Auto-Crosslinking, Cathodically Depositable Binders

The present invention relates to paint binders. More particularly this invention is concerned with cathodically depositable binders for paints which can be deposited by the electrodeposition process.

U.S. Pat. No. 4,147,676 discloses a process for producing binders whereby 2 moles of a diepoxy resin are reacted with 1 mole of a primary-secondary or secondary-secondary diamine and 2 moles of a saturated and/or unsaturated monocarboxylic acid. In order to attain sufficient crosslinking, it is necessary in most cases to introduce the double bonds necessary for crosslinking by employing unsaturated isocyanate compounds. Under unfavorable stoving conditions, these isocyanate compounds can be split off, and, as a result, crosslinking sites are lost and stoving equipment is soiled with the decomposition products. Furthermore, it has shown that the binders disclosed in U.S. Pat. No. 4,147,676 do not have sufficient basicity for obtaining neutralization at optimum conditions, which, in turn, owing to a necessarily higher degree of neutralization for achieving dilutability with water, may lead to enhanced corrosion of the coating equipment.

Surprisingly, it has now been found that it is possible to avoid the disadvantages of the coating compositions obtained according to U.S. Pat. No. 4,147,676, and, in addition, to considerably improve other important properties, such as electrochemical characteristics, surface quality, and corrosion protection, by using a modified resin composition.

The present invention, therefore, describes auto-crosslinking, cathodically depositable paint binders and a process for the production of such binders based on amine-modified epoxy resins, characterized in that:

(A) 2 moles of a polyepoxy compound with from 2 to 10, preferably 2.5 to 5, epoxy groups per molecule and an epoxy equivalent of from 100 to 1000 are reacted with

(B) 0.1 to 2.0, preferably from 0.2 to 1.0, moles of an aliphatic and/or cycloaliphatic and/or aromatic diamine, the nitrogen atoms of which are either secondary-secondary or primary-tertiary linkages, and

(C) from 1 to 10, preferably from 2 to 5, moles of an alpha, beta-unsaturated monocarboxylic acid, optionally in admixture with subordinate quantities of saturated and/or unsaturated monocarboxylic acids with from 2 to 18 carbon atoms, and

(D) from 0.5 to 10, preferably from 1 to 5, moles of a dialkanol amine and/or of a monoalkanol monoalkyl amine, optionally in admixture with dialkyl amines, and

(E) the obtained product is rendered water-dilutable through partial or total neutralization with inorganic and/or organic acids,

the weight ratios being chosen in order that the sum of epoxy-reactive hydrogen atoms of components (B) to (D) substantially correspond to the number of epoxy groups of component (A) and that the final product contains from 0.5 to 2.5 basic nitrogen atoms and from 0.5 to 2.5 polymerizable bonds per 1000 molecular weight units. The coating compositions produced based on the products of this invention provide, even with a low degree of neutralization, good dilutability with water, high basicity, low corrosion of the installations, high deposition equivalent, and low energy consumption, respectively. The compositions can be deposited at high voltage without surface defects and exhibit enhanced throwing power.

Suitable components (A) are polyepoxy compounds with from 2 to 10 epoxy groups per molecule and with an epoxy equivalent weight of from 100 to 1000. In this group the polyglycidyl ethers of phenol or cresol novolaks or their hydration products are the preferred compounds. Diepoxy compounds such as the diglycidyl ethers of Bisphenol A or glycidyl esters of aliphatic, aromatic, or cycloaliphatic polycarboxylic acids or polyols preferably are used in conjunction with higher functional epoxy resins.

Suitable diamines for use according to this invention having either one primary and one tertiary amine group or two secondary amine groups (component (B)) include 3-dimethylaminopropylamine, 3-diethylaminopropylamine, 4-dimethylaminobutylamine, 4-diethylaminobutylamine, 3-diethanolaminopropylamine, N,N-dimethylisophorone diamine, N,N-diethylisophorone diamine, N-(2-aminoethyl)-morpholine, N-(3-aminopropyl)-morpholine, N,N'-diethylpropylene diamine, N,N'-diethanolpropylene diamine, N,N'-diethylisophorone diamine, and N,N'-diethanolisophorone diamine.

Suitable unsaturated monocarboxylic acids (component (C)) are alpha,beta-unsaturated acids such as acrylic, methacrylic, or crotonic acid as well as semiesters of maleic acid or itaconic acid with saturated monoalcohols or semiesters of other aliphatic, cycloaliphatic, or aromatic dicarboxylic acids with alpha,beta-ethylenically unsaturated monoalcohols. For example, semiesters obtained through reaction of succinic acid anhydride, phthalic acid anhydride, tetra- or hexahydrophthalic anhydride with hydroxyalkyl methacrylates or esters of methacrylic acid with di-, tri-, or polyalkylene glycols can be used to advantage. Together with the above-mentioned alpha,beta-ethylenically unsaturated carboxylic acids, subordinate quantities of other unsaturated or saturated monocarboxylic acids with from 2 to 18 carbon atoms may be used, exemplified by acetic acid, n-capronic acid, isononanoic acid, palmitic acid, dehydrated castor oil fatty acid, oleic acid, and stearic acid.

Preferred suitable secondary amines (component (D)) are the dialkanol amines or monoalkanolmonoalkyl amines. Preferred components include diethanolamine, dipropanolamine, dibutanolamine, as well as methylmethanolamine, ethylethanolamine, propylethanolamine, methylpropanolamine, ethylpropanolamine, propylpropanolamine, or their homologues. Dialkyl amines, such as diethylamine, dipropylamine, or dibutylamine, may be coemployed.

Type and quantity of polyepoxy compound, diamine, unsaturated or saturated monocarboxylic acid, and secondary amine are chosen in order that, on the one hand, the number of epoxy reactive hydrogen atoms from the diamine, monocarboxylic acids, and secondary amines is practically equal to the number of epoxy groups in the reaction batch and, on the other hand, that after the addition reaction the product carries in 1000 molecular weight units from 0.5 to 2.5 of basic nitrogen groupings and from 0.5 to 2.5 polymerizable double bonds.

The process of the invention is preferably carried out in order that the polyepoxides are reacted at from 50.degree. to 150.degree. C. simultaneously with the diamines, secondary amines, and monocarboxylic acids while coemploying inert solvents and polymerization inhibitors. The preferred reaction temperature ranges between 100.degree. and 120.degree. C. Other choices of conditions can be selected, including without inert solvents, to obtain the essential reaction.

The amine groups of the coating composition according to the invention are neutralized partially or totally with organic and/or inorganic acids, including with formic acid, acetic acid, lactic acid, and phosphoric acid. The neutralized composition is then diluted with water. The degree of neutralization depends in the individual case upon the properties of the binder employed. The binder composition, with judicious selection of the components, permits dilution with or dispersion in water with a pH-value of from 5 to 8, and preferably from 6 to 7. The concentration of the binder in water depends upon the parameters utilized during application by electrodeposition and may range from 3 to 30% by weight, preferably 10 to 20% by weight. The composition which is applied may optionally contain various additives, such as pigments, extenders, paint surface active agents, and the like.

Upon deposition, the aqueous coating composition containing the binders of the invention is brought into contact with a wired anode and cathode, with the coating composition depositing on the surface of the cathode. A variety of electrically conductive substrates can be coated, especially metal substrates, such as steel, aluminum, copper, and the like, but also metallized substrates or other substrates covered with a conductive material can be used. After deposition, the coating is cured at elevated temperature. For curing, temperatures of from 130.degree. to 200.degree. C., preferably 150.degree. to 180.degree. C., are employed. Curing times are from 5 to 30 minutes, preferably 10 to 25 minutes.

The following examples illustrate the invention without limiting its scope. All parts and percentages given refer to parts or percentages by weight.

A reaction vessel is equipped with stirrer, addition funnel, thermometer, and reflux condensor. The polyepoxy compound and, if employed, an inert solvent are charged to the reaction vessel. The solvent, if used, can be ethyl glycol acetate. The monocarboxylic acid, the secondary amines and diamines are added and the blend is reacted for from 1 to 3 hours at from 100.degree. to 110.degree. C., coemploying a polymerization inhibitor, until the epoxy value and the acid value have fallen to practically 0. Quantity and type of the components are listed in Table 1.

100 g resin solids of each of the binder samples were mixed with the quantity of acid listed below and made up to 1000 g with deionized water while stirring. The 10% solutions were deposited on steel panels as the cathode. Deposition time in all cases was 60 seconds. The coated substrates were then rinsed with deionized water and cured at elevated temperature. Average film thickness of the cured films was from 13 to 17 .mu.m. Conditions and evaluation results are listed in Table 2.

For the evaluation, degreased, non-pretreated steel panels were coated with a pigmented paint composed of 100 parts by weight of resin solids, 20 parts by weight of aluminum silicate pigment, and 2 parts by weight of carbon black.

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.