1. Field of the Invention
The present invention relates to cationic electrodeposition baths and to the use of these baths in cationic electrodeposition of metal articles.
2. Brief Description of the Prior Art
Electrodeposition of aqueous organic coating compositions has risen to industrial prominence in recent years. The process has many advantages including uniformity and completeness of coating, even on intricate shapes. The process results in labor savings and pollution abatement. Virtually any electrically conductive substrate can be coated. There are, however, a number of disadvantages that attend the electrodeposition process.
In anionic electrodeposition wherein coatings deposit on the anode, less than desirable properties of corrosion resistance and appearance are obtained. These shortcomings have been ascribed to the nature of the process. Specifically, it is believed that these shortcomings are due, in part, to metal dissolution from the anodes, caused by the electrolytic action of the bath.
Particular interest has recently arisen in cationic electrodeposition wherein coatings deposit on the cathode. For, when the article is the cathode, there is greatly reduced metal ion dissolution which commonly causes poor corrosion resistance and staining in the anionic systems.
Examples of cationic electrodeposition are described in U.S. Pat. Nos. 3,799,854 and 3,984,299, both to Jerabek, and 3,947,338 to Jerabek and Marchetti. The cationic solubilizing groups in these resins are amine acid salts. Also suitable for cationic electrodeposition are quarternary ammonium salt group-containing resins such as those described in U.S. Pat. Nos. 3,962,165 and 4,001,156 to Bosso and Wismer.
Although cationic electrodeposition offers signficant improvement over anionic electrodeposition, there are still problems that attend the presence of ions in electrodeposition baths. The ions present in cationic electrodeposition baths are generally from sources different from or besides those present in anionic electrodeposition baths, as discussed above. Paradoxically, the ions are sometimes purposefully introduced into the bath so as to improve some other properties of electrodepositable resins.
It has, for example, been noticed that for maximum corrosion-resistant coatings over ferrous metal, the metal must be pretreated, e.g., by phosphating, before it is electrocoated. The pretreatment operations, although employed on many industrial electrodeposition lines, are undesirable for a number of reasons, such as cost, pollution, and non-uniform deposition particularly on complexed shaped articles. To compensate for the shortcomings or deficiencies in the pretreatment process, or even do away with them, lead ions (Pb.sup.+2) are introduced into the electrodeposition bath by adding water-soluble lead compounds to the bath, as described in U.S. Pat. No. 4,115,226.
Another source of the ions found in the bath, it is believed, is iron dissolution from bath accessories such as plumbings and pumps which are typically made from mild steel, when certain acid-solubilized resins are employed in the electrodeposition.
The presence of metal ions in the bath poses a significant problem in the application of coating compositions of "light" colors such as white or pastels. It has been noticed that when the coating compositions of light colors are applied cathodically, they can appear stained or discolored. For aesthetic or functional reasons, the discoloration is a matter of particular concern in instances when one-coat application of light color is desired.
By the present invention, it has been discovered that during cationic electrodeposition, when staining or discoloration due to the presence of metal ions, particularly from sources afore-described occur, the discoloration or staining can be prevented or significantly reduced.