Migration (paint)
In paint technology , the term migration stands for unwanted movements of a paint component, mostly pigments or plasticizers , out of the surrounding paint layer. The occurrence of migration phenomena is considered a formulation error.
Definitions
Under blooming refers to the migration of paint constituents of the paint surface and the local crystallization.
In contrast, migration into a layer of lacquer applied later and crystallization in the same is referred to as bleeding . The successful prevention of bleeding is referred to as fastness to overcoating if the pigment property is meant. This is usually checked by painting over the pigmented lacquer layer with a white lacquer layer, since the migrated pigment is easier to see there than in a clear lacquer . However, there is no industry-wide standardized test method. This property of a pigment must be differentiated from that of a paint , as other properties (e.g. intercoat adhesion ) must be taken into account when testing whether a paint can be overpainted .
The efflorescence must be differentiated from the so-called plate-out , an effect that essentially occurs in the manufacture of plastics and also occurs in powder coatings . In the case of a plate-out, there is insufficient wetting of the pigment by the binder and, as a result, insufficient pigment absorption capacity. The pigment cannot be completely absorbed, is deposited on the surface and remains there. In contrast to blooming, this is a one-time process.
Under the terms of flooding and floating , which are often used interchangeably with each other, in contrast, is meant separation phenomena within the coating film. So these effects are not migration phenomena.
mechanism
The occurrence of migration phenomena is a function of the paint system (components with a dissolving effect), pigment chemistry, temperature and concentration . If the temperature is increased when the varnish is baked, some of the pigment will dissolve in the medium. The concentration of dissolved pigment must be so high that a supersaturated solution forms on cooling . Pigments that are mobile enough in the paint layer move out of it to the surface (blooming) or into a higher paint layer (bleeding) and recrystallize there. If you rub the surface of a paint affected by blooming, the effect will reappear. Pigments that bloom usually bleed too. The reverse is not always true.
Typical influencing factors
There is a lower and an upper concentration limit for migration. If the concentration is too low, there is no oversaturated solution and therefore no tendency to move out of the paint layer. If, on the other hand, the concentration is too high, the tendency to form new crystals is low and the dissolved molecules are more likely to crystallize on existing crystals.
The tendency to migrate is typically stronger at higher temperatures.
Large or polycyclic pigments and inorganic pigments are rarely affected by migration phenomena. There are big differences when it comes to azo pigments . Simple monoazo pigments are more frequently affected by migration than azo pigments with large or even polycyclic residues (e.g. benzimidazolones ). Both the solubility and the mobility of the pigments have an influence here.