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Sunday, 31 May 2020

Floating of pigments on 100% solid epoxy-coating

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Monday 28 June 1999 1:00:00 am

We have problems with the floating of pigments in a low viscosity (500 mPa.s) epoxy-coating. We use oxyd-yellow, oxyd-brown and oxyd-black and titanium white.
When aplying it, it looks okay but after 15-30 minutes it turns darker.
Can anybody help us?

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Friday 16 July 1999 1:00:00 am

If the colour of solvent-free epoxy coatings alters in this way during film formation, this may be due to an accumulation of darker pigments in the zone of the film near the surface. The non-homogeneous distribution of the pigments mentioned comes about in the film when their particles have different mobilities in the dispersion medium. The mobility of the pigment particles in any particular dispersion medium is determined largely by their size, but also by their morphology and the way they interact with one another. Larger particles sink under the effect of gravity much more quickly than smaller ones, even if their specific weight is lower than that of the others.
This effect is all the more pronounced the lower the viscosity of the dispersion medium and the smaller the PVC. At high pigment volume concentrations, the interaction between the various types of pigment particles is greater, as a result of which the difference between the rates of sinking of larger and smaller particles become less. The flooding effect thus becomes more pronounced the greater the differences in particle sizes in the pigment mixture, the lower the viscosity of the dispersion medium, and the smaller the interaction of the particles. The circulation streams (Benard cells) caused by evaporation during film formation of solvent-based coatings are less pronounced in solvent-free coatings, so that the floating effect otherwise caused by this is of no importance.
There are several possibilities of limiting and even eliminating the flooding effect. If the nature and amount of pigmentation is to be retained and the viscosity of the dispersion medium is not to be altered substantially, special wetting and dispersing agents may be of assistance. These polymer additives with a particular structure act by firmly anchoring their adhesive groups to the polar surfaces of the inorganic pigments and stretching out their polymer chains towards the binder molecules that are compatible with them. Not only do the polymer additives stabilise the pigments against flocculation, they also counteract the flooding effect. Wetting and dispersing agents of this kind are sold by various producers, and include in particular those for low-solvent and solvent-free coatings. Further details about the additives and their use should be obtained from the manufacturers.

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