Reaping the benefits of talc in high solids coatings.

The extender that you use can lend important qualities to the coating you produce. Experiments have investigated how low oil absorption talc can enhance the corrosion resistance of high solids coatings, and have discovered that it can enhance the paint system's performance in several areas.

Talcum powder (also known as soapstone or steatite) is a hydrous magnesium silicate with triple-sheet crystalline units, with the chemical formula: [Mg.sub.3][Sil.sub.4][O.sub.10][(OH).sub.2] Pure talc mineral is characterised by softness (Mohs scale 1), hydrophobic surface properties, flakiness and chemical inertness.

Its softness and flakiness are explained by the structure of the crystal lattice (Figure 1), which consists of a brucite sheet sandwiched between two silica sheets to form talc layers, which are superimposed indefinitely. Each layer is electrically neutral. The adjacent layers of the structure are held together by weak Van der Waals forces.

[Figure 1 ILLUSTRATION OMITTED]

Morphology of talc

The crystal structure differs from source to source and can also be influenced by using a certain milling technique. The morphology for flaky material is expressed in an aspect ratio number, which is the ratio between the average equivalent diameter of the flakes and the average thickness of these flakes.

Microcrystalline talc products have an aspect ratio up to 20; macrocrystalline talc, however, is characterised by an aspect ratio of 20 or higher.

Sources for microcrystalline talcs are rare, and currently they-only exist at Three Springs, Australia, and Montana, US. This quality enables the manufacturer to produce a special product with a low oil absorption value because it is possible to remove the ultra-fine particle fraction. The obtained low oil absorption is therefore a result of the microcrystalline structure and a sophisticated method of processing.

Figures 2 and 3 demonstrate the difference between a microcrystalline talc with low oil absorption (Westmin D30E) and a (standard) macrocrystalline product.

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Talc is a functional extender in industrial coating applications because it:

* improves sandability

* can be used as a rheology modifier

* improves application properties (sprayabitity, brushability)