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Anodizing Q&A: Important Factors in the Light Fastness of Organic Dyes

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Q. We are working on changing the structure of one of our outdoor products to aluminum and have been having difficulty getting the UV colorfastness to satisfactory levels with a gold and an olive-green color. The blue we are using is OK. I am having trouble determining if this is a thickness issue, dye issue or another process. What are the most critical aspects of colorfastness in anodized aluminum in order of priority?

A. The most important factors in determining the light fastness of any organic dye are as follows:

-Anodic coating thickness.

-Time in the dye bath

-Dye bath conditions:

-pH per manufacturer recommendations

-Temperature according to manufacturer instruction

-Dye concentration, per manufacturer instructions

-Use of high quality DI water to make up the dye bath, or less than 5 microSeimens (<5 µS/cm) specific conductance (equivalent to over 200,000 ohms resistance)

-Activity of the dye bath, concerning the amount of contaminants dragged into the bath over time.

-The quality of the seal

-The inherent lightfastness of the dye itself

Many organic dyes are not lightfast, so all of these factors can be controlled, though the dye may still fade quickly. Dyes are graded on a scale from 1 to 8, or a scale from 1 to 10, with the higher number being more lightfast. Certain organic dyestuffs have sufficient lightfastness to be used in outdoor architectural applications. Certain blue dyes may be among this group, though I don’t believe olive green is one of them. Inorganic gold dye ferric ammonium oxalate is commonly used in outdoor applications, whereas organic golds/yellows are not particularly lightfast.

See also: short.pfonline.com/blackdye 

Originally published in the September 2015 issue.

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