Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 413 of 707

ly, following a thorough rinsing. For most effective rinsing, three tanks should be used. In this way the final tank, usually deionized water, will remain relatively free of acid. The variables in the dyebath are time, temperature, concentration, and pH. Time and temperature are readily controlled in plant practice; however, regula- tion of concentration presents some difficulties. Fortunately, in the case of most single component dyes, concentration control is not very critical, a varia- tion of 100% causing little change in depth of shade. The usual dyebath concentration for full shades is 2 g/L except for black, which requires from 6 to 10 g/L. In the case of pastel shades concentrations of con- siderably less than 2 g/L may be required in order that the shade does not become too deep. This reduction in concentration will have a negative effect on the dye lightfastness. Control of pH is important and a daily check (more often in smaller tanks or where high volume is a factor) should be made. The pH range between 6.0 and 7.0 gives the best results with the majority of dyes; however, a few are more effective at values close to 5.0. Initial adjustments should always be made since it is not practical for the manufacturer to standardize the dyes with respect to the pH of their solutions. These adjustments are made by addition of small amounts of acetic acid to lower the pH value and dilute sodium hydroxide or acetate to raise it. Solutions may be buffered against possible carry-in of sulfuric acid by adding 1 g/L of sodium acetate and adding sufficient acetic acid to reduce the pH to the desired value. COLORFASTNESS OF THE DYED COATING Of the many dyes that color anodized aluminum, possibly several hundred, it should be understood that only a few possess sufficient inherent resistance to fading to be considered for applications where exposure to direct sunlight is intended. Where items of long life expectancy are involved, for example, archi- tectural components, even greater selectivity must be imposed, since all organ- ic colorants now known will exhibit some fading when subjected to sunlight of sufficient intensity and duration. Also, the parameters of application as well as the colorant are involved in the resistance to premature loss or change of col- or. The following additional factors are considered by most authorities as affecting the lightfastness of the dyed coating. Coating Thickness and Penetration of the Dyestuff Accelerated and long-term exposure tests and practical experience both here and abroad verify that an anodic oxide thickness in the order of 0.8 mil (20 microns) and its complete penetration by the colorant is required for optimum resistance to fading and weathering. This means that, in some applications, the dye time may be extended to 30 minutes for complete dye saturation. Intensity of Shade Usually, the greater the amount of dye absorbed, the better its resistance to fading. Also, whatever fading may occur will be less apparent to the observer. Pastel shades may, therefore, be expected to exhibit inferior light and weather fastness as compared to full strength dyeing. Type and Degree of Sealing Those dyes that are reactive with the nickel or cobalt salts present in the sealing 412

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