When Black Dye is Not Black
Q. We are sending aluminum extrusions to our outside anodizer for Type II anodize and black dye, and we are encountering several problems, notably that the newly black anodized parts exhibit a greenish tint.
After about six months, most of the parts have faded to a purplish color. The end use for these parts is interior modular framing. Parts are stored indoors at room temperature, but are exposed to indirect sunlight coming from nearby windows. Samples were sent to an outside laboratory for testing and the coating thickness was found to be within the spec we called out at 10–15 µ.
What causes these problems and what specification can we use to prevent this from happening? P.D.
A. Many organic dyes for aluminum, in fact most, are not very fade-resistant (lightfast). Exposure to a UV light source such as the sun or fluorescent lighting can cause the dyed colors to fade over time. Sometimes this happens within a matter of days or weeks. There are several causes for this. Here is some background information you might find useful.
There are a few aluminum dyes that rate very high on the lightfastness scale. Most of these were especially developed for use as an architectural “over-dye” to be used in conjunction with the architectural bronze colors that have been popular for the past 50 years, or on their own without the bronze. These dyes—black, red, blue, green—have been used successfully in this application. The method did not really catch on in North America, but was very popular in Europe and still is. Besides being high on the scale of lightfastness, these dyed colors were produced within very strict processing conditions in order to enhance their lightfast properties over a long period of time. More about that in a minute.
As for black dye in particular, there are many different black dyes and some are more lightfast than others. There are many hues of black. There is black-red, black-blue, black-green, black-brown, black-black, and so forth. The anodizer should choose either the most lightfast, and/or the black-black. Probably the best black dyes for the properties you desire are Deep Black HBL and Deep Black H3LW. Black MLW has been a standard for many years, but is not quite as good as the first two.
All black dyes may eventually fade to a purplish color over time. This is dependent on two primary factors. These are the processing conditions and prolonged exposure to ultra-violet light. Let’s look at the processing conditions that offer the best chance of producing a lightfast black dyed anodic coating. These are:
The minimum coating thickness should be 0.07 mil (0.0007”). The specification for architectural dyes is 1.0 mil. A thicker anodic coating has the ability to absorb more dye. The more dye absorbed the more fade resistant the color.
Dye bath conditions should be optimal when dyeing parts. Bath should be clean and free of contaminants such as sulfates, chloride, hard water and just plain dirt that falls into the bath over time. This means that the bath should be relatively new (perhaps less than 6 months old). And it should be well cared for. High quality deionized (DI) water should be used to make up the bath. A dedicated pre-dye rinse of clean, high quality DI water should be used before dyeing the parts. Dye concentration, pH and temperature should be optimum, etc., etc. See and follow manufacturer instructions.
Quality of seal is just as important as the anodizing and dyeing steps. Again, high quality DI water comes into play. Sealing bath should be clean and free of contaminants. The anodizer should know all about this. Quality of seal can be tested with the procedure stated in ASTM B680 (Acid Dissolution Test), or at the very least ASTM B136 (Modified Stain Test).
If you absolutely need a permanent, fade resistant black color, then it should be obtained by electrolytic coloring instead of dyeing. This is a totally different way of coloring and it is lightfast. Your anodizer either has the electrolytic coloring process, or he doesn’t.
Anodizing for pre-prep bonding bridges the gap between the metallic and composite worlds, as it provides a superior surface in many applications on aluminum components for bonding to these composites.
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