Benefits of Transparent Iron Oxides
The high durability and weatherfastness of transparent iron oxides make them the natural choice for wood stains and furniture finishes. They are resistant to acids, alkalis and solvents, and are non-bleeding/non-migratory. They exhibit excellent weatherfastness and gloss retention properties as confirmed by industry standard external exposure testing.
Transparent iron oxide differs from the opaque iron oxide principally in terms of particle size and shape. These differences, however, impart significantly different pigmentary properties, similar to water in the form of rain or fog! The opaque pigments offer higher tinting strength. The transparent iron oxides offer higher UV protection and, important for the wood finish formulator, offer far greater aesthetic value to the finished article.
The heat stability of yellow iron oxides is lower than that of red iron oxides. The chemical composition of yellow iron is a hydrated form of the red iron oxide and this dehydration process occurs at a relatively low temperature. Since the surface area of transparent iron oxide is significantly higher than opaque iron oxide, the thermal stability is reduced slightly further. The stability of yellow transparent iron oxide is both temperature and time dependent. The stability is generally recognized as being approximately 180ºC, and is therefore suitable for wood applications.
As discussed previously, transparent iron oxides offer extremely good UV protection of the supporting medium and the substrate. Although pigments such as opaque iron oxide, carbon black and zinc oxide can also be used as UV absorbing materials, the recommended usage level renders these products to range between opaque and translucent, thus impairing the aesthetics of the wood grain finish. For UV-cure wood applications, it may be necessary to slightly increase the loading of the photoinitiator.
Transparent iron oxide pigments, because they are inorganic, offer excellent permanence. They do not migrate or bleed from the finished article and are of a size small enough to penetrate into the pores of the wood structure. Greater cost effectiveness and permanence can be achieved by using transparent iron oxide pigments rather than organic dyes in conjunction with expensive organic UV absorbers, which tend to be sacrificial by their mechanism of absorption.
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