Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 284 of 331

a simple film casting from a fluid-flow technique. Coatings on Substrates The coating on a substrate is generally applied for a wide variety of tests for ap- pearance and physical properties. The color and opacity are commonly evaluat- ed on coated-paper charts, although there are unsealed-paper charts with a porous surface available as well. Coating on substrate to simulate actual use is common as well, so the paint lab has all manner of steel, aluminum, wood, or oth- er panels in stock as needed by the customer of the paint company. Plastic pan- els are rarer but can be made available when needed. Several companies now offer such products. The casting technique on a cardboard chart or panel is a skill one learns through apprenticeship. First, the panel needs to be clean and dust free. For horizontal castings (a way to have good, flat films), one must ensure that the sur- face is level during casting and drying. Tape the chart or panel to the level surface so that it does not curl as the film tries to shrink. Unsealed charts will curl con- vexly as the water in the coating swells the paper fibers, and the substrate will try to curl concavely when the film tries to shrink across the top surface. Vacuum plates are available that hold down the paper, foil or cardboard chart quite well during the film casting process. Application of the film with a draw bar nominally gives a good continuous film, but the spacing between the bar and the substrate may have little to do with the film thickness obtained. There are other influences that govern final film thickness. Speed of pulling the draw bar over the fluid will affect the film thickness because there is a rheologic factor in how the fluid responds. You see the extremes in the fraction of nominal blade spacing compared with resultant film thickness from 0.17 to 0.83 µm or 17% to 83% of spacing. One assumes better control and may estimate 50% of nominal spacing in what is cast. Measure film thickness with a micrometer when there are suspicions that the yield is not 50%. For very thin films, the wire-wound rods are quite good at applying films. Spray application (even by weight on a panel hanging on a balance in a spray booth) can be done with good precision. Good brush out application on panels can be done by those skilled in the art. Other Tips on Practice of the Art Dust is always a problem, especially in formulation labs that have pigment dust- ing in the lab and plant. Cover cast films immediately to keep the dust off. The easiest cover is the top of a box from a typewriter paper or file folder shipment. Simply cut out a ½-in. (or 1-cm) strip from two, three, or four sides for free flow of air. Keep the cover on top of the lab refrigerator or bookshelf so that it is al- ways at hand. Make sure that the film dries in the appropriate temperature and humidity. Something in the formulation may respond poorly to the condensing moisture that forms droplets on the surface as the rapidly evaporating solvent cools the system. In urethanes, the condensing moisture may react with the isocyanates to modify the degree of cure, which reduces strength, solvent resistance, etc. In nonwater-reactive systems, one may still get pits, pinholes, or haziness from the condensing water. Paint applicators are learning to pay attention to humidity vari- ation effects on the end-product surface. 283

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