Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 154 of 331

tected against corrosion, as long as the solution has wet the surface. Typical coating thicknesses are about 15 to 30 mm (0.6 to 1.2 mils). FILM PROPERTIES Analysis of a cured autodeposited film shows the presence of iron throughout the organic layer. It is believed that the positively charged iron reacts with anion- ic sites (e.g., sulfonate groups and carboxyl groups), formulated into the polymer backbone, to effect the deposition. No additional cross-linking agents are required. Although many organic emulsion polymers can be autodeposited, there actions are the same; however, the properties of the cured film depend on the chemical nature of the polymer. For example, polyvinylidene chloride (PVDC) latex emul- sions are the most widely used at present in the commercial practice of autode- position. Films deposited show excellent resistance to the penetration of mois- ture and oxygen to the base metal and, hence, offer superior corrosion resis- tance, as well as the excellent hardness, formability, and adhesion characteristic of paint films using this type of resin. Another commercial autodeposition process utilizes acrylic resin polymers to produce films that are resistant to high temperatures (>400°F) in the presence of aggressive fluids (e.g., alkaline polyglycols). Carbon black pigments, which are effectively encapsulated by the polymers and thus deposit simultaneously with the resin, are highly effective. A nonpigmented version of the polyvinylidene chloride latex is commercially available as a primer for subsequent top coating. While other colored pigments have been successfully evaluated on a laboratory basis, none are at present commercially available. Zinc and zinc-alloy coated steels can be effectively painted by autodeposition by varying the process chemistry. PROCESS SEQUENCE Commercial autodeposition systems employ movement/transfer of the work package from stage to stage by either a continuous conveyor or by an indexing hoist. Conveyorized systems offer the advantage of assured agitation (to bring fresh reactants to the metal surfaces) due to the movement of the work through the coating tank (as well as the cleaning and rinsing tanks). Hoist-operated systems conserve space due to decreased transfer distance. A typical process sequence is shown below. (Contact times in each stage vary from 30 seconds to 2 minutes, with the exception of the oven where 15 to 30 min- utes is common). Stage 1: Spray alkaline cleaning Stage 2: Immersion alkaline cleaning Stage 3: Immersion water rinse (plant water) Stage 4: Immersion (or spray) deionized water rinse Stage 5: Autodeposition (immersion) Stage 6: Immersion water rinse (plant water) Stage 7: Immersion sealing rinse Stage 8: Cure Cleaning Good cleaning is essential to successful autodeposition. Any residual soils, which hinder the solubilization of metal ions, can prevent or reduce coating forma- 153

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