Method of electrolyte bath stabilization

Details Method of electrolyte bath stabilization Method of electrolyte bath stabilization

1999-07-22

2001-02-06

Dawson, Robert (Department: 1712)

Chemistry: electrical and wave energy

Processes and products

Electrophoresis or electro-osmosis processes and electrolyte...

C204S504000, C523S404000, C525S525000, C525S530000

Reexamination Certificate

active

06183616

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method of stabilizing electrolyte bath, which facilitates the control of the electrolyte bath comprising a cationic electrocoating composition.
PRIOR ART
Cationic electrocoating can provide substrates or coating target articles with coatings every nook and cranny even when said substrates are complicated in form or shape and can be performed automatically and continuously. Therefore, it is in general use as a technique of forming an undercoat on substrates, such as automotive bodies, which have a large size and are complicated in form or shape and are required to be highly resistant to rusting. Furthermore, said technique is very highly efficient from the coating composition consumption viewpoint, hence economical, as compared with other methods of coating and, therefore, it has come into wide use as an industrial method of coating.
In such cationic electrocoating, electrocoating compositions mainly containing polyaminated resin having a skeleton of epoxy resin or acrylic resin have been in general use. Generally, they are positively charged as a result of neutralization with organic acid.
Cationic electrocoating is carried out by immersing a substrate, which is to serve as a cathode, in a cationic electrocoating composition and applying a voltage. The deposition of a coating layer on the substrate involves electrochemical reaction and the coating layer that has deposited on the substrate surface upon voltage application has an insulating property. Therefore, with the progress of deposition of the coating in the process of coating, hence with the increase in deposit layer thickness, the electric resistance of the coating layer increases in proportion to the increase in layer thickness. As a result, the deposition of the coating composition at that site decreases and, instead, the deposition of a coating layer on some other uncoated site begins. In this way, the deposition of emulsion particles of the coating composition takes place successively from uncoated site to uncoated site. In the present specification, such successive formation of a coating on uncoated sites of a substrate is referred to as “throwing power”.
However, when cationic electrocoating compositions having such neutralized amino groups as hydratable group are used, the ion release from the amino group is reversible, and the deposition behavior and/or film characteristics are changeable sensitively depending on the temperature conditions of the coating bath. This state of affairs is shown in FIG.
1
and FIG.
2
. As is evident from
FIG. 1
, the thickness of the deposit film changes widely as the bath temperature changes. Further, as is evident from
FIG. 2
, film smoothness can be maintained only in a narrow temperature range. Therefore, for attaining good throwing power and maintaining the desired deposit film thickness and smoothness, it is necessary to control the bath temperature within limits of variability of about ±2° C., for instance. The burden of bath control is thus very heavy.
Meanwhile, WO 98/03701 discloses a cationic electrocoating composition differing from the conventional electrocoating compositions having neutralized amino groups as hydratable group. Said composition comprises a basic resin containing sulfonium group and triple bond, such as an ethynyl or nitrile group, within the molecule. However, the above-cited publication does not disclose any relationship between the bath temperature and the deposit film thickness and smoothness, although it describes that said cationic electrocoating composition is excellent in throwing power.
SUMMARY OF THE INVENTION
In view of the current state of the art as mentioned above, the present invention has its object to provide a method of stabilizing electrolyte bath by which excellent throwing power can be obtained and bath control can be facilitated for maintaining the desired deposit film thickness and smoothness.
The present invention provides a method of stabilization of electrolyte bath in electrocoating for obtaining electrodeposition coat by immersing a substrate in the electrolyte bath filled with a cationic electrocoating composition containing a basic resin and an amine compound and carrying out electrocoating; wherein said basic resin in said electrolyte bath contains, per 100 grams of the resin solids, 10 to 300 millimoles of sulfonium group and 50 to 2,000 millimoles of carbon—carbon unsaturated bond, and said amine compound is contained in an amount of 1 to 50 millimoles per 100 grams of the basic resin solids in the electrolyte bath, said amount being not less than 5 mole percent relative to the sulfonium group content in said basic resin, so that the sulfonium group remaining in said electrodeposition coat amount to not more than 50% relative to the sulfonium group contained in the electrocoating composition in said electrolyte bath.
According to an advantageous aspect of the invention, carbon—carbon triple bond of propargyl group accounts for at least 15% of said carbon—carbon unsaturated bond.


REFERENCES:
patent: 5003025 (1991-03-01), Dobbelstein et al.

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