| Micaceous iron oxide (MIO) is a crystalline
form of iron oxide that differs from the more familiar red,
yellow, and brown forms of iron oxide pigments. Its crystals are very
easily fractured into thin flakes, as shown in Fig 1, giving it a
physical form similar to mica. This similarity gives rise to the term
"micaceous." MIO is normally dark gray to black in color,
although finer grades have a rod tint. All grades are character rind by
,t metallic luster or sparkle that can easily to seen with the naked
eye. Like the other forms of iron oxide, MIO is a very inert material.
It is insoluble in water, organic solvents, and alkalis, ant! is only
slightly soluble in strong acids at elevated temperatures. It is
un-reactive to most chemicals and is heat stable up to it's melting point
of over 1,00 C (2,700 F). It is non-toxic, non-oxidizing, non-corrosive,
and non-flammable. With such health and environmental properties, it is
not surprise? that formulators consider MIO to he a key weapon in their
anti-corrosive arsenal. The use of coatings containing MIO pigments is
increasing rapidly, accelerated 1>y globalization of the
manufacturing and specifying industries. This article discusses how
these pigments work, the factors that affect performance, and key
specifying criteria.
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
This alignment enhances protection by providing harrier protection,
ultraviolet (UV) light absorption, film reinforcement, and increased
intercoat adhesion
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.
| Spherical
particles do not provide a barrier to the movement of molecules
through the coating |
With lamellar
MIO the permeability is reduced |
|
Fig.3 Barrier effect Created By MIO is shown on right
|
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.
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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. |
However, over the past 20 years, the MIO market and global conditions have
changed considerably. New sources of high quality MIO have become
available, and greater competition has reduced prices. Barriers to trade
have been removed and importation costs greatly reduced. Coating
manufacturers have changed out of all recognition, with many companies now
being truly global in technology, manufacturing, and purchasing policies.
In addition, stronger links have been forged between national specifying
associations. Many specifying industries, such as power and railways,
increasingly operate in a number of countries. Therefore, facility owners
and coating specifiers in the U.S. now have access to a wider range of
coating systems and more information on performance.
In practice, these changes mean that specifiers can seriously evaluate
MIO-containing coating systems against traditional systems that do not
contain MIO. |
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