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The hardness of minerals is usually measured according to the Mohs’ scale, as used in mineralogy. This scale is based on the ability of one mineral to scratch another and goes from talc (softest) with a hardness of 1 to diamond with a hardness of 10. The scale is approximately logarithmic and so small, numerical differences can have a significant impact.

The hardness of the main fillers is given in Table 2. Most commonly used fillers have a Mohs’ hardness below 4. The hardness of fillers is important to polymer processing, where soft fillers are preferred to minimize equipment wear and the accompanying pickup of metal contamination. The harder fillers can also lead to undesirable degradation of fiber length when used in conjunction with glass fiber reinforcements. Hard fillers, such as crystalline silicas, are used when high abrasion resistance is required, particularly in thermosets. Solid surfaces for use in kitchen sinks are a good example.

Morphology (Particle Size and Shape)

The size and shape of filler particles are critical factors in the processing and final properties of particulate fillers. Although they seem to be simple concepts, their adequate measurement and description provide significant challenges. One reason for this is that except in rare instances, the particles exhibit a wide range of size and shape within any given sample. Another is the effect that composite processing can have in significantly altering the particle size and shape from that in the original powder on which most measurements are usually based.

Fig. 2 The concept of primary particles, agglomerates, and aggregates (with permission of Smithers Rapra)

The terminology used can also give rise to some confusion; this is particularly noted with the description of particle types as primary particles, aggregates, and agglomerates. The term primary particle normally refers to the smallest unit that the filler can be sensibly thought to be composed of. With very fine fillers, these primary particles are often strongly attached to each other, usually by some intergrowth or fusion, and these assemblies of particles are referred to as aggregates. Both primary particles and aggregates can then form weaker structures which are easily broken down and these are referred to as agglomerates. (Care must be exercised here, as the terms aggregate and agglomerate are sometimes reversed in the literature.) These concepts are illustrated in Fig. 2.

The larger size solid glass sphere fillers provide an example of a simple type which only consists of primary particles (and probably very weak agglomerates). Carbon blacks provide a good example of the two extremes. The larger thermal blacks are essentially composed of single, nonfused particles. The furnace blacks, on the other hand, show various levels of strong aggregation caused by partial fusion (this is discussed further in the section dealing with shape).

As briefly mentioned above, processing can affect the size of particles. This is especially true where aggregates are involved (such as can occur with high-structure carbon blacks). The degree to which these are broken down and dispersed will depend on their strength relative to the processing conditions. Hollow glass spheres provide an extreme example. In some processing, the spheres survive intact and can be regarded as primary particles, while in others they can be broken down into fragments.

The measurement and application of size and shape are discussed in more detail below.

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