Dyeing of the anodic oxide layer (Topic: 14068)

Dye colouring or adsorption dyeing is a two-step process, and is carried out by means of dipping (dip colouring) the profile in a colouring solution after anodising and rinsing. The temperature in the bath is dependent of the colour/dye, and usually between 55 and 65°C. Some dyestuffs could be used in cold baths (20ƒC), but the weathering resistance of such a coloured oxide coating is very poor. The colouring pigments (metal complex dyes) are either inorganic or organic. These pigments penetrate only into the outer part of the pores in the anodic oxide coating, and therefore oxide coatings coloured by dyeing have not the same permanence as oxide coatings coloured by electrolytic colouring where the pigment is at the bottom of the pores in the anodic film.

Many colours are available, for instance blue, red, yellow, green, and black. Dye colouring has been improved during recent years, but still the dye coloured oxide layer do not obtain the same light fastness as other colouring methods. Dye colouring should be restricted to colouring of anodised profiles for indoor use. They should only be used for external application where unavoidable.

Most of the dye will be adsorbed in the first part of the dyeing time. Therefore control of colour uptake is much more difficult if short dyeing times are employed. An important implication with dye colouring is that it will always be more difficult to secure good colour matching with short dyeing times, since small variations in temperature, pH and agitation can have a disproportionate effect on the rate of dye uptake. Further, since the formation of chemical bonds proceeds slowly, lightfastness will be reduced with short dyeing times.