Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1999-01-21
2001-11-13
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S501000, C204S504000, C204S506000
Reexamination Certificate
active
06315882
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a basic cationic electrodeposition coating composition with which cationic electrodeposition coating can be carried out under basic conditions and a methods for producing it.
PRIOR ART
Cationic electrodeposition coating permits coating of a substrate automatically and continuously without leaving any details uncoated even when the substrate has a complicated shape of surface and, therefore, has been used broadly in the undercoating of automotive bodies and other substrates having large and complicated shape of surfaces and demanding a high level of corrosion resistance. Moreover, since it is more economical than any other coating technique in the viewpoint of the application efficiency of coatings, cationic electrodeposition coating has found widespread application as an industrial method for coating.
In such cationic electrodeposition coating, the acid-neutralized aminoepoxy type electrodeposition coatings are most widely used. Generally, such acid-neutralized aminoepoxy electrodeposition coatings are composed generally of a polyamino resin prepared by reacting an epoxy group with an amine. or a resin available upon modifying said polyamino resin. However, since the tertiary amino groups of the resin have been neutralized with hydrogen ions (protons) by adding an organic acid such as formic acid, acetic acid, lactic acid, or the like in order to obtain a stable dispersion of the resin in water, the hydrogen ion exponent (pH) of the electrodeposition bath is inevitably on the weakly acidic side near pH 6.
When such an electrodeposition coating is used in a coating equipment not having corrosion resistance, the electrodeposition tank and associated piping may be corroded. Therefore, in order that a stable coating operation may be insured, it is necessary to provide a special coating equipment using corrosion-resistant materials such as stainless steel or the like material. However, such corrosion-resistant materials are so expensive that the practice of said electrodeposition has an economic disadvantage. Therefore, there has been a demand for development of a cationic electrodeposition coating composition capable of coating with the coating equipment using inexpensive sheet steel.
In order to inhibit corrosion of the electrodeposition tank and associated piping, it can be considered to make the coating bath weakly basic at least. However, when a basic compound such as an amine is added to a cationic electrodeposition coating composition to make it weakly basic, agglomeration and/or precipitation of coating particles will occur to detract from both the storage stability of the coating and/or the bath stability in the electrodeposition tank. This phenomenon is generally accounted for as follows. Thus, because the conventional cationic electrodeposition coating composition is of the proton hydration type such that the tertiary amino group has been neutralized with an acid, the basic substance added for pH adjustment extracts the neutralizing acid from the tertiary amino group. Therefore, it is practically impossible to make a cationic electrodeposition coating composition of the proton hydration type basic.
SUMMARY OF THE PRESENT INVENTION
In the light of the above-mentioned art, the present invention has for its object to provide a basic cationic electrodeposition coating composition which does not detract from either storage stability or bath stability even though it is basic and, does not make the coating equipment such as the electrodeposition tank and associated piping corroded, and a method for producing it.
The present invention is directed to a basic cationic electrodeposition coating composition having a hydrogen ion concentration within the range of 7.5 to 12.0 in terms of hydrogen ion exponent (pH).
The present invention is further directed to a method for producing a basic cationic electrodeposition coating composition which comprises preparing an aprotic hydration functional group-containing resin composition and adding thereto a basic organic compound.
The present invention is now described in detail.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The basic cationic electrodeposition coating composition of the present invention has a hydrogen ion concentration within the range of 7.5 to 12.0 in terms of hydrogen ion exponent. The relationship of hydrogen ion concentration to pH is expressed by the following equation.
pH=−log[H
+
]
The pH usually denotes the hydrogen ion exponent of an aqueous solution but as used in this specification, the term pH means the value of a cationic electrodeposition coating composition as measured with a pH meter in the routine manner as if it were an aqueous solution. While the basic cationic electrodeposition coating composition of the present invention is an aqueous solution or water dispersed liquid, the pH measurement of the water dispersed liquid is carried out under constant stirring. In this specification, pH represents the value measured under ordinary conditions.
If the pH of the basic cationic electrodeposition coating composition is less than 7.5, the corrosion of the coating equipment such as the electrodeposition tank and associated piping cannot be so sufficiently inhibited that the effect of the present invention is likely not to be obtained. On the other hand, if the pH exceeds 12.0, the strong basicity causes various troubles. Therefore, the above-mentioned range should be respected. The preferred range is pH 7.8 to 10.0.
In the present invention, it is of great importance that the pH of the cationic electrodeposition coating composition be controlled within the basic region of pH 7.5 to 12.0. Thus, the pH of the electrodeposition bath must not be less than 7.5 in order that the corrosion of the coating equipment such as the electrodeposition tank and the like are successfully precluded. However, it was not conceivable, from the standpoint of storage stability and bath stability of the coating, to make the coating bath basic in the cationic electrodeposition coating composition heretofore in widespread use. In fact, the conventional cationic electrodeposition coatings could not be made basic. The inventors of the present invention discovered that when a resin which is stable in a weakly basic environment is used as the main binder of a cationic electrodeposition coating composition, a sufficiently stable bath for cationic electrodeposition coating can be obtained even through it is weakly basic. The present invention has been developed on the basis of the above finding.
The basic cationic electrodeposition coating composition of the present invention is preferably a composition containing an aprotic hydration functional group-containing component for controlling the hydrogen ion exponent (pH) of the composition in the above-mentioned range. The aprotic hydration functional group mentioned above is a functional group required for insuring an acceptable polymer dispersion stability in water and containing no proton.
The aprotic hydration functional group is not particularly restricted but includes onium salts, for example, such as sulfonium salt, ammonium salt, phosphonium salt, and selenium salt. In consideration of the ease of production, the sulfonium salt and ammonium salt are preferred. Moreover, in consideration of throwing power, the sulfonium salt is more preferred.
The preferred proportion of said aprotic hydration functional group is 10 to 300 millimoles based on 100 grams of resin solids in the basic cationic electrodeposition coating composition. If its proportion is less than 10 millimole/100 g, the desired electrodeposition characteristics cannot be obtained and the bath stability will become also poor. If its proportion exceeds 300 millimole/100 g, no satisfactory electrodeposition on the substrate surface will be obtained. The more preferred proportion is 20 to 100 millimole/100 g.
In the present invention, the component containing said aprotic hydration functional group is a main binder for the basic cationic electrodeposi
Kawakami Ichiro
Kawanami Toshitaka
Kokubun Takayuki
Saito Takao
Sakamoto Hiroyuki
Connolly Bove Lodge & Hutz
Mayekar Kishor
Nippon Paint Co. Ltd.
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