Aqueous Polymer Dispersions for Coating Wood

The present invention relates to aqueous polymer dispersions essentially containing a copolymer of

a) from 85 to 98.5% by weight of methacrylates,

b) from 0.5 to 4% by weight of acrylic acid and/or methacrylic acid,

c) from 0.5 to 4% by weight of acrylamide and/or methacrylamide,

d) from 0.5 to 4% by weight of an .alpha.,.beta.-ethylenically diunsaturated or polyunsaturated compound and

e) from 0 to 3% by weight of a carbonyl-containing .alpha.,.beta.-ethylenically unsaturated compound,

and a process for its preparation and its use for coating wood.

Wood surfaces, such as those of paneling, furniture and doors, are frequently coated with transparent finishes for protection and for aesthetic reasons. To date, nitro finishes, acid-curable alkyd-amino finishes, finishes based on unsaturated polyesters, and polyurethane finishes have been used. The stated traditional finishes are unacceptable for reasons relating to occupational hygiene and environmental protection, because of the solvents which are present therein and which pass into the atmosphere during processing and drying.

German Laid-Open Application DOS 2,535,660 describes a process for the preparation of crosslinkable polymers and copolymers containing methylolated carboxamide groups. European Patent 29,144 discloses the use of N-methylol(meth)acrylamide-containing aqueous polymer dispersions for coating wood surfaces. The methylol groups may react with one another at above 100.degree. C., i.e. the finish may undergo crosslinking and hence an improvement in its resistance to chemical influences. However, the disadvantage is that the temperature required for the reaction of the methylol groups is frequently too high when the substrate is wood, and that formaldehyde is eliminated in this crosslinking reaction.

Furthermore, these coatings give surfaces which do not fully meet practical requirements with regard to scratch resistance, hardness, flexibility, sandability, stackability and resistance to chemical influences, for example drinks and cleaners.

It is an object of the present invention to provide polymer dispersions for the preparation of aqueous wood finishes which do not have the abovementioned disadvantages.

We have found that this object is achieved by aqueous polymer dispersions essentially containing a copolymer of

a) from 85 to 98.5% by weight of methacrylates,

b) from 0.5 to 4% by weight of acrylic acid and/or methacrylic acid,

c) from 0.5 to 4% by weight of acrylamide and/or methacrylamide,

d) from 0.5 to 4% by weight of an .alpha.,.beta.-ethylenically or polyunsaturated compound and

e) from 0 to 3% by weight of a carbonyl-containing .alpha.,.beta.-ethylenically unsaturated compound.

Particularly suitable components a) are methacrylates of C.sub.1 -C.sub.12 -monoalcohols, preferably C.sub.1 -C.sub.8 -monoalcohols, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butanol, cyclohexanol, 2-ethylhexanol and lauryl alcohol. n-Butyl methacrylate or a mixture of n-butyl methacrylate with up to 25% by weight of methyl methacrylate is particularly preferred. The methacrylates are used in amounts of from 85 to 98.5, preferably from 90 to 97, % by weight.

Components c) and d) are each used in amounts of from 0.5 to 4, preferably from 0.5 to 2.5, % by weight.

.alpha.,.beta.-ethylenically diunsaturated or polyunsaturated compounds are used as component d), in amounts of from 0.5 to 4%, preferably from 0.5 to 2.5, % by weight. Examples of suitable compounds are divinylaromatics, such as divinylbenzene, diacrylates and dimethacrylates of

C.sub.2 -C.sub.24 -diols, such as butanediol diacrylate, butanediol dimethacrylate and hexanediol diacrylate, divinylureas, such as divinylethyleneurea, and di- and triallyl compounds, such as diallyl phthalate and triallyl isocyanurate. Divinylbenzene, butanediol diacrylate and butanediol dimethacrylate are preferably used.

Carbonyl-containing .alpha.,.beta.-ethylenically unsaturated compounds e) may also be present. Where they are used, from 1 to 3% by weight are preferred. Examples of suitable compounds are acrolein, methacrolein, diacetoneacrylamide, diacetonemethacrylamide, 2-butanonemethacrylates and vinyl acetoacetate.

From 0.5 to 1 mole of a dihydrazide of an aliphatic dicarboxylic acid, preferably of a C.sub.4 -C.sub.16 -dicarboxylic acid, can be added to the dispersion per mole of carbonyl-containing monomer, as an additional crosslinking agent where compounds (e) are present. Examples of suitable dihydrazides are adipodihydrazide, glutarodihydrazide and succinodihydrazide.

The novel polymer dispersions can be prepared by conventional emulsion polymerization processes using initiators, such as hydrogen peroxide, sodium persulfate, potassium persulfate, organic peroxides or conventional redox initiator systems, regulators, such as mercaptans, emulsifiers/protective colloids, such as Na salts of C.sub.12 -C.sub.18 -paraffinsulfonic acids or salts of acidic sulfuric half esters of ethylene oxide (propylene oxide) adducts, and other conventional assistants, so that further information is not necessary.

The novel dispersions usually have solids contents of from 20 to 60, preferably from 40 to 50, % by weight and LT (light transmission) values of from 70 to 98%, preferably from 90 to 96%.

Wood finishes prepared from the novel polymer dispersions may contain conventional additives, such as waxes based on paraffin and polyethylene. The waxes are usually added as 20-60% strength by weight dispersions during or after the emulsion polymerization. The waxes used have softening points of about 50.degree.-250.degree. C., preferably 70.degree.-130.degree. C. The size of the wax particles may be from 0.01 to 0.5 .mu.m.

To improve film formation, film forming assistants, such as butylglycol, mixtures of diisobutyl esters of adipic, glutaric and/or succinic acid and other solvents may be added in amounts of from 3 to 8% by weight.

Flatting agents, such as silica compounds, in amounts of from 0.5 to 2% by weight, and thickeners based on polyurethane and/or polyacrylates may also be present.

The wood finishes prepared using the novel dispersions can be processed by brushing, spraying and curtain coating.

Drying is carried out at room temperature but may also be effected at elevated temperatures of up to 80.degree. C. in conventional tunnel dryers.

The coatings thus obtained are readily sandable and have good stacking properties and high resistance to chemical influences.

The initially taken mixtures stated in each of Examples 1 to 4 were heated to 85.degree. C., while stirring, in a reaction vessel equipped with four feed vessels. 5% by weight of each of the feeds 1 and 2 were added, and initial polymerization was carried out for 15 minutes. The remaining amount of feed 1 was then added in the course of 2.5 hours, and the remaining amount of feed 2 introduced in the course of 2.75 hours. When feed 1 was complete, polymerization was continued for a further 1.75 hours, after which the mixture was cooled to 20.degree. C. The pH was brought to 8 with 25% strength by weight aqueous ammonia solution. In Examples 1 and 2, the particular feed 3 stated there was then added dropwise in the course of 5 minutes, and stirring was continued for 55 minutes. The feeds 4 described in Examples 1 to 4 were finally each introduced in the course of 15 minutes, while stirring. The characteristic properties of the polymer dispersions are shown in the relevant Examples.

20 g of xylene,

10 g of 3,3,5-trimethylpentane-1,3-diol isobutyrate,

3 g of the Na salt of a sulfated oleic acid dibutylamide (about 40% strength by weight in water),

4 g of an antifoam based on mineral oil,

21 g of water,

25 g of a 55% strength by weight polyurethane thickener in butyldiglycol/water (reaction product of an oxyethylated tallow fatty alcohol with hexamethylene diisocyanate),

15 g of a flatting agent based on silica,

34 g of ethylene glycol mono-n-butyl ether,

40 g of diethylene glycol monoethyl ether,

806 g of polymer dispersion according to Example 1, 2, 3 or 4,

2 g of a C.sub.16 -C.sub.19 -oxoalcohol containing 35 propylene oxide and 5 ethylene oxide units,

20 g of 50% strength by weight Ca stearate dispersion in water and

20 g of a 35% strength by weight polyethylene wax emulsion (dropping point of the solid 130.degree. C.)

were mixed at room temperature, with stirring, to give a wood finish.

Table 1 summarizes the test results for the wood surfaces (light oak) coated with this wood finish. The wood finish was applied by spraying on two coats (about 80 g/m.sup.2 of finish in each case) and sanding between the coats, and was dried at room temperature.