Processing Conditions for Anodized and Black Dyed parts

Question:

I am curious to know if there is any information on light-fastness of black dyed hardcoat vs. the concentration, time and/or temperature of the nickel acetate seal. JM.

Answer:

I’m not aware of any specific test results that have been published for the parameters you’re speaking of, but I can tell you what to expect.

There are many different black dyes, and one of the best for lightfastness is MLW. New blacks are being developed all the time, so there may be others that are just as good.

If a study of the characteristics and relationships you speak of is something that you need, you may have to develop those numbers yourself by testing. There are several things that are essential for maximum degree of lightfastness with any dye. It starts with coating thickness. A 1-mil anodic coating thickness is the place to start. The heavy coating can absorb the maximum amount of dye under the right dyeing conditions.

The best light-fastness is obtained with a new bath that is not contaminated with dirt and drag-in from other tanks.  Of course pH, bath concentration, dyeing time and temperature all factor in. The best bath concentration is the range recommended by the dye supplier. Best temperature is usually 140°F. The longer the parts are dyed, up to a point, the more dye is absorbed. The more dye is absorbed, the better the light-fastness. Probably a dyeing time of 15–20 min would be the absolute maximum. Too much dye time can cause the parts to start sealing. This, of course, is self-defeating from a dye uptake standpoint, and it can also cause some sealing smut on the parts right in the dye tank.

The concentration of nickel acetate in the seal is important. There are two methods of sealing that can be used. The “old fashioned” method would be to seal for 3–5 min in a bath containing 3–5 g/L nickel acetate at 208–210°F, pH about 5.8–6.2, followed by 10 min in 210–212°F DI water, pH 5.6–5.8.  You probably know that the DI sealing bath must be buffered with sodium acetate to maintain pH control.

As an alternative to the above sealing method, you could use a proprietary “mid temp” nickel acetate seal. These usually run at about 1–3 g/L at 165–175 °F.  Seal for at least 10 min; up to 20 min would be better as long as it can be done without creating sealing smut.

The above conditions are optimum for maximum dye uptake and sealing. Something less than this will most likely produce a lesser degree of light-fastness. If you are truly interested in developing a table of these parameters, testing light-fastness with a xenon light source instrument would give you values of that property.

If you do elect to conduct this testing, I would be interested in the results.