Aqueous Coating Compositions for Wood Surfaces

This invention is directed to aqueous coating compositions. More particularly, this invention is directed to aqueous coating compositions for wood surfaces which can be used to obtain clear open and/or closed pore coatings. The coating compositions are based on polymer dispersions containing N-methylol acrylamide units. Alternatively, the dispersions containing N-methylol acrylamide units can be processed together with a paint composition based on a water-soluble acrylic copolymer.

The painting of wood with clear varnishes is described in detail in "Lehrbuch der Lacke und Beschichtungen," H. Kittel, Vol. V, pages 305-317, edition W. A. Colomb, 1977. As stated thereby, nitrocellulose lacquers (cellulose nitrate lacquers at times referred to as NC-lacquers) and acid curing alkyd-amine resin enamels, polyurethane paints and paints based on unsaturated polyesters are primarily used in varnishes for use on wood. All these coating compositions require a large amount of organic solvent for processing and application or of polymerizable monomers, which, at least in part, can have a narcotic effect. Accordingly, there is a concerned effort to obtain coating compositions based on water as a solvent, eliminating substantial environmental pollution, and where the paints correspond to nitrocellulose lacquers in performance.

Polymer dispersions containing N-methylol acrylamide units are known in the literature and have been used for various purposes. U.S. Pat. No. 3,856,734 describes the preparation of dispersions from vinylacetate, N-methylol acrylamide, and acrylic acid. The dispersions are used as adhesive for plywood. Acrylic dispersions which, in addition to the acrylates or methacrylates, can also optionally contain N-methylol acrylamide are disclosed in U.S. Pat. No. 3,790,520. The dispersions are used in the textile industry, and for the surface protection of plastics, metals, and wood. Dispersions described in Deutsche Auslegeschrift No. 10 47 431 which contain methylol acrylamide units are used for crosslinking polymer particles during production of the dispersion. Deutsche Offenlegungsschrift No. 25 35 660 describes the preparation of auto-crosslinking polymer dispersions containing methylolated carboxylic acid amides and polymer solutions wherein emulsion polymers containing acrylamide are produced which are subsequently reacted with methylolated carboxylic acid amides. These known dispersions, when stoved at the conditions applicable to wood, give coatings which do not meet the requirements for practical use from the standpoint of mar resistance, hardness and flexibility, and pile strength.

Accordingly, it is a primary object of the present invention to provide coating compositions for wood surfaces, particularly for furniture coatings, which will replace the conventional nitrocellulose lacquers and contain low levels of organic solvents.

The requirements of the furniture industry for coatings for wood applications include a short cure time at from 70.degree.-90.degree. C.; good hardness and mar resistance in order to allow the objects to be stacked within a short time without damage to the coated surface. Furthermore, the coating should not mask but rather should bring out the wood grain and color of the wood. Finally, the finish obtained should have enhanced resistance, particularly to water. These requirements are met by the compositions of the present invention which include polymer dispersions which contain copolymers with a high glass transition temperature, in conjunction with specifically selected coalescing agents, and provide an auto-crosslinking mechanism.

The present invention is, therefore, concerned with the use of polymer dispersions containing N-methylol acrylamide, and having the formulation:

30-65% by weight of methylmethacrylate;

10-17% by weight of ethylacrylate;

10-30% by weight of butylmethacrylate;

5-20% by weight of styrene;

3-10% by weight of N-methylol(meth)acrylamide,

and containing as coalescing agent 5 to 15% by weight (of the dispersion) of a blend having the formulation

20-65% by weight of a high boiling paint solvent such as 2,2,4-trimethylpentane-1,5-diol-monotrimethylacetate, and

80-35% by weight of ethylene glycol monoethyletheracetate, and/or ethylene glycol monobutyletheracetate.

The compositions can optionally be utilized in conjunction with basic varnishes based on cationic water-soluble acrylic copolymers, for open pore or closed pore coatings for wood surfaces.

In a particularly advantageous embodiment of the invention the dispersions are prepared from

45-55% by weight of methylmethacrylate;

12-15% by weight of ethylacrylate;

15-25% by weight of butylmethacrylate;

5-10% by weight of styrene, and

7-9% by weight of N-methylol methacrylamide,

with the addition of a coalescing agent (calculated on the emulsion) having the formulation

4-6% by weight of the mentioned high boiling paint solvent,

7-9% by weight of ethylene glycol monoethyletheracetate, or

3-5% by weight of ethylene glycol monobutyletheracetate.

The coating compositions of the invention, after drying for 5 to 10 minutes at 75.degree. to 90.degree. C., form coatings on wood which have excellent appearance, excellent hardness and mar resistance, and very good water resistance. The overall performance is in all cases equal to the optimum performance of a nitrocellulose lacquer, with the additional essential advantage of non-flammability and non-toxicity. The toxicity rating of the dispersions of the invention corresponds to YSAM, group 0. Furthermore, the pile strength of the coatings is excellent after a short cooling period subsequent to the curing procedure. The compositions are particularly suited for open pore coatings.

The preparation of the polymer dispersions is effected in known manner by emulsion polymerization in the presence of anionic and nonionic emulsifiers and free radical initiators. Water and the emulsifiers are charged to the polymerization reactor and heated to 85.degree. to 88.degree. C. The monomer blend, the Methylolmethacrylamide and the initiator solution are added from three separate addition funnels simultaneously over a period of 90 minutes. After the end of the addition, the batch is allowed to react at 88.degree. to 90.degree. C. for about 40 minutes.

The polymer dispersions of the invention are prepared from

30-65, preferably 45-55% by weight of methylmethacrylate;

10-17, preferably 12-15% by weight of ethylacrylate;

10-30, preferably 15-25% by weight of butylmethacrylate;

5-20, preferably 5-10% by weight of styrene, and

3-10, preferably 7-9% by weight of N-methylol(meth)acrylamide.

Suitable anionic emulsifiers are alkylarylethoxylate sulfates, the sulfates of ethoxylated fatty alcohols, dodecylbenzol sulfonate, or sulfosuccinamates. Nonionic emulsifiers such as the ethoxylated alkylphenols can be used with advantage in conjunction with the anionic emulsifiers. Furthermore, small quantities of sodium vinyl sulfonate can be copolymerized as an additional emulsifier. Suitable coalescing agents are blends of 2,2,4-trimethylpentane-1,5-diol-monotrimethylacetate with ethylene glycol monoethyletheracetate or with ethylene glycol monobutyletheracetate in a ratio of from about 20:80 and 65:35. The coalescing agents are mixed before addition and the blend is very slowly added to the dispersion with good stirring. Storage stability of the dispersion, also with the coalescing agents, is very good. The level of coalescing agents is from 5 to 15% by weight of the dispersion.

The optical appearance of clear varnish coated wood, such as the accentuation of the grain and the uniformity of the coating, can be still further enhanced through the use of a basic varnish of special water-soluble acrylic copolymers. The acrylic copolymers suitable for this purpose are amine adducts of copolymers carrying glycidyl groups, which can be rendered water soluble due to the neutralization of their cationic groups with inorganic or organic acids. The copolymers have the following monomer composition:

20-40% by weight of glycidylmethacrylate;

40-80% by weight of methylmethacrylate, and

0-40% by weight of other (cyclo)alkylmethacrylates, the homopolymers of which have glass transition temperatures of over +40.degree. C.

The copolymers, produced in known manner, are reacted with secondary amines carrying 1 or 2 hydroxyalkyl radicals, the ratio being 1 mole of a secondary amine per 1 glycidyl group. The copolymers for the basic paint are prepared in known manner through solution polymerization in ethanol or isopropanol. The method includes slowly adding the monomer blend mixed with the chain transfer agent to the charged solvent and initiator. When a conversion of practically 100% is reached, the amine is slowly added in drops at 80.degree.-85.degree. C. The obtained adduct is practically infinitely dilutable with water upon neutralization with an organic or inorganic acid. Suitable amines are secondary mono- or dialkanol amines, such as diethanol amine, di-(iso)propanolamine, cyclohexylethanol amine, or ethylethanol amine. The basic paint shows excellent penetration of the wood and, upon drying at normally applied conditions of 3-10 minutes at 70.degree.-90.degree. C., has the hardness, flexibility, and sanding characteristics as required by the industry. The polymer dispersions and the optionally used basic paint can be applied by brushing, curtain coating, spray gun, and the like.

The following examples illustrate the invention without limiting its scope. Parts and percentages are by weight.

A polymerization reactor equipped with reflux condensor, thermometer, and inert gas supply is charged with 1322 parts of deionized water, 18 parts of the sodium salt of an octylphenol reacted with 2 moles of ethylene oxide and sulfated, 12 parts of an octylphenol reacted with 16 moles of ethylene oxide and 7.2 parts of sodium vinyl sulfonate. The charge is heated to 85.degree.-87.degree. C. While continuously stirring, the following ingredients are added simultaneously, from three separate addition funnels, within a period of 90 minutes:

1. A monomer blend consisting of 645 parts of methylmethacrylate, 267 parts of n-butylmethacrylate, 180 parts of ethylacrylate, and 66 parts of styrene;

2. 246 parts of a 36% aqueous solution of N-methylolmethacrylamide, and

3. a solution of 3 parts of ammonium peroxodisulfate in 90 parts of water.

A temperature of 86.degree.-88.degree. C. is maintained during polymerization. After the end of the monomer addition, the batch is reacted at 88.degree.-90.degree. C. for another 40 minutes. The dispersion is cooled, and, while stirring well, a blend of 143 parts of a paint solvent based on 2,2,4-trimethylpentane-1,5-diol-monotrimethylacetate (boiling range 180.degree.-182.degree. C. at 125 mm Hg, such as Texanol.RTM., a product of Eastman Kodak Prod. Inc.) and 228 parts of ethylene glycol monoethylether acetate or 114 parts of ethyleneglycolmono butyletheracetate is added. A stable, fine particle dispersion of about 41% solids is obtained. For testing, the paint formulation is applied to various types of wood, i.e., oak, walnut, and mahogany panels, by spraying and curtain coating. After a flash-off of 1 minute, the coatings are dried at 85.degree. C. for 5 minutes.

The obtained coatings have a film thickness of about 30 .mu.m. They are very hard, have good mar resistance, and an excellent appearance. The Buchholz test (Erichsen, DIN 53 153) gives a value of 1.7 mm. Color and the wood grain of all types of wood are accentuated.

The process of Example 1 is repeated using an initial reaction charge of 1268 parts of deionized water, 18 parts of the sodium salt of an octylphenol reacted with 2 moles of ethylene oxide and sulfated, 12 parts of an octylphenol reacted with 16 moles of ethylene oxide and 7.2 parts of sodium vinylsulfonate. At 85.degree.-87.degree. C. the following ingredients are added:

1. A monomer blend of 481 parts of methylmethacrylate, 342 parts of n-butylmethacrylate, 144 parts of ethylacrylate, and 156 parts of styrene;

2. 282 parts of a 36% aqueous solution of N-methylolmethacrylamide; and

3. a solution of 3 parts of ammonium peroxodisulfate in 90 parts of water.

A blend of 111 parts of the high boiling solvent used in Example 1 and 84 parts of ethylene glycol monobutyletheracetate are used as the coalescing agent. The obtained dispersion has substantially the same properties as the dispersion obtained according to Example 1.

The water-soluble cationic copolymers used in suitable base paints are prepared as follows:

A reactor equipped with stirrer, thermometer, and reflux condensor is charged with 40 parts of isopropanol containing dissolved therein 2.5 parts of azobisisobutyronitrile. The charge is heated to about 80.degree. C. Then the monomer blends listed in Table 1 are added, and 2 parts of dodecylmercaptan or a solution of 2.5 parts of azobisisobutyronitrile in 20 parts of isopropanol and ethyl acetate (1:1) are added simultaneously and continuously from separate vessels. The addition is made at a rate so as to guarantee a conversion of 90% at any time of the polymerization. The polymerization is finished when, at the end of the addition, the polymer content corresponds to a conversion of 98-100%. Then, at 40.degree.-50.degree. C., a solution of diisopropanol amine in isopropanol is added slowly, as listed in Table 1. The exothermic reaction is monitored in order that the temperature does not exceed 80.degree. C. After the end of the addition, a temperature of 80.degree. to 85.degree. C. is held for 2 hours.

In Examples B and C tert.butylperhexanoate was used as a replacement for the azobisisobutyronitrile. The copolymers have a slightly yellow color and are infinitely dilutable with water upon neutralization with acetic acid to a pH-value of from 4 to 6.

In preparing a base paint the resin solution is neutralized with diluted acetic acid, about 30%, to a pH-value of from 5 to 6, and diluted with deionized water to a solids content of 15 to 30%.

The wood types mentioned in Example 2 were coated with a double coat system, the base coat consisting of the copolymers A-H of Table 1. The finishing coats were the dispersions of Examples 1 and 2. The base coat was applied in a quantity of about 30 g/m.sup.2 and dried for 5 minutes at 85.degree. C. The coated surface could be sanded well (emery paper No. P 320). After sanding, the top dispersion coat was applied and also cured for 5 minutes at 85.degree. C. The coating with a thickness of 30 .mu.m had an excellent appearance, with fine gloss and great hardness and mar resistance. The differences between the various coatings were only slight. Piled panels did not stick together, even with loads. The Buchholz test gave a result of 1.7 mm in all cases. Color and wood grain were strongly accentuated. The performance of the coatings corresponds to that of an optimum quality nitrocellulose lacquer. On water soak a slight swelling could be observed after 2 hours.

Substantially the same results were obtained with dispersions prepared according to the method of Example 1 from 1314 parts of deionized water, 18 parts of the sodium salt of an octylphenol reacted with 2 moles of ethylene oxide and sulfated, 12 parts of an octylphenol reacted with 16 moles of ethylene oxide and 7.2 parts of sodium vinyl sulfonate, with the following three components:

1. A monomer blend of 697 parts of methylmethacrylate, 192 parts of n-butylmethacrylate, 180 parts of ethylacrylate, and 60 parts of styrene;

2. 203 parts of a 36% aqueous solution of N-methylolmethacrylamide; and

3. a solution of 3 parts of ammonium peroxodisulfate in 90 parts of water.

The coalescing agent is a blend of 139 parts of the high boiling paint solvent used in Example 1 and 194 parts of ethylene glycol monoethyletheracetate.

As will be apparent to one skilled in the art, various modifications can be made within the scope of the aforesaid description. Such modifications being within the ability of one skilled in the art form a part of the present invention and are embraced by the appended claims.