Barriers, pigmentation effect

Akbarinezhad, E., Ebrahirru, M., Shar if, F Attar, M.M., and FaridL H.R. (2011) Synthesis and evaluating corrosion protection effects of emeraldine base PAni/clay nanocomposite as a barrier pigment in zinc-rich ethyl sibcate primer. Prog. Org. Coat., 70, 39-44. 

In particular, micaceous iron oxide (MIO) contributes to the outstanding protective properties of barrier pigments through a parasol effect the organic binder beneath the flakes is shielded from harmful ultraviolet (UV) radiation and therefore the flakes remain on the surface although the binder above or around is degraded with time. Figure 1.7 illustrates the UV protection by MIO. 

Flake zinc pigments have a barrier effect and also act by a cathodic anticorrosive mechanism. Compared with zinc dust coatings, flake zinc pigments are formulated with lower pigment volume concentrations [5.178]. 

From a practical point of view, the attainment of the specified dry film thickness is important. In many cases, a minimum thickness must be achieved or protection will fail in a short period. This aspect demands skill in application and close inspection and control. For metallic zinc paints, the thickness would normally be of the order of 60-100/im, for anticorrosive pigmented paints, together with top coats, 100-200 //m, and for inert barrier types 150-300/im. For special environments or projects, these thicknesses may be varied but it should not be presumed that protection will always be improved or be more effective over a longer period of time by increasing the film thickness. 

The barrier effect can be achieved, for example, by using pigments with a platelet-like or lamellar particle shape. This allows them to form a wall of flat particles within a paint film and therefore water and electrolytes have to take an extended/less direct route through the paint film to the substrate. 

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