Patent GB-933175-A (GB)
Title: Improvements in an electrophoretic coating process
IPC: C25D13/18
Filed: 1961-12-22
Abstract: 933,175. Electrophoretic coating processes. FORD MOTOR CO. Ltd. Dec. 22, 1961 [Dec. 27, 1960], No. 46086/61. Class 41. The conducting surface of an article is electrophoretically coated by contact with an aqueous conducting bath having a dispersion of charged coating material therein, said surface serving as a first electrode, and causing a second electrode to contact said aqueous bath thereby depositing the said colloidal material from the bath on to the surface of said article by establishing an electrical potential between the first and second electrodes. The electrical potential is maintained for at least a substantial part of the deposition process above the value 1.2 to 1.9 volts, at which the electrolysis of water occurs, and the polarity is then reversed between the electrodes to remove the excess of colloidal material from the surface of the article. The process is a method of electrophoretically coating articles to provide a uniform coat of paint on all surfaces. The electrolyte solution comprises emulsified paint particles in a colloidal state dispersed in a conducting-liquid medium. In a mechanized unit in which a further article may constitute a second electrode articles 11 for coating are transported by conveyer 18 to insulating tank 13 containing the electrophoretic coating electrolyte 12; articles 11 are slung from conveyer 18 by paint hook 19 having an electrical insulator 20 to isolate the articles from earth, but fitted with contact plate 21 below insulator 20. Article 11a, for coating, is earthed when its contact plate 21 connects with earthed bus-bar 22. Article 11b, already previously coated, acquires negative polarity when its contact plate 21 contacts bus-bar 23 connected to the negative terminal of power source 17. Electrode 26 is likewise connected to the negative terminal. Colloidal paint particles carrying a negative charge are repelled from the vicinity of the cathodes comprising coated article 11b and electrode 26 and migrate towards the earthed article 11a, acting as anode, to irreversibly deposit an electrophoretic film thereon. Articles 11a and 11b remain submerged until a sufficient paint film is deposited on the surface of article 11a; article 11b is then substantially without undeposited colloidal particles adhering to its coated surface, due to depletion of colloidal particles in the vicinity of the cathode during the coating of the anode. Coated article 11b is then conveyed to oven 24 to cure the deposited paint film; mixer 25 keeps the colloidal material uniformly dispersed. The potential required to rinse colloidal particles away from the coated surface of article 11b is lower than that required to electrophoretically coat the surface of article 11a, and coated article 11b has a relatively high resistance due to its insulating paint coat. A second uncoated electrode 26, acting as supplementary cathode permits the otherwise required potential for coating article 11a to be reduced. For certain formulations of the coating bath wherein the dispersed colloid is positively charged, deposition occurs on an article surface acting as a cathode. The electrical potential applied, substantially above the decomposition potential of waterranges between 50-1000 volts at a current density of 0.1-5.0 amps. per sq. ft. for electrophoretic deposition of a paint coating; for rinsing away colloidal particles adhering to a coated article voltages of 5-1000 volts are mentioned.