The Use of Mixed Metal Oxide Pigments in Industrial Coatings

Mixed metal oxide pigments offer a multitude of benefits for the coatings formulator, including excellent outdoor durability, chemical resistance and heat stability.

Mixed metal oxide (MMO) pigments, also known as complex inorganic color pigments (CICPs), have been around since the early 1800s. Their use in the ceramic and pottery industries is well known due to their overall inertness, which contributes to outstanding heat, chemical and UV resistance. The use of MMOs in industrial coatings is less common and more for special purposes. This paper will discuss the benefits of formulating coatings with MMOs as a replacement for, or in conjunction with, the more common organic types.

Mixed metal oxide pigments are compounds comprised of a group of two or more metals and oxygen. The most common crystal structures are rutile (MeO2) hematite (Me2O3) or spinel (Me3O4). Metals commonly present include: cobalt, iron, trivalent chrome, tin, antimony, titanium, manganese and aluminum. Different metal combinations produce a wide spectrum of hues ranging from black to brown to green, blue, yellow and red. All MMOs are produced by a calcination process consisting of an intimate mixture of appropriate metal precursor materials being fired at temperatures of 800 to 1300 °C. It is this calcining process that creates the extremely stable metal oxide bonds. The chemical stability of these bonds affords the outstanding durability of this class of color pigments.

UV Durability

The chemical inertness of inorganic MMOs renders their excellent resistance to UV radiation and the elements encountered in the most extreme outdoor environments. Most organic pigments degrade when exposed for more than a few years in UV-intense tropical environments. High-performance organic pigments that do provide acceptable durability are typically very expensive, commanding two to four times the cost of a metal oxide pigment. MMOs are therefore the colorant material of choice for architectural coatings requiring extremely high durability such as the performance specified in AAMA 2605-05 (Architectural Aluminum Manufacturers Association). This specification calls for maintenance of color and gloss after 10 years exposure in south Florida.

Chemical Resistance

The exceptionally stable chemical bonds characteristic of MMO pigments make them insoluble in most chemicals including strong acid and alkali, and virtually all organic solvents. Because of this insolubility, coatings formulated with MMOs will not lose color due to pigment degradation even with the most extreme exposure to aggressive chemicals. Atmospheric pollution including acid rain, volcanic fallout and waste incineration does not chemically attack these pigments. Moreover, coatings requiring resistance to strong chemical exposure such as laboratory and chemical manufacturing facilities can use MMOs and be assured of color stability.

Heat Resistance

The thermal stability of MMOs is well known in the ceramic and pottery industries. This class of pigments is processed for hours at temperatures ranging from 800 to 1300 °C in their manufacture. Consequently, they are chemically and color stable at these temperatures in service. As mentioned, the ceramic industry has used mixed metal oxides for color glazes for centuries. The glazes used in ceramics are regularly fired at temperatures of 985 to 1300 °C (1800 to 2350 °F) without significant color shift. The use of MMOs in thermally stable coating formulations such as those based on silicone (polysiloxane) resins brings a palette superseding the traditional black and silver high-heat choices. This brings an attractive array of color possibilities to the designer of specialized and sport transportation such as motorcycles, ATVs and jet skis.

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