Fig. 1 left The crystals of MIO are easily fractured into thin flakes 
Fig. 2 right When the thin MIO flakes in a coating align parallel to a substrate, they produce a shield of overlapping plates.

Protective Action 

MIO has wined widespread use in protective coatings around the world because of its anti-corrosive properties, which stem From the unique nature of its flake-like particles. 

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, as shown in fig. 2.1 

Barrier Effect 

Because the flakes are impermeable, a physical harrier is formed that prevents the ingress of water, oxygen, and ions-and thus prevents Corrosion of the steel and degradation of the hinder. The harrier effect is illustrated in Fig.3 

Ultraviolet Light Protection 
MIO flakes are strong UV light absorbers and are very weather resistant. These properties protect the surface of the binder system from the degrading action of UV and other weathering elements. The shape and alignment of the particles permit MI0 to be much more effective than conventional granular pigments. Erosion rates and chalking are Greatly reduced when MIO is present, and other film properties such as flexibility are retained. 

Film Reinforcement/Adhesion Promotion
MIO reinforces the binder matrix. The aspect ratio (the ratio of length to thickness) and alignment of flakes in a coating toughen and strengthen the film. Coatings formulated with MIO can show greatly improved resistance to blistering and increased substrate adhesion. For example, MIO is used to promote adhesion in coatings formulated for galvanized.  

Increased Intercoat Adhesion 
MIO has found wide use in binders, such as epoxies, that form very hard surfaces and that are difficult to recoat due to the lack of a suitable key cu? profile. The incorporation of MIO produces, after weathering, a surface with an excellent physical profile for subsequent coats. Alternatively, coatings may be formulated with higher levels of MIO to boost the intercoat adhesion of freshly applied systems.

Pigment Quality 
Natural MIO is obtained from mineral deposits of specular hematite, found in many countries, including Morocco, Turkey, Australia, and Austria. However, there is tremendous variation in the physical and chemical properties of different sources, and not all MIO is suitable for protective coatings. 
Two of the most important criteria are the amount of thin flakes and the chemical purity. These properties, however, vary widely from source to source and even within a single deposit. In addition, the processing of the crude ore to remove impurities and control particle shape, size, and size distribution is absolutely critical to final performance. This fact has been recognized by formulators and specifiers, leading in 1994 to the publication of ASTM D 5532, standard Specification for MIO for Paint. 
The standard is summarized in Table 1. It requires MIO pigments to have a minimum thin flake content of 50%, with a further classification into two types. Type 1 is of the highest quality with a thin flake content greater than 65%. In addition, the standard sets maximum levels for impurities and a minimum level of iron oxide content. These requirements are designed to ensure that a specifies receives material suitable for protective coatings applications. The demand for consistent, high quality MIO was one of the key forces driving the development and commercialization of a new synthetic process in the late 1980s. The process provides for consistency and flexibility. Flake size and aspect ratio can be closely controlled, impurities can be kept at very low levels, and properties can be tailored for specific applications. The process is based on readily available raw materials: liquid chlorine, caustic soda, iron flakes, and salt. 

Coating Performance 
The quality of MIO is only one of many factors that determine the performance of coating systems containing MIO. It is therefore appropriate to make only general comments that may be of use to the specifier. MIO pigments, being inert and pH neutral, are compatible with all of the major binder types. They have been used successfully in phenolics, alkyds, urethane alkyds, epoxy-esters, chlorinated rubbers, styrene acrylics (acrylated rubber), vinyl copolymers, polyvinyl butyral resins, vinyl chloride-vinyl isobutyl ether copolymers, epoxy resins cured with polyamines or polyamides, moisture-cured polyurethanes, and two-component polyurethanes. In all cases, the protective nature of a hinder will be enhanced by inclusion of an optimized level of MIO. 
Proper pigment loading and make-up are key to obtaining the benefits of MIO. MIO has a much lower oil absorption than other lamellar pigments. Formulations, therefore, must contain much higher levels of MIO than would be the case with, for example, aluminum, talc, or mica. 
Specifiers who are not familiar with coatings may be surprised to see formulations at 25"% to 50% pigment volume concentration (PVC) with MIO forming greater than 80% of the pigment volume. The Optimum PVC will normally be in the above range but depends on the binder system and type of application. It is crucial to choose a formulation with the correct PVC. 
The size of MIO flakes (nominally 4050 micrometers [1.6 to 2 mils] long and 5-10 micrometers [0.2 to 0.4 mils] thick) dictates a dry film thickness in excess of 50 micrometers (2 mils) to achieve a shield of overlapping flakes. For this reason, MIO is normally found in high-build intermediate and finishing coats. A typical system will consist of a primer containing corrosion-inhibiting pigments; one or two coats of MIO intermediate; and, if a high gloss or bright color is required, a finishing coat based on a low chalking binder. Recent developments in MIO technology have led to the introduction of a range of pigments with a flake thickness of only 1 to 2 micrometers (0.04 to 0.08 mils). These pigments are finding increased use in thin film applications, particularly shop primers, where they provide cost and environmental benefits. 

Coating Specification  It is clear from the above discussion that specifications based on composition will only go part of the way to satisfying the specifier's need for quality.  Many specifiers use existing compositional specifications to ensure a minimum standard and assist quality assurance on site. But the same specifiers also complement the compositional guidelines with their own performance-based specifications, which call for laboratory testing, accelerated natural weathering, and site trials.  Of course, the best evidence of a coating system's durability is a proven track record on a similar structure exposed to a similar environment. MIO-based coatings have a proven track record on some of the world's most famous structures

(Table 2). MIO-containing coatings have been the mainstay of facility owners in many parts of the world, under all extremes of climatic conditions, often for many decades,  At present in the U.S., MIO seems to be regarded as a specialty material and is strongly associated with moisture curing urethane binders. In Europe and many other parts of the world, however, MIO is regarded as giving added durability to a coating system regardless of binder type, from alkyds to two-pack systems.

Global Changes May Change Use of MIO in U.S. Coating formulators in many parts of the world have chosen MIO as the solution to their coating problems. So why is it that MIO has not been adopted in the same way in the U.S.? Table 3 shows how little MIO is used in the U.S. compared to other regions of the world. The reasons, we believe, have been the lack of a good quality local source and the cost and complexities of importing a key raw material. In the past, it has not always been cost-effective to use MIO, and U.S. formulators have gone down alternative formulation routes to achieve the desired performance.