This is the most widely used parameter for filler morphology but is subject to a lot of misunderstanding and needs careful explanation. Particle size is usually measured in microns (10~6 of a meter), and despite the fact that on its own, it is fairly meaningless, many data sheets give this as a single number usually called the average particle size. To make some attempt at allowing for irregular sizes, this number is often what is
Fig. 3 How two different shapes can have the same ESD (reproduced with permission of Smithers Rapra)
Fig. 4 How different particle size distributions can have the same average particle size (reproduced with permission of Smithers Rapra)
known as the equivalent spherical diameter (ESD), which is the diameter of a sphere having the same volume as that of the particle. As shown in Fig. 3, the same ESD can be derived for a plate and a sphere, although they clearly have different sizes.
The average can also be determined by weight or by number, with the by weight method being the commonest and giving the larger value. The term D50 is often used for the average, and this is the size with 50% of the particles above and 50% below (usually by weight, but can be by number if specified). As shown in Fig. 4, markedly different size distributions can give the same average value. Sometimes an indication of the width of the size distribution is also given, usually by providing a top and bottom size value. The top size is frequently given as a value such as D98, which means that 98% of the particles are below this number. The bottom size, on the other hand, is usually given by the percent less than a certain fixed value. The fixed value is often a convenient sieve size.