GB1562243A

GB1562243A - Radiation-curablecoating compositions

Info

Publication number: GB1562243A
Application number: GB46941/76A
Authority: GB
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1975-11-12
Filing date: 1976-11-11
Publication date: 1980-03-05
Also published as: FR2331606B1; NL7612331A; FR2331606A1; DE2550740A1; BE848267A

Description

(54) RADIATION-CURABLE COATING COMPOSITIONS (71) We, BASF AKTIENGESELL SCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to radiationcurable coating compositions which comprise a polymerizable, unsaturated aminoplast resin which carries acrylic or methacrylic groups and acts as a binder.

It has been disclosed that coating compositions which contain prepolymers, carrying acrylate groups as binders can be cured by high energy radiation and thereby give coatings possessing good adhesion, a smooth surface and good abrasion resistance and impact strength.

According to German Published Application 1,745,540, polymerizable aminoplast resins are manufactured by reacting melamine/formaldehyde condensates with hydroxyacrylates. Cation exchange resins or silicates are employed as catalysts for this reaction. Th use of hydrochloric acid is alleged to cause the reaction mixture to gel during the reaction. German Published Application 2,132,318, discloses acrylate-based paints and coating compositions which are curable by electron beams; these materials are of very complicated composition and contain, inter alia, a reaction product of an amine/formaldehyde condensate with a hydroxyacrylate. The reaction is carried out in a weakly alkaline medium or in a weakly acid medium, for example in the presence of acrylic acid.The acrylate-modified melamine/ formaldehyde condensates, curable by electron beams, which are disclosed in Swiss Patent 560,729, are also manufactured by reaction in a weakly acid range, for example in the presence of formic acid.

Experiments have shown that such reaction products, manufactured in a weakly acid range, of amine/formaldehyde condensates with hydroxyacrylates or hydroxymethacrylates have a relatively low reactivity, so that when they are cured with ultraviolet light, long exposure times are needed, or if they are cured with electron beams, high radiation doses are required.

The present invention seeks to provide radiation-curable coating compositions which comprise, as binder, a polymerizable unsaturated aminoplast resin which has a high reactivity and is simple to manufacture.

We have found that good results may be achieved if the polymerizable unsaturated aminoplast resin used is one in whose manufacture an amine/formaldehyde condensate, which may be completely or partially etherifled, is reacted with an acrylic or methacrylic compound containing one or more hydroxy groups or amino groups, in the presence of an organic or inorganic acid having a pK of < 3.0.

According to the present invention there is provided a radiation-curable coating composition consisting of or containing, as the sole binder or one of the binders, a polymerizable unsaturated aminoplast resin which carries acrylic or methacrylic groups and has been manufactured by reacting A) an amine/formaldehyde condensate which is optionally completely or parti ally etherified, and B) an acrylic or methacrylic compound containing one or more hydroxy groups and/or amino groups, from 1/3 to 1 mole of the acrylic compound being employed per equivalent of cH.2O group of the amine/formaldehyde con densate, in the presence of C) an organic or inorganic acid haying a pK in aqueous solution of < 3.0 as the catalyst, the acid being employed in an amount of from 2 to 60 milliequi valents per equivalent of CH group of the amine/formaldehyde con densate, and in the presence or absence of D) a polymerization inhibitor.

From the disclosure of German Published Application 1,745,540, it was not to be expected that this reaction with strong acids would be feasible at all.

The amine/formaldehyde condensates of component A in the manufacture of the resin binder are basically condensation products of aminoplast precursors, containing one or more amino, imino or amido groups, with from 1 to 2 moles of formaldehyde per amino, imino or imido group. Examples of aminoplast precursors include melamine, urea, benzoguanamine, acetoguanamine, acetylenediurea, ethyleneurea, proplyeneurea and also carbamates and urones. These compounds may be condensed with formaldehyde in a weakly alkaline medium, and, depending on the reaction conditions, simple reaction products or more highly condensed polymeric compounds may be produced. They are characterized by the -N-CH, group, which may carry 1 or 2 -CH2O- groups on the nitrogen.

These condensation products can now be used directly for the reaction according to the invention. However, it is preferred to employ them in a completely or partially etherified form. This etherification is generally carried out in an acid medium, using monoalcohols of 1 to 6 carbon atoms. Condensation produts of melamine with from 3 to 6 molecules of formaldehyde, which condensation products are partially etherified with monoalcohols, pre ferably with methanol or butanol, are preferred. Component A may be employed undiluted or in aqueous or alcoholic solution.

To form the aminoplast resin the amine/ formaldehyde condensate A is reacted with one or more acrylic or methacrylic compounds B carrying one or more hydroxy groups or amino groups. For this purpose, monoesters of acrylic acid or methacrylic acid with alkanediols of 2 to 4 carbon atoms, e.g. hydroxyethyl acrylate or methacrylate, hydroxypropyl acrylate or methacrylate with butanediol monoacrylate, are preferred as the compounds B.

N - methylolacrylamide, N - methylolmeth- acrylamide, and the same compounds etherified with butanediol, may also be used. Suitable compounds possessing amino groups are acrylamide and methacrylamide. Particularly good results are obtained by employing a mixture of at least one acrylic or methacrylic compound containing one or more hydroxy groups and at least one acrylic or methacrylic compound containing one or more amino groups, in a molar ratio of from 4:1 to 1:4.

The ratio of the reactants A and B for the reaction to form the aminoplast resin is so chosen as to correspond to from 1/3 to 1 mole of acrylic compound per equivalent of -CH2-O- group of the amine/formaldehyde condensate.

The principal essential feature of the invention is that the reaction is carried out in the presence of a strong organic or inorganic acid.

Accordingly, the pK of the acid in aqueous solution is required to be less than 3.0, preferably less than 2.0 (examples of pK tabulations are to be found in the Handbook of Chemistry and Physics, 48th edition, 1%7/68, D90/D91). The pK is the negative logarithm of the dissociation constant, the critical dissociation constant being the one relating to the first stage where polybasic acids are concerned. Examples of suitable acids are hydro- chloric acid, sulfuric acid, para-toluene-sulfonic acid, oxalic acid, maleic acid and phthalic acid, hydrochloric acid being preferred. The acids are employed in amounts of from 2 to 60 milliequivalents, preferably from 3 to 40 milliequivalents, per equivalent of -CH2-O- group of the amine/formaldehyde condensate.

The reaction is preferably carried out in the presence of one or more inhibitors D, which are intended to prevent the polymerization of the acrylic compounds, and are generally used in amounts of from 0.01 to 0.5% by weight, based on compounds A and B.

Examples of suitable compounds are hydroquinone, hydroquinone monoalkyl ethers, phenothiazine or salts of N-nitrosocyclohexylamine.

The reaction temperature is usually from 50 to 120"C, preferably from 70 to 100"C.

Water and/or volatile monoalcohols formed in the condensation are, in general, distilled off in the course of the reaction. This can be done either by working under reduced pressure, e.g. at from 50 to 200 mm Hg, or by carrying out the reaction in the presence of azeotropic entraining agents, e.g. cyclohexane.

The reaction in general requires from 10 minutes to 5 hours. It may be terminated by neutralizing the catalyst acid with a base, e.g.

sodium hydroxide solution, sodium carbonate or an amine.

The resulting polymerizable unsaturated aminoplast resins usually have a viscosity of from 400 to 10,000 mPas at 20"C. Resins of low viscosity may in themselves be used as coating compositions. More viscous resins may be diluted with copolymerizable organic compounds, advantageously rhose which are liquid at room temperature. These reactive diluents may be employed in amounts of from 0 to 60% by weight, preferably from 10 to 50% by weight, based on the coating composition.

Examples are butanediol diacrylate, trimethylolpropane triacrylate, N-vinylpyrrolidone, divinylurea or 2-ethylhexyl acrylate.

In order to achieve special effects, inorganic or organic pigments, e.g. titanium dioxide, or fillers, e.g. talc, may be added to the coating compositions, generally in amounts of from 0 to 80% by weight, preferably from 3 to 60% by weight, based on the coating composition.

The organic additives conventionally used in the surface-coatings industry, e.g. thixotropic agents1 flow control agents, de-aerating agents or lubricants, may also be present usually, in amounts of up to low by weight.

In addition to the polymerizable unsaturated aminoplast resins, the coating compositions may contain from 0 to 50% by weight of one or more other polymerizable unsaturated binders, eg. unsaturated polyesters or acrylatemodified epoxy resins, polyurethanes and saturated polyesters.

The coating compositions of the invention may be used for finishing substrates such as wood, metal, paper or plastic, and may also be used as trowelling mixes or as printing inks.

They are cured by polymerizing the acrylic or methacrylic groups, as the case may be.

Polymerization may be initiated either by means of electron beams having an energy of from 50 to 500 KeV, or by UV light. In the latter case, photoinitiators, advantageously in amounts of from 0.5 to 10% by weight, are added to the coating compositions.

In the Examples, parts and percentages are by weight.

EXAMPLE 1 390.0 parts of hexamethoxymethylmelamine, 432.0 parts of butanediol monoacrylate, 0.5 part of hydroquinone, 4.7 parts of concentrated hydrochloric acid and 240.0 parts of cyclohexane are used.

The reaction mixture is heated to the boil and a mixture of water and methanol is removed by means of an azeotrope separator.

The temperature rises from 70 to 85"C. After neutralizing the mixture with aqueous sodium hydroxide solution, the cyclohexane is removed by distillation or vacuum distillation at up to 90"C. About 700 g of a resin having a viscosity of about 1,500 mPas remain in the flask.

Curing: a) Curing by means of an electron beam: a 120 sLm coating of the resin was applied to sheet metal and irradiated with an electron beam having an energy of 350 KeV to provide a dose of 0.9 Mrad. The resulting surface-coating film was nail-hard and glossy.

b) Curing with ultraviolet light: 4 Ó of benzoin butyl ether were added to the resin, and a 15 ,am thick layer was irradiated, on a belt travelling at 20 m/minute, under nitrogen, using a 20 cm long mercury high-pressure tube having an output of 80 Watt/cm (HOK 2-ultraviolet tube). Here again the resulting film was nail-hard and glossy.

EXAMPLE 2 550.0 parts of hexabutoxymethylamine, 71.0 parts of acrylamide, 144.0 parts of butanediol monoacrylate, 1.0 part of oxalic acid and 0.5 part of hydroquinone are used.

About 135 g are distilled off in the course of one hour under reduced pressure at 100"C.

After neutralizing with sodium hydroxide solution, the viscous resin is diluted with 45 parts of 2-ethylhexyl acrylate.

EXAMPLE 3 350.0 parts of a 70 ,Ó strength aqueous solution of a partially methanol-etherified tetramethylolmelamine, 390.0 parts of hydroxypropyl acrylate, 0.5 part of hydroquinone and 2.4 parts of concentrated hydrochloric acid are used.

About 125 g are distilled off in the course of 3.5 hours under reduced pressure at from 70 to 80"C. The product is neutralized with sodium hydroxide solution. The resin, mixed with 4% of benzoin butyl ether, gives a nailhard, glossy film when irradiated with an HOK 6 ultraviolet lamp (tube length 60 cm), at a belt speed of 10 m/minute.

EXAMPLE 4 1,140.0 parts of an 80 ó strength solution of partially methanol-etherified hexamethylolmelamine in methanol, 426.0 parts of acrylamide, 1,170.0 parts of hydroxypropyl acrylate, 1.5 parts of hydroquinone and 7.2 parts of concentrated hydrochloric acid are used.

In the course of 4.5 hours, a total of about 410 g is distilled off under reduced pressure at from 70 to 120"C. The resin which remains is neutralized and then filtered.

On adding 4Oió of benzoin butyl ether to the resin and irradiating a 30 jim thick film with an HOK 6 ultraviolet lamp, in the presence of air, at a belt speed of 7 m/minute, a hard, glossy surface-coating film is obtained.

Comparative Example according to German Published Application 1,754,540 EXAMPLE 5 390.0 parts of hexamethoxymethylmelamine, 130.0 parts of hydroxypropyl acrylate, 0.2 part of hydroquinone and 26.0 parts of strongly acid cation exchanger are used.

The mixture was heated at 97"C for 4 hours; contrary to (the statement in) the German Published Application, no distillate passed over.

If 4% of benzoin butyl ether are added to the filtered resin, no curing occurs under an HOK 6 ultraviolet lamp even at a belt speed of only 2 rn/minute.

EXAMPLE 5 300 parts of an aqueous solution, of about 70% strength, of a partially methanol-etherified urea-formaldehyde resin (corresponding to about 2 moles of urea), 520 parts of hydroxypropyl acrylate, 0.5 part of hydroquinone and 1.7 pans of concentrated hydrochloric acid are used.

The mixture is subjected to a distillation under reduced pressure for 2.5 hours at a temperature rising from 60 to 90 , then for 6 hours at from 90 to 95" and finally for 1 hour at 100 .

After adding 3 parts of 50% strength sodium hydroxide solution, about 670 g of a resin having a viscosity of 1,120 mPas (at 20 ) are obtained.

EXAMPLE 6 a) According to the invention: 390 parts of hexamethoxymethylmelamine, 390 parts of hydroxypropyl acrylate, 0.5 pan of hydroquinone and 3.4 parts of concentrated hydrochloric acid are used.

Material is distilled off under reduced pressure for about 90 minutes at 70" and then for about 25 minutes at 90" (weight loss about 90 g). After adding 6 parts of 50% strength sodium hydroxide solution, 700 g of a resin having a viscosity of 2,480 mPas (at 20 ) are obtained.

b) Comparative Experiment according to Swiss Patent 560,729, Example 2.

390 pans of hexamethoxymethylmelamine, 390 parts of hydroxypropyl acrylate, 0.65 part of hydroquinone and 4.5 parts of formic acid are used.

About 40 g are distilled off under reduced pressure at 110 in the course of 4 hours.

745 g of a resin having a viscosity of 200 mPas (at 20 ) are obtained.

Curing: The resin from Example 6a was diluted with about 20% of hydroxypropyl acrylate to a viscosity of about 350 mPas (at 20 ). 3% of benzil dimethyl keral were added to this solution and to the resin from Example 6b, and a 100 am thick layer of each was applied to paper and irradiated by means of an HOK 6 ultraviolet lamp. At 7 m/tube.minute the resin according to the invention gave a scratchresistant glossy film, whilst under the same conditions the comparative resin showed no curing whatsoever.

WHAT WE CLAIM IS: 1. A radiation-curable coating composition comprising a polymerizable unsaturated aminoplast resin which carries acrylic or methacrylic groups, acts as a binder and has been manufactured by reacting A) an amine/formaldehyde condensate which is optionally completely or par tially etherified, and B) an acrylic or methacrylic compound containing one or more hydroxy groups and/or amino groups, from 1/3 to 1 mole of the acrylic compound being employed per equivalent of CH2 0 group of amine/formaldehyde con densate, in the presence of C) an organic or inorganic acid having a pK in aqueous solution of < 3.0 as the catalyst, the acid being employed in an amount of from 2 to 60 milliequivalents per equivalent of H2-O- group of the amine/formaldehyde condensate, and in the presence or absence of D) a polymerization inhibitor.

2. A coating composition as claimed in claim 1, wherein component A in the manufacture of the resin is a condensation product of an aminoplast precursor carrying one or more amino, imino or amido groups, with from 1 to 2 molecules of formaldehyde per amino, imino or amino group.

3. A coating composition as claimed in claim 1 or 2, wherein the component A in the manufacture of the resin is an amine/formaldehyde condensation product which is completely or partially etherified with a monoalcohol of 1 to 6 carbon atoms.

4. A coating composition as claimed in claim 3, wherein component A in the manufacture of the resin is a condensation product of melamine and from 3 to 6 molecules of formaldehyde, which product is partially etherified with methanol or butanol.

5. A coating composition as claimed in any of claims 1 to 4, wherein component B in the manufacture of the resin is a monoester of acrylic acid or methacrylic acid with an alkanediol of 2 to 4 carbon atoms, or is acrylamide or methacrylamide.

6. A coating composition as claimed in any of claims 1 to 4, wherein component B in the manufacture of the resin is a mixture of a monoester of acrylic acid or methacrylic acid with an alkanediol of 2 to 4 carbon atoms and of acrylamide or methacrylamide, in a molar ratio of from 4:1 to 1:4.

7. A coating composition as claimed in any of claims 1 to 6, wherein component C in the manufacture of the resin is an acid having a pK in aqueous solution of less than 2.0.

8. A coating composition as claimed in any of claims 1 to 6, wherein component C in the manufacture of the resin is hydrochloric acid.

9. A coating composition as claimed in any of claims 1 to 8, wherein the reaction of the components A and B in the manufacture of the resin has been carried out at from 50 to 120"C, and any water and volatile monoalcohols formed during the reaction have been distilled off under reduced pressure.

10. A coating composition comprising a

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Claims

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EXAMPLE 5

300 parts of an aqueous solution, of about 70% strength, of a partially methanol-etherified urea-formaldehyde resin (corresponding to about 2 moles of urea), 520 parts of hydroxypropyl acrylate, 0.5 part of hydroquinone and 1.7 pans of concentrated hydrochloric acid are used.

The mixture is subjected to a distillation under reduced pressure for 2.5 hours at a temperature rising from 60 to 90 , then for 6 hours at from 90 to 95" and finally for 1 hour at 100 .

After adding 3 parts of 50% strength sodium hydroxide solution, about 670 g of a resin having a viscosity of 1,120 mPas (at 20 ) are obtained.

EXAMPLE 6 a) According to the invention:

390 parts of hexamethoxymethylmelamine, 390 parts of hydroxypropyl acrylate, 0.5 pan of hydroquinone and 3.4 parts of concentrated hydrochloric acid are used.

Material is distilled off under reduced pressure for about 90 minutes at 70" and then for about 25 minutes at 90" (weight loss about 90 g). After adding 6 parts of 50% strength sodium hydroxide solution, 700 g of a resin having a viscosity of 2,480 mPas (at 20 ) are obtained.

b) Comparative Experiment according to Swiss Patent 560,729, Example 2.

390 pans of hexamethoxymethylmelamine, 390 parts of hydroxypropyl acrylate, 0.65 part of hydroquinone and 4.5 parts of formic acid are used.

About 40 g are distilled off under reduced pressure at 110 in the course of 4 hours.

745 g of a resin having a viscosity of 200 mPas (at 20 ) are obtained.

Curing: The resin from Example 6a was diluted with about 20% of hydroxypropyl acrylate to a viscosity of about 350 mPas (at 20 ). 3% of benzil dimethyl keral were added to this solution and to the resin from Example 6b, and a 100 am thick layer of each was applied to paper and irradiated by means of an HOK 6 ultraviolet lamp. At 7 m/tube.minute the resin according to the invention gave a scratchresistant glossy film, whilst under the same conditions the comparative resin showed no curing whatsoever.

WHAT WE CLAIM IS: 1. A radiation-curable coating composition comprising a polymerizable unsaturated aminoplast resin which carries acrylic or methacrylic groups, acts as a binder and has been manufactured by reacting A) an amine/formaldehyde condensate which is optionally completely or par tially etherified, and B) an acrylic or methacrylic compound containing one or more hydroxy groups and/or amino groups, from 1/3 to 1 mole of the acrylic compound being employed per equivalent of CH2 0 group of amine/formaldehyde con densate, in the presence of C) an organic or inorganic acid having a pK in aqueous solution of < 3.0 as the catalyst, the acid being employed in an amount of from 2 to 60 milliequivalents per equivalent of H2-O- group of the amine/formaldehyde condensate, and in the presence or absence of D) a polymerization inhibitor.
2. A coating composition as claimed in claim 1, wherein component A in the manufacture of the resin is a condensation product of an aminoplast precursor carrying one or more amino, imino or amido groups, with from 1 to 2 molecules of formaldehyde per amino, imino or amino group.
3. A coating composition as claimed in claim 1 or 2, wherein the component A in the manufacture of the resin is an amine/formaldehyde condensation product which is completely or partially etherified with a monoalcohol of 1 to 6 carbon atoms.
4. A coating composition as claimed in claim 3, wherein component A in the manufacture of the resin is a condensation product of melamine and from 3 to 6 molecules of formaldehyde, which product is partially etherified with methanol or butanol.
5. A coating composition as claimed in any of claims 1 to 4, wherein component B in the manufacture of the resin is a monoester of acrylic acid or methacrylic acid with an alkanediol of 2 to 4 carbon atoms, or is acrylamide or methacrylamide.
6. A coating composition as claimed in any of claims 1 to 4, wherein component B in the manufacture of the resin is a mixture of a monoester of acrylic acid or methacrylic acid with an alkanediol of 2 to 4 carbon atoms and of acrylamide or methacrylamide, in a molar ratio of from 4:1 to 1:4.
7. A coating composition as claimed in any of claims 1 to 6, wherein component C in the manufacture of the resin is an acid having a pK in aqueous solution of less than 2.0.
8. A coating composition as claimed in any of claims 1 to 6, wherein component C in the manufacture of the resin is hydrochloric acid.
9. A coating composition as claimed in any of claims 1 to 8, wherein the reaction of the components A and B in the manufacture of the resin has been carried out at from 50 to 120"C, and any water and volatile monoalcohols formed during the reaction have been distilled off under reduced pressure.
10. A coating composition comprising a

polymerizable unsaturated aminoplast resin substantially as described in any of the foregoing Examples.
11. A radiation-curable coating composition as claimed in any of claims 1 to 10, which comprises: from 20 to 100% by weight of a poly merizable unsaturated aminoplast resin as defined in any of claims 1 to 10, as the sole binder or one of the binders, from 0 to 60% by weight of a copoly merizable organic compound which is liquid at room temperature, from 0 to 80n e by weight of a pigment or filler, from 0 to 10% by weight of one or more other organic additives for surface coat ings, and from 0 to 50% by weight of one or more further polymerizable unsaturated binders, the total of the percentages being 100.
12. Radiation-curable coating compositions as described in the Examples.
13. A method of coating a substrate comprising applying a layer of a coating composition as claimed in any of claims 1 to 12 to the substrate and curing the composition.
14. A method as claimed in claim 13 wherein the curing is effected with an electron beam having an energy of 50 to 5,000 KeV.
15. Wood, metal, paper or plastic coated by a method as claimed in claim 13 or 14.