GB1592678A - Binders for aqueous baking finishes - Google Patents

Description

(54) BINDERS FOR AQUEOUS BAKING FINISHES (71) We, BASF AKTIENGESELLSCHAFT, 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 binders for aqueous baking finishes, which binders possess quaternary ammonium groups, and to their use for cathodic electrocoating.

Binders which possess quaternary ammonium groups for aqueous baking finishes have been disclosed. For example, German Laid-Open Application DOS 2,163,143 describes epoxy resins which contain at least one epoxide group per molecule, at least 0.05% of chemically bonded nitrogen in the form of quaternary ammonium bases and from 0.01 to 8% of boron in the form of boric acid. According to this disclosure, it is evidently difficult to manufacture aqueous dispersions, especially electrocoating baths, which are stable at an alkaline pH.

In addition, German Laid-Open Application DOS 2,339,398 discloses organic synthetic resins which have been rendered soluble by the presence of onium groups, and which are used in combination with melamine resins or phenolic resins for electrocoating and crosslink with the said resins on baking. Here, the difficulty is above all to maintain a constant ratio of organic synthetic resin to melamine resin or phenolic resin in the electrocoating baths.

The present invention seeks to provide binders possessing quaternary ammonium groups for baking finishes, which binders are self-crosslinking and can be manufactured in a simple manner from easily accessible starting materials. It would be a further advantage if the binders were to be suitable for electrocoating and remained stable in the coating bath for lengthy periods at a neutral or slightly alkaline pH.

According to the present invention, there is provided a binder possessing quaternary ammonium groups for use in aqueous baking finishes, which binder has been obtained by reacting (A) at least one aminoplast resin which contains at least one tertiary basic nitrogen atom per molecule obtained by reaction of a secondary amine with an aminoplast resin, with (B) at least one epoxy compound which contains at least 2 epoxide groups per molecule, at from 30 to 1300C, in the presence of an acid and in the presence or absence of one or more solvents, with the proviso that the components (A) and (B) are employed in such amounts as to provide from 1 to 1.2 tertiary basic nitrogen atoms of component (A) per epoxide group of component (B).

Components (A), which are reaction products of an aminoplast resin, which may or may not be etherified, with a secondary amine, may be prepared in the presence of a basic compound.

Particularly preferred binders contain, as component (A), a reaction product of an amine/formaldehyde resin, e.g. a urea, melamine and/or benzoguanamine resin, with a secondary amine.

The binders of present invention may be used for cathodic electrocoating.

The following may be noted with respect to the individual components from which the binders according to the invention are made up: (A) Examples of aminoplast resins with at least one tertiary basic nitrogen atom per molecule are the products which have been manufactured in a conventional manner by reacting urea, melamine, benzoguanamine or another amino compound capable of aminoplast formation with formaldehyde and preferably then at least partially etherifying the methylol groups with an alcohol of 1 to 12.carbon atoms, which products are then reacted with a secondary amine, examples of secondary amines which may be used being dialkylamines, dialkoxyalkylamines and dialkanolamines, e.g. diethanolamine, diisopropanolamine, di-2-methoxyethylamine, dicyclohexylamine, diethylamine, di-n-propylamine, diisopropylamine, dibutylamine, diisobutylamine, di-2-ethylhexylamine, N-ethylbutylamine and N-methylbutylamine, as well as cyclic secondary amines, eg. imidazole, benzimidazole and oxazolidine.

The reaction of the aminoplast resin with the secondary amine is preferably carried out in the presence of a basic compound, eg. NaOH, KOH, NH40H or Ca(OH)2, at room temperature or at up to about 1200C, as described, for example, in Angewandte makromolekulare Chemie, 46 (1975), 171 - 186, in the presence or absence of a solvent, eg. an alcohol or an inert solvent, e.g. xylene, using from 0.25 to 6, preferably from 0.5 to 3, moles of secondary amine per mole of aminoplast resin.

(B) Examples of epoxy compounds which contain at least 2 epoxide groups per molcule are especially those having epoxy values of from 0.01 to 0.7 mole of epoxide/100 g of substance.

Examples of such compounds are reaction products of epichlorohydrin and bisphenol A, of epichlorohydrin and a polyalcohol, e.g. butane-1,4-diol, trimethylolpropane or pentaerythritol, and of epichlorohydrin and an aliphatic, cycloaliphatic or aromatic polycarboxylic acid.

In order to increase the molecular weight or in order deliberately to produce branches, such an epoxy compound can be reacted with a less than equivalent amount of a polyamine, eg. hexamethylenediamine, 1,3-diaminopropane or ethylenediamine, or of a polyphenylol compound, eg. bisphenol A or a resol, or of a polycarboxylic acid, eg. adipic acid, phthalic acid or isophthalic acid, or of a polyisocyanate, eg. hexamethylene diisocyanate, toluylene diisocyanate or isophorone diisocyanate.

Other epoxy compounds (B) which can be used are copolymers with oxirane side groups, e.g. glycidyl methacrylate homopolymers and copolymers. Epoxidized oils, eg. epoxidized polybutadiene oils, may also be employed.

The binders according to the invention are manufactured by reacting the individual components (A) and (B) at from 30 to 1300C, preferably from 50 to 1000C, in the presence of an acid and in the presence or absence of one or more solvents, the components (A) and (B) being employed in such amounts as to provide from 1 to 1.2, preferably about 1, tertiary basic nitrogen atoms of the component (A) per epoxide group of the component (B), so that resins which possess quaternary ammonium groups are formed. Accordingly, the binder no longer contains any free epoxide groups.

Examples of acids for the reaction of (A) with (B) are mineral acids, e.g. phosphoric acid, and, preferably, organic acids, especially carboxylic acids, e.g. acetic acid, oxalic acid and carbonic acid, which are usually employed in amounts of from 1 to 100, preferably from 10 to 50, mole %, based on epoxide groups of component (B). The reaction of (A) and (B) can be carried out in an organic solvent, eg. i-propanol, i-butanol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diacetone-alcohol, toluene, xylene, methyl isobutyl ketone, n-butanol or a mixture of any of these. The use of aqueous solvent mixtures, for example of mixtures of i-butanol, ethylene glycol monoethyl ether and water or of i-propanol, ethylene glycol monobutyl ether and water, has proved particularly advantageous.

Binders within the invention are dispersible in water. The dispersions are alkaline and have a pH of from 8 to 9. They can be brought to a pH of from 7 to 8 by adding an acid, for example, acetic acid. These dispersions are stable over prolonged periods.

The auxiliaries conventionally used in the surface-coating industry, such as catalysts (eg.

2-ethylhexyl tosylate or diphenyl cresyl phosphate), flow control agents (eg. relatively high-boiling solvents) and plasticizers can be added to the binders. Further suitable additives are resins which are compatible with the present binders and which, for example, act as soft resins which control the elasticity of the baked finishes. It may also be advantageous to use a soft resin in order to disperse pigments and fillers therein and then admix this paste to the binder.

The binders may be used as clear finishes or for pigmented finishes (in which latter case the pigments conventionally employed in the surface-coating industry, eg. titanium dioxide, talc, carbon black, strontium chromate, organic colored pigments and iron oxides, and fillers, may be present) for finishing metals, wood or plastics. They may be applied by conventional methods, eg. spraying, dipping or casting. Electrocoating is preferred. The baths may be operated at a slightly alkaline pH; values of from 6 to 9, especially from 7.5 to 8.5, are preferred. Baking is carried out under the conventional conditions at from 150 to 210 C.

It is a particular advantage of the binders that they may be used at a neutral or slightly alkaline pH and hence to not corrode the coating equipment. A further advantage is that they constitute a self-crosslinking binder and hence no phase separation of resin and curing agent takes place during electrocoating.

The coatings obtained generally possess good elasticity and are very hard. They are very resistant to solvents. The binder may be used particularly advantageously for the cathodic electrocoating of automotive components.

In the text which follows and in the Examples, parts and percentages are by weight, unless stated otherwise.

(A) Manufacture of an aminoplast resin containing tertiary basic nitrogen.

1,400 parts of a melamine-formaldehyde-methanol resin, which has a molar ratio of melamine : formaldehyde of about 1 : 4, and about 1 mole of non-etherified methylol groups per mole of melamine, are concentrated under reduced pressure (the amount of distiwlate obtained being about 170 parts). 420 parts of diethanolamine and 12 parts of 50% strength NaOH are then added to the melamine resin and the mixture is heated for 1 hour at 50"C under reduced pressure. 15 parts of concentrated hydrochloric acid are then added and the mixture is diluted to a solids content of about 62% with 190 parts of ethanol. The viscosity at 200C is about 4,000 mPas.

The content of unconverted diethanolamine in the reaction product is shown to be less than 0.5% by potentiometry. Urea-formaldehyde and benzoguanamine-formaldehyde resins which are etherified with ethanol, propanol or butanol can be manufactured similary.

(B) Manufacture of an acrylate resin containing epoxide groups (resin BI): 300 parts of n-butanol are introduced into a round flask equipped with a condenser, dropping funnel, thermometer and stirrer, and are heated to the reflux temperature (118"C) under N2. A mixture of 350 parts of n-butyl acrylate, 210 parts of glycidyl methacrylate, 70 parts of styrene, 70 parts of hydroxypropyl acrylate and 28 parts of azodiisobutyronitrile is then added in the course of 2 hours. The polymerization is then continued for a further 3 hours and at intervals of 1 hour 3.5 parts of azodiisobutyronitrile are added twice.

Solids content = 69.1% K value (3So strength in acetone) = 15.4 Epoxy value: 0.13 mole of epoxide/100 g of substance Manufacture of a bisphenol A resin containing epoxide groups (resin BII): 469 parts of an epoxy resin (manufactured from pentaerythritol and epichlorohydrin, and having an epoxy value of 0.65 mole of epoxide/100 g of substance), 94 parts of phenol and 114 parts of bisphenol A are stirred, in a round flask equipped with a condenser, thermometer and stirrer, at 1700C under N2, until a melt viscosity of 1,200 mPas (measured with a plate and cone viscometer from Epprecht Instruments + Controls) is reached. This is the case after about 42 hours. The product is then cooled to about 1200C and diluted with 290 parts of iso-butanol.

Solids content = 70.0% K value (28% strength in DMF) = 22.3 Epoxy value: 0.07 mole of epoxide/100 g of substance Example I 119 parts of an epoxy resin prepared from bisphenol A and having an epoxy value of 0.21 mole of epoxide/100 g of substance are dissolved in 119 parts of ethylene glycol monoethyl ether. 125 parts of melamine resin (A) containing basic nitrogen, 6 parts of glacial acetic acid and 7.2 parts of water are then added to the solution and the mixture is stirred under N2 for 2 hours at 80"C.

Solids content = 53.6% K value (38% strength in DMF) = 19.3 Viscosity at 25"C = 1,600 mPas The binder is dispersible in water (giving a pH of 9). The dispersion is stable and does not sediment.

Example 2 162.5 parts of acrylate resin containing epoxide groups (resin BI), 103.8 parts of melamine resin containing basic nitrogen, 7.5 parts of glacial acetic acid and 9 parts of water are stirred under N2 for 2 hours at 800C.

Solids content = 68.8% K value (3% strength in DMF) = 16.1 Viscosity at 750C = 280 mPas The binder is dispersible in water (giving a pH of 9). The dispersion is stable and does not sediment.

Example 3 193.7 parts of bisphenol A resin containing epoxide groups (resin BII), 83 parts of melamine resin containing basic nitrogen, 6 parts of glacial acetic acid and 7.2 parts of water are stirred under nitrogen for 2 hours at 80"C.

Solids content = 70.3% K value (0.3% strength in DMF) = 21.6 Viscosity at 75"C = 640 mPas Content of quaternary nitrogen: 0.38% The binder is dispersible in water (giving a pH of 9). The dispersion is stable and does not sediment. Testing the binders in cathodic electrocoating finishes The binders are combined with a non-ionic organic compound as described in German laid-open Patent Application Dos 2,606,831 which is a reaction product of an alkylated amine-fcxmaldehyde condensation product and a modified epoxy resin in a weight ratio of 9:1 to 1:9, having a viscosity from 500 to 10,000 mPa, and should be contained in the finish in an amount of 10 to 80 weight %) and with diphenyl cresyl phosphate, and are coated onto Zn-phosphatized sheet steel.

Binder according to according to according to Example 1) Example 2) Example 3 ) Deposition voltage (2 minutes' duration) 50 V 100 V 150 V Baking conditions 20 minutes/190 C 20 minutes/190 C 20 minutes/190 C Coating thickness 24 m 18 m 15 m Pendulum hardness 179 seconds 115 seconds 120 seconds Erichsen deep-drawing value 7 mm 6 mm 10 mm Bending test 3 1 1 Acetone resistance ± + + 1) 80 parts of binder + 20 parts of non-ionic compound (as described in laid-open German Patent Application Dos 26 06 831 2) 80 parts of binder + 20 parts of non-ionic compound + 5 parts of diphenyl cresyl phosphate 3) 75 parts of binder + 25 parts of non-ionic compound + 5 parts of diphenyl cresyl phosphate

Claims (10)

WHAT WE CLAIM IS:

1. A binder possessing quaternary ammonium groups for use in aqueous baking finishes, which binder has been obtained by reacting (A) at least one aminoplast resin which contains dt least one tertiary basic nitrogen atom per molecule obtained by reaction of a secondary amine with an aminoplast resin, with (B) at least one epoxy compound which contains at least 2 epoxide groups per molecule, at from 30 to 1300C, in the presence of an acid and in the presence or absence of one or more solvents, with the proviso that the components (A) and (B) are employed in such amounts as to provide from 1 to 1.2 tertiary basic nitrogen atoms of component (A) per epoxide group of component (B).

2. A binder as claimed in claim 1, wherein component (A) has been obtained by reacting an aminoplast resin, which may or may not be etherified, with a secondary amine in the presence of a basic compound.

3. A binder as claimed in claim 1 or 2, wherein component (A) is a reaction product of a urea formaldehyde, melamine-formaldehyde and/or benzoguanamine-formaldehyde resin with a secondary amine.

4. A binder as claimed in any of claims 1 to 3, wherein component (B) has an epoxy value of from 0.01 to 0.7 mole of epoxide per 100 g of substance.

5. A binder as claimed in any of claims 1 to 4, wherein the reaction is carried out in the presence of a carboxylic acid in an amount of from 10 to 50 mole % based on epoxide groups of component (B).

6. A binder as claimed in any of claims 1 to 5, wherein the solvent for the reaction consists at least partially of water and/or of an organic solvent containing hydroxyl groups.

7. A binder which possesses quaternary ammonium groups, substantially as described in any of the foregoing Examples.

8. An aqueous dispersion of a binder as claimed in any of claims 1 to 7 either having a pH of 8 to 9 or having added thereto an acid and having a pH of 7 to 8.

9. Use of a binder, which possesses quaternary ammonium groups, as claimed in any of the preceding claims, for the cathodic electrocoating of electrically conductive articles.

10. Articles which have been coated with a composition comprising a binder as claimed in any of claims 1 to 7 followed by baking at 150 to 210 C.