Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 150 of 331

by the applied coating. Electrocoat solids deposit initially in the areas closest to the counter electrode and, as these areas become insulated to current, solids are forced into more recessed, bare metal areas to provide complete coverage. This phenomenon is known as throwing power and is a critical aspect of the electro- coat process and materials. Electrocoat bath solids are deposited electrically via a system that includes a number of components: the rectifier, which supplies a DC charge to the bath enabling deposition of ionic species; circulation pumps to maintain proper paint bath uniformity; a heat exchanger and chiller to con- trol the temperature of the bath; filters, which remove dirt particles introduced into the systems; and ultrafilters that produce permeate for rinsing and allow for recovery of excess paint solids. Postrinsing As the part exits the bath, excess paint solids not deposited electrically cling to the part and must be rinsed off to maintain process efficiency and optimal aesthetics. Rinse material is supplied from the ultrafilters and is called permeate. The permeate, containing low molecular weight organics and some solvent, is used to rinse the drag- out from the parts; the excess solids and permeate are returned to the bath in a coun- terflow fashion, affording superior levels of transfer efficiency. Baking After exiting the postrinses, the coated parts enter the bake oven for curing and cross-linking of the paint film, resulting in a high-quality finish void of runs, drips, and sags (see Fig. 3). Bake temperatures range from 180 to 375°F depending on the type of electrocoat applied. TYPES OF ELECTROCOAT PRODUCTS Electrocoat products are referred to as either anodic or cathodic, indicative of where coating deposition takes place (see Fig. 4). The first electrocoats devel- oped in the late 50s and early 60s were anodic systems. Cathodic systems were de- veloped later and were initial- ly commercialized in the ap- pliance industry in the late 60s and early 70s. Fig. 3. Typical electrocoat finish after baking. Anodic Electrocoats Anodic electrocoating involves the use of negatively charged paint particles, which are de- posited onto positively charged metal substrates. The polymer species are acid func- tional and amine solubilized. Anodic electrocoats offer eco- nomical finishes with excel- lent color and gloss control. The anodic deposition process leads to some dissolu- tion of metal ions from the parts being coated. These ions become trapped in the depo- 149

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