U.S. Steel Prepainted Sheet consists of a substrate of U.S.Steel Hot Rolled, Cold Rolled, Hot-Dip Coated or Electrolytic-Coated Steel Sheet to which any of a variety of organic finishes is applied. Paint systems include polyesters, siliconized polyesters, plastisols and polyvinyl fluorides. To achieve both a high level of corrosion resistance and a pleasing aesthetic appearance, optimum combinations of metallic coating (type and thickness) and paint film (type, color and thickness) can be tailored for specific applications. The painting is performed on coil painting lines using a continuous process similar to that used for applying hot-dip and electrolytic coatings.
The incoming coil of steel sheet, either metallic coated or uncoated steel surface, is inspected for thickness, width, shape and surface as it enters the continuous coil coating line. The flat strip proceeds through closely monitored cleaning and chemical treatment sections of the line ahead of the paint application step. Extensive cleaning is required to achieve good adhesion between the paint and steel sheet, and a thin chemical treatment is applied to promote paint adhesion during subsequent forming of the sheet and to improve the corrosion resistance of the coating (reduce the tendency for undercutting corrosion, i.e., paint delamination) during service in the environment.
After the treatment is applied, the strip passes to the coating section where the paint is applied. Most often, a compatible primer coating is applied before application of the finish (color) coating. The primer is applied using rubber-covered metering rolls to apply a controlled thickness of primer coating to both surfaces. Then, the primer is baked in an oven to about 450oF to 500oF to cure and harden the primer layer. Curing ovens are designed to allow the strip to "float" in the oven to fully cure (dry and harden) the paint before the strip is allowed to contact any rolls. Subsequently, the topcoat layer is roll-applied onto the primer and it, too, is baked at similar high temperatures in the same type of oven to cure and harden the paint layer.
The heart of the process is the roll-applicator equipment. It has to be closely monitored and finely tuned to achieve a uniform application of the primer and topcoat layers. Applicator-roll equipment is very sophisticated today so that excellent control of the paint and primer thicknesses is achievable. This equipment as well as the drying ovens and the heated air in the ovens must be maintained very cleanly to achieve a high quality finish and appearance.
Paint color matching is another step that has to be closely monitored, not only by the coating line personnel but also by the supplier of the paint.
Paint chemistry for these thermosetting paints is a well developed science today. Paints that exhibit controlled amount of gloss (high or low) and retain their color for many years with only minimal fading are commercially available.
Color and gloss uniformity are assured through both visual and instrumental comparisons with standard panels. This ensures coil-to-coil and run-to-run uniformity.
