Iron oxide primer
Influence of PVC and Ratio of Zn Dust and MIO-Results of Drying TimeIn tests of single-layer coatings and paint systems in two corrosive environments, it was found that a partial replacement of spherical zinc dust with micaceous iron oxide is advantageous from both an ecological and a cost perspective. The MIO positively influenced the adhesion of paints after exposure in corrosive cabinets.
MIO pigment is used in paints to provide long-term durability. MIO paints have been used continuously since the early 1900s to protect steel1.
MIO, also known as specular haematite and flaky haematite, is a natural mineral ore that, after it's refined, consists mainly of Fe2O3. MIO differs in form and shape from the well-known iron oxides that are used as pigments for red, yellow, brown and black pigmentation. MIO shows a flaky, lamellar structure and is dark gray with a metallic sheen. The term "micaceous" is used because the lamellar particles are similar to mica2, 3.
MIO is insoluble in water, organic solvents and alkalis and is only slightly soluble in strong acids at elevated temperatures. It is nonreactive with most chemicals and is heat-stable up to its melting point of over 1, 500°C.
MIO is used widely in protective coatings due to its outstanding anticorrosive properties. When MIO is incorporated into a coating at an appropriate level, the flakes align parallel to the substrate surface, producing a shield or barrier of overlapping plates. The flakes are impermeable-a physical barrier is formed to the ingress of the water, oxygen and ions that corrode steel and degrade the binder system. MIO flakes are strong UV absorbers and very weather-resistant. This protects the surface of the binder system from the degrading action of UV and other weathering elements. MIO reinforces the binder matrix and increases the substrate adhesion and inter-coat adhesion.
The size of MIO flakes (normally 40 to 50 µm long and 5 to 10 µm thick) dictates a dry-film thickness in excess of 50 µm to achieve a shield of overlapping flakes. For this reason, MIO is normally found in high-film-build intermediate and finishing coats.
Recent developments in MIO technology have led to the introduction of a range of ultra-thin pigments with a flake thickness of only 1 to 2 µm. They are increasingly being used in film applications, particularly shop primers, where they provide cost and environmental benefits4.
Ultra-thin MIO pigments can also replace zinc dust in zinc-rich primers. The unique chemical and physical properties of the pigment may produce a packing structure that would enable zinc levels to be reduced. This pigment offers formulators an opportunity to reduce environmental impact while maintaining anticorrosion performance, and-unlike many environmentally friendly technologies-at a significantly reduced formulation cost5.Figure 2: Influence of PVC and Ratio of Zn Dust and MIO-Results of Pendulum Hardness (after 28 days)
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