Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Patent
1998-12-03
2000-11-14
Gorgos, Kathryn
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
523412, 523415, C08F 258, C08K 320
Patent
active
061465127
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for preventing foaming in cationic electrophoretic coating using special polyalkylene oxide compounds which have a solubility of more than 50 parts by weight in 100 parts by weight of water. The invention also relates to electrophoretic coating baths containing special polyalkylene oxide compounds which are distinguished by a particularly low tendency to foam on agitation.
2. Discussion of Related Art
The coating of conductive substrates by cataphoresis or even cathodic electrodeposition coating has been adopted in almost all areas of mass painting. In most countries, its main application is in the automotive industry where the major advantages of this coating process are particularly relevant. Thus, substantially defect-free uniform coatings can be obtained by dip coating and even the coating of voids is readily possible.
In cathodic electrodeposition coating (CEDC), the substrate is dipped into an aqueous bath, the so-called electrodeposition bath or ED bath, and connected as the cathode. By applying a direct current, the binder is then deposited from the bath onto the substrate. The binder thus deposited is then hardened by baking or other methods.
Cationic electrophoretic coating (CEC) installations are extended dip coating installations. The workpiece is provided with electrodes and coated in the dip tank over a contact time of about 1 to 4 minutes at voltages of 100 to 500 V and current densities of 0.01 to 5 A/dm.sup.2. In the automotive industry, for example, the tank volumes are between 50 and 450 ton.
After returning to the surface, the coated objects pass through a rinse zone to remove any non-deposited lacquer film and then through blowing and drying zones.
The production and use of aqueous polymer systems is often accompanied by foaming. The foam is produced through the presence of surfactants and stabilized. Corresponding surface-active substances are present in virtually every water-based paint or lacquer formulation. Thus, this group of substances includes, for example, emulsifiers, wetting agents, dispersants and polymeric surfactants, such as for example the charged binders used in CEC coating compositions. Surfactants develop their effect by migrating to the phase interface between two generally immiscible or substantially immiscible components where they reduce surface tension. However, a side effect of this mechanism--necessary for the production of dispersions--is its tendency to stabilize and disperse trapped gas bubbles, especially air bubbles. Accordingly, the introduction of gases into a surfactant-containing solution often results in foaming. Foaming can take place in various ways, for example during production when the coating composition is thoroughly mixed or during the application of the coating composition. Thus, in the case of electrophoretic coating, air can be introduced in dispersed form, for example during the introduction or removal of the workpiece, during the washing of the workpiece and during filling of the dip tank with coating composition, often leading to the formation of a voluminous, stable foam in coating compositions known from the prior art.
Depending on the stability of the foam, foaming such as this can lead to unwanted effects, for example overflowing of the dip tank, surface soiling of the workpieces, uneven application of the paint and, hence, breaks in production.
This problem has been overcome by adding defoamers to coating compositions. A defoamer normally contains substances which intervene in the stabilizing mechanisms of the foam. The active substances normally used develop their effect by spreading out at the original phase interface as a result of their incompatibility. Another mechanism for destroying unwanted foam consists in the absorption of surfactants at the phase interface by introduction of hydrophobic silica. In many cases, a combination of these mechanisms is also achieved in mixed preparations. For example, aliphatic and aromatic mineral oils a
REFERENCES:
patent: 4036795 (1977-07-01), Tominaga
patent: 5021502 (1991-06-01), Patzschke et al.
patent: 5059293 (1991-10-01), Sugishima et al.
patent: 5124074 (1992-06-01), Uchiyama et al.
patent: 5451305 (1995-09-01), Ott et al.
Journal of Coatings Technology, "Influence of Defoamers on the Efficiency of Waterborne Coating Systems" vol. 66, Feb. 1994, pp. 47-53.
"Elektrophorese Lacke" ,Kurt Weigel:Stuttgart (1967) pp. 197-199.
Frommelius Harald
Hoefer Rainer
Schulte Heinz-Guenther
Drach John E.
Gorgos Kathryn
Grandmaison Real J.
Henkel Kommanditgesellschaft auf Aktien
Tran Thao
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