Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1988-09-23
1990-07-31
Niebling, John F.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
523404, 523414, C08L 6300, C25D 1306
Patent
active
049448552
DESCRIPTION:
BRIEF SUMMARY
The invention relates to aqueous electrocoating baths for cathodic paint deposition which contain, as cathodically depositable binders, modified epoxide-amine adducts and are prepared by reacting, in the presence of basic amine catalysts, hydroxyl groups bonded to aromatic and/or (cyclo)aliphatic molecular fragments, to form epoxide group-containing intermediates, and further conventional modifiers and finally with tertiary amines, the resultant reaction products being neutralized with acids, if appropriate, dispersed in an aqueous medium and these dispersions further processesd by well-known methods to form electrocoating baths.
Cathodic paint deposition is a painting method frequently employed particularly for priming, in which water-dilutable synthetic resins carrying cationic groups are deposited on electrically conducting bodies by direct current.
Electrocoating baths of the type described above are disclosed, for example, in the following patent documents U.S. Pat. No. 3,799,854, U.S. Pat. No. 3,984,299, U.S. Pat. No. 4,031,050, U.S. Pat. No. 4,252,703, U.S. Pat. No. 4,332,711 and in German Patent No. 3,108,073.
It is known that paint coatings of excellent quality can be obtained by using paint systems of this type.
The electrocoating baths under discussion unfortunately and undesirably also contain chloride ions which cause corrosion of the electrocoating plant, in particular of the anodes (for example pitting corrosion of stainless steel electrodes), when the concentration of these ions exceeds a limit that depends on several parameters (for example the pH of the electrocoating bath).
The interfering chloride ions originate mainly from the polyepoxides used as component (A) in the synthesis of the binders and contaminated with chlorine-containing byproducts.
These byproducts can contain, for example, the following structural elements: ##STR1##
Because of the presence of such byproducts, the chlorine content of commercial polyepoxides is between 0.15 and 0.5%.
German Auslegeschrift No. 2,751,498 states that the addition of nitrate and/or nitrite ions to the electrocoating bath results in a reduction of the corrosion damage due to the presence of chloride ions.
This process as well as purification of the polyepoxides contaminated with chlorine-containing byproducts or removal of the chloride ions from the ready-to-use aqueous electrocoating paint by means of known methods of separation are not carried out in practice because of economic and/or technical difficulties.
The object forming the basis of the present invention consists in preparing electrocoating baths of the type described at the outset which have chloride ion concentrations lower than the baths known in the current state of the art.
Surprisingly, this object can be achieved by preparing electrocoating baths of the type described above, wherein the basic amine catalyst which catalyzes the reaction between the components (A) and () is neutralized, prior to further reaction of the intermediates formed from the components (A) and (B), with a Bronsted acid having a pK.sub.s value of the first stage of dissociation, measured at 20.degree. C., of below 3.7, preferably below 3.5
The invention also relates to a process for the preparation of aqueous electrocoating baths for cathodic paint deposition, in which with hydroxyl groups bonded to aromatic and/or (cyclo)aliphatic molecular fragments, to form epoxide group-containing intermediates, and further conventional modifiers and finally with tertiary amines, the resultant reaction products are neutralized with acids, if appropriate, dispersed in an aqueous medium and these dispersions are further processed by well-known methods to form electrocoating baths, wherein the basic amine catalyst which catalyzes the reaction between the components (A) and (B) is neutralized, prior to further reaction of the intermediates formed from the components (A) and (B), with a Bronosted acid having a pK.sub.s value of the first stage of dissociation, measured at 20.degree. C., of below 3.7, preferably below 3.5.
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REFERENCES:
patent: 4476263 (1984-10-01), Owens
patent: 4624975 (1986-11-01), Pham
patent: 4698141 (1987-10-01), Anderson
patent: 4707232 (1987-11-01), Batzill
Geist Michael
Ott Gunter
Schon Georg
BASF Lacke & Farben Aktiengesellschaft
Hsing Ben C.
Niebling John F.
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