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Basic Properties Relevant to Filler Applications
The specific gravity of pure calcite is 2.7, while that of aragonite is slightly higher at 2.8-2.9. Calcite and aragonite are both birefringent with refractive indices sufficiently different from most common polymers for them to give some white pigmentation. The two crystal forms are relatively soft. Calcite has a Mohs hardness of three, while that of aragonite is slightly higher at 3.5-4. Calcite usually occurs as rhombohedral crystals, while aragonite frequently occurs in acicular or needlelike form. Vaterite is a relatively rare crystal form and not met with as a polymer additive.
Ground (Natural) Calcium Carbonates (GCC)
The majority of the filler market is taken by the natural forms of calcium carbonate, with calcite being the main crystal form encountered. Most natural deposits of calcium carbonate are largely derived from sediments made up of the accumulation of minute calcite crystals derived from marine microorganisms. These deposits come in three distinct forms: chalk, limestone, and marble. All three forms are readily converted into useful polymer fillers, usually by grinding and some purification (often referred to as beneficiation). Poor color and the presence of other mineral phases are the main limitations of many potential deposits. Drying is almost always necessary to create a useful polymer filler, and simple surface treatments, notably fatty acids, are frequently also applied to hydrophobize the surface and to make the finely milled particles dispersible in, and compatible with, particular polymer systems.
Chalks are the softest and least altered form of the original sediment and still contain the recognizable remains of the original marine microorganisms, with the individual calcite crystallites easily distinguished (see Fig. 1). These deposits are
Fig. 1 An SEM image of a coccolith microfossil from a UK chalk deposit - each individual calcite crystallite making up the spherical body is around 3 |rm in dimension
Fig. 2 Fine chalk-based filler formed by breaking apart the calcite crystallites in a wet-grinding process
readily ground to release the basic crystals which, being only a few microns in size, are ideal for most polymer applications without needing to be further broken down (see Fig. 2). Chalk-derived powders are frequently referred to as whitings.
Limestones have undergone significant densification, as a result of geological processes. They are thus much more compact, harder to grind to fine size, and with no recognizable marine organisms present.
Marble is a metamorphic rock and has undergone the severest geological transformation. Marbles vary markedly in color and find many decorative uses in bulk form. Filler grade marble is usually produced from industrial mines tailored for industrial mineral applications but can also be sourced from the fine dust from cutting marble for architectural use - in this case it usually comes from processing of the whiter grades.
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