to 15% by weight, based on the total weight of the silicatic coating composition, in particular 1 to 15% by weight, is advantageous. Particularly preferred is a polymer content of 3 to 10 wt.%. The polymer is usually incorporated in the form of a dispersion in the silicate coating composition. The solids content of the polymer dispersion is preferably from 20 to 80% by weight. Preferably, the polymer is a (meth) acrylate homopolymer or copolymer. Particularly preferred is a butyl acrylate-methyl methacrylate copolymer.

Ammonium compound (B-4)

wobei R¹, R² und R³ jeweils unabhängig eine Alkylgruppe mit 1 bis 20 Kohlenstoffatomen, die gegebenenfalls mit einer funktionellen Gruppe substituiert sein kann, oder Wasserstoff darstellen, R⁴ eine Alkylgruppe mit 1 bis 20The silicatic coating composition which can be used according to the invention can be admixed with an organic ammonium compound (B-4) for stabilization against gel formation. For this purpose, suitable ammonium compounds are in DE 100 01 831 and include compounds of formula (I):

wherein R ¹ , R ² and R ³ each independently represent an alkyl group having 1 to 20 carbon atoms which may be optionally substituted with a functional group, or hydrogen, R ^{4 represents} an alkyl group having 1 to 20

Carbon atoms, hydrogen or - (CH ₂ ) _X -N + R ⁵ R ⁶ R ⁷ Y ^- , wherein R ⁵ , R ⁶ and R ⁷ each independently represent an alkyl group of 1 to 20 carbon atoms optionally substituted with a functional group may be hydrogen or, and wherein at least one of R ¹ , R ² , R ³ and R ^{4 is} not hydrogen, x is a number between 1 and 6 and X ^- and Y ^- each represents an anion. The functional group as a substituent of the alkyl group may be, for example, a hydroxy group, an amino group, a thiol group, preferably a hydroxy group. The choice of anion is not limited so long as it does not diminish the effect of the organic ammonium compound, eg the anion can be F ^- , Cl ^- , Br ^- , I ^- or OH ^- .

wobei R¹, R², R⁵ und R⁷ jeweils unabhängig eine Alkylgruppe mit 1 bis 20 Kohlenstoffatomen oder Wasserstoff darstellen, R³ und R⁶ jeweils unabhängig eine Hydroxy-substituierte Alkylgruppe mit 1 bis 6 Kohlenstoffatomen sind, x eine Zahl zwischen 1 und 6 ist und X^- und Y^- gleich oder verschieden sein können und jeweils ein Anion, z.B. F^-, Cl^-, Br-, I^- oder OH^-, darstellen.Particularly preferred is an organic ammonium compound of the formula (II):

wherein R ¹ , R ² , R ⁵ and R ⁷ each independently represent an alkyl group having 1 to 20 carbon atoms or hydrogen, R ³ and R ⁶ each independently is a hydroxy-substituted alkyl group having 1 to 6 carbon atoms, x is a number between 1 and 6 and X ^- and Y ^- can be identical or different and each represents an anion such as F ^-, Cl ^-, Br-, I ^- or OH ^-, which represent.

Preferably, the alkyl groups of formulas (I) and (II) contain from 1 to 6 carbon atoms, and selected examples are methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, pentyl, hexyl and cyclohexyl. Selected examples of a hydroxy-substituted alkyl group having 1 to 6 carbon atoms are hydroxymethyl, hydroxyethyl, 1-hydroxypropyl and 2-hydroxypropyl.

Above all, preferred is an organic ammonium compound in which R ¹ , R ² , R ⁵ and R ⁷ each represent a methyl group, R ³ and R ^{6 are} each a 2-hydroxypropyl group, x is a number between 1 and 3 and X ^- and Y ^- each OH ^- are.

The organic ammonium compound is preferably contained in the silicate coating composition at 0.1 to 3% by weight, based on the total weight of the silicate coating composition.

Other ingredients

The siliceous coating composition may contain other ingredients as long as they are not detrimental to the object of the invention.

Further constituents which may be used are, in particular, the additives customary in the prior art, for example thickeners, water repellents, dispersants and / or defoamers.

Examples of thickeners are polysaccharides, cellulose, bentonite and xanthan. Their content may be from 0.1 to 5% by weight, based on the total weight of the siliceous coating composition.

The hydrophobing agent may e.g. Polysiloxanes and especially amino-functional polysiloxanes include. The hydrophobing agent may be present at 0.1 to 5 weight percent, based on the total weight of the siliceous coating composition.

As the dispersant, e.g. Tetrasodium N- (1,2-dicarboxyethyl) -N-alkylsulfosuccinamid be used. The dispersant is preferably present at 0.1 to 0.5 wt.%, Based on the total weight of the siliceous coating composition.

As defoamers hydrophobic silica, liquid hydrocarbons, nonionic emulsifiers and / or synthetic copolymers may be present in the silicate coating composition. The preferred amount of antifoaming agent is from 0.1 to 1% by weight, based on the total weight of the silicatic coating composition.

Furthermore, the silicate coating composition may contain water, preferably in a content of 20 to 50 wt.%, Based on the total weight of the silicate coating composition.

Preparation of the silicate coating material

The silicatic coating composition which can be used according to the invention can be prepared by first dispersing the mineral component (B-2) in water, then optionally adding the organic ammonium compound (B-4) and optionally subsequently the polymer (B-3) in the form of a dispersion , After good dispersion, the water glass or the mixture of water glass and silica sol is added, then optionally further additives, such as thickeners and / or water repellents. By further stirring a very homogeneous dispersion is obtained.

Alternatively, a silicatic coating composition which can be used according to the invention can be prepared as follows: First, the waterglass or the mixture of waterglass and silica sol is initially introduced and optionally mixed with the organic ammonium compound (s). For this purpose, the mineral component is then added, wherein optionally usually first the pigments and then the fillers are stirred. Subsequently, if appropriate, a polymer dispersion can be stirred in. Last, the addition of other additives can be done, such. As a thickener and / or a hydrophobing agent.

example 1 Alkyd Primer 1

50% by weight of tall oil fatty acid alkyd resin (Uralac AD97, manufacturer DSM NeoResins) are diluted with 39.99% by weight of white spirit (Varsol 40, manufactured by Exxon Mobil Chemical). For this purpose, 10% by weight of diatomaceous earth (Dicalite WB 6, manufacturer Dicalite Europe) is mixed on the dissolver for 30 minutes to a fineness of less than 25 μm. The fineness is determined by grindometer.

As a desiccant, 0.01% by weight of cobalt 2-ethylhexanoate is added.

Silicate paint 1

A mixture of 3.8% by weight of a 30% potassium silicate solution and 17% by weight of a silica sol having a particle size of about 9 nm and a solids content of 20%, which has a molar ratio of 10 moles of SiO ₂ per mole of potassium oxide , Is presented and dispersed therein 10.5 wt .-% iron oxide for 5 minutes on the dissolver.

Subsequently, 29.3% by weight of layered silicate mixture (kaolinite, muscovite, chlorite) and 6% by weight of 2-ethylhexyl acrylate / methyl methacrylate copolymer (as a 50% dispersion in water) are added and homogenized.

After stirring in 0.2% by weight of bentonite, 0.1% by weight of xanthan gum and 1.5% by weight of an aminofunctional polydimethylsiloxane emulsion (Wacker 851 306), 31.6% by weight of water turns to ready-to-use consistency set.

application

The primer described above is applied by brush or airless on planed pinewood boards. After drying for 24 hours under standard conditions (23 ° C and 50% relative humidity), the boards are coated with the above-described silicate paint with a brush or roller.

A wood sample produced in this way was initially stored for two weeks at standard climate and then tested in a short-term weathering according to DIN EN 927-6. At the end of the 2000 hour test period prescribed in the standard, the coating was still completely intact.

Comparative Example 1

The silicate paint 1 described above was applied directly to the planed pinewood boards without previous priming. Subsequently, the wood sample thus prepared, as described in Example 1, stored for two weeks under standard conditions and then tested in a short-term weathering according to DIN EN 927-6.

After 48 hours of short-term weathering, color chips were observed along the wood grain.

Comparative Example 2

The alkyd primer 1 described above is applied by brush or airless on planed pinewood boards. After drying for 24 hours under standard conditions (23 ° C. and 50% relative humidity), the boards are coated with a commercial dispersion silicate paint according to DIN 18 363 2.4.1, which has the following composition: 30% by weight Potassium water glass with a molar ratio of SiO ₂ to potassium oxide of 2.55 4% by weight Butyl acrylate / methyl methacrylate dispersion 10% by weight TiO ₂ 40% by weight Filler mixture of calcium carbonate and mica Remaining amount: water

A wood sample thus produced was then stored for two weeks under standard conditions as described in Example 1 and then tested in a short-term weathering according to DIN EN 927-6.

After 500 hours, there was a strong lightening and washout and severe chalking.

Comparative Example 3

In a further comparative sample, a commercially available alkyd paint system (Xyladecor wood protection glaze) was twice applied with a brush to planed pine wood boards. The wood sample was then, as described in Example 1, stored for two weeks under standard conditions and then tested in a short-term weathering according to DIN EN 927-6.

After 500 hours, a strong darkening was observed, and after 2000 hours, the coating was almost completely weathered off.

Example 2 Alkyd primer 2

50% by weight of tall oil fatty acid alkyd resin (Uralac AD97, manufactured by DSM NeoResins) is diluted with 39.99% by weight of white spirit (Varsol 40, manufactured by Exxon Mobil Chemical). For this purpose, 10% by weight of silica (Acematt HK 125, manufacturer Evonik) are mixed. As a desiccant 0.01 wt .-% cobalt-2-ethylhexonoate are added.

Silicate paint 2

22 wt .-% e i nes filler (chlorite-quartz-mica adhesion) and 10% by weight of titanium dioxide are dispersed with 23 wt .-% water. 2% by weight of tetramethylammonium hydroxide (25% in water) are added. Subsequently, 7% by weight of an aqueous dispersion of a butyl acrylate / methyl methacrylate copolymer is added at 50%.

A mixture of 18% by weight of silica sol (particle size 5-8 nm, solids content 30% by weight) and 6% by weight of a 29% potassium silicate solution are stirred in. After dispersion, 10% by weight of a polysaccharide solution (solids content 5% by weight) are added. As a further additive, 2% by weight of a 50% emulsion of an aminoalkyl-substituted polydimethylsiloxane is added.

application

The primer described above is applied by brush or airless on planed pinewood boards. After drying for 24 hours under standard conditions (23 ° C and 50% relative humidity), the boards are coated with the above-described silicate paint by brush or roller.

The pinewood boards thus produced were stored for two weeks under standard conditions and then, as described in DIN EN 927-3, free-weathered. After expiry of the test period of one year prescribed in the standard, no flaking or other paint damage was observed.

Comparative Example 4

The silicate paint 2 described above was applied directly to the planed pinewood boards without prior priming. Subsequently, the wood sample thus prepared, as described in Example 2, stored for two weeks under standard conditions and then freeze-weathered according to DIN EN 927-3.

After 7 days, flakes appeared along the wood grain.

Comparative Example 5

Pinewood boards were coated with an alkyd resin primer (DELTA Impregnation Primer 1.02) containing no mineral fillers and dried for 24 hours. Subsequently, the boards were coated with the silicate paint 2 described above. After two weeks of drying under standard conditions, cross-cut tests according to EN 927-3 Annex C were carried out. The silicate paint peeled off along almost the entire cut length.

Claims (15)

1. Combination coating for wood comprising:

   - an alkyd resin primary coat which can be obtained by applying an alkyd resin coating composition (A) to the wood and allowing it to dry; and

   - a siliceous coating which can be obtained by applying a siliceous coating composition (B) to the alkyd resin primary coat;
   wherein the coating compositions (A) and (B) have the following compositions:

   (A) alkyd resin coating composition comprising:

   (A-1) a long-oil alkyd resin having a fatty acid proportion indicated relative to the weight proportion of the oil (oil length) of at least 60%; and an iodine number of the oil of 100 or more;

   (A-2) a mineral component;

   (A-3) solvent;

   (B) siliceous coating composition comprising:

   (B-1) water glass or a mixture of silica sol and water glass, wherein the molar ratio of SiO₂ to alkali metal oxide is 5 to 30 moles of SiO₂ per mole of alkali metal oxide; and

   (B-2) a mineral component.
2. Combination coating according to claim 1, wherein the alkyd resin coating composition (A) also contains a drying agent (A-4).
3. Combination coating according to claim 1 or 2, wherein in component (A-1) the iodine number of the oil is at least 160.
4. Combination coating according to one or more of claims 1 to 3, wherein component (A-2) is present in a quantity of 10-40 wt.%, relative to the weight of component (A-1).
5. Combination coating according to one or more of claims 1 to 4, wherein in component (B-1) the molar ratio of SiO₂ to alkali metal oxide contains 15 to 25 moles of SiO₂ per mole of alkali metal oxide.
6. Combination coating according to one or more of claims 1 to 5, wherein in component (B-1) the alkali metal oxide is potassium oxide.
7. Combination coating according to one or more of claims 1 to 6, wherein in component (B-1) the silica sol has an average particle size of < 10 nm.
8. Combination coating according to one or more of claims 1 to 7, wherein component (B-2) comprises one or more fillers which are selected from calcite, layered silicates and barite.
9. Combination coating according to one or more of claims 1 to 8, wherein the siliceous coating composition (B) also contains a polymer (B-3).
10. Combination coating according to claim 9. wherein the polymer (B-3) is a (meth)acrylate homopolymer or (meth)acrylate copolymer.
11. Combination coating according to claim 10, wherein the polymer (B-3) is present in a proportion of 3-10 wt.% relative to the weight of component (B).
12. Combination coating according to one or more of claims 1 to 11, wherein siliceous coating composition (B) also contains an organic ammonium compound (B-4).
13. Use of the combination of an alkyd resin coating composition (A) and a siliceous coating composition (B), as defined in one or more of claims 1-12, for coating wood, wherein the alkyd resin coating composition (A) is used for producing a primary coat for the siliceous coating composition (B).
14. Method for producing the combination coating according to one or more of claims 1-12 which comprises the following steps:

    1. primary coating of the wood by applying one or more layers of the alkyd resin coating composition (A) and allowing them to dry

    2. applying one or more layers of the siliceous coating composition (B) to the primary coat and allowing them to dry.
15. Combination coating kit comprising the alkyd resin coating composition (A) and the siliceous coating composition (B), as defined in one or more of claims 1-12, wherein the coating compositions (A) and (B) are separated from one another spatially.