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The particle sizes relevant for inorganic pigments stretch between several tens of nanometers for transparent pigment types to approximately two micrometers. For practical applications it is very desirable to determine not only the mean particle size but also the whole distribution. These parameters must not be confused with the crystal size determined by X-ray diffraction, as pigment particles usually are not monocrystals.

The determination of the particle size distribution is a complex issue and the subject of voluminous monographs [1.15] so only an introduction to the questions relevant for applications concerning inorganic pigments can be given.

Measuring and counting the particles shown on a suitable electron micrograph is the most straightforward means of determination of the particle size distribution of inorganic pigments. The necessary number of particles (2500-10000) renders this method too costly and time consuming for day-to-day business, although the process can be automated to a certain degree. Another disadvantage is the length of time for the process, which makes it impossible to apply this method for production control. An advantage is of course that additional information about particle shape and morphology can be obtained in this way.

Two methods are mainly used for the determination of particle size of inorganic pigments: Sedimentation methods (centrifuges) and Fraunhofer diffraction with additional correction due to Mie scattering.

When evaluating the results of these measurements one has to remember that a property of the particles (light scattering or the velocity of sedimentation) is de­termined. With models relying on a number of assumptions (for example that all particles are spherical) and further input (for example the complex index of refraction or the density) the particle size distribution is calculated in the final step. Applying the results of the measurement this and other deviations from the model have to be taken into account. Different measurement techniques usually result in different results for the measurements of particle size distributions.

The main advantage of centrifuges is the high resolution these instruments usu­ally deliver, making it possible to differentiate between particle sizes that are very closely spaced. Instruments using Fraunhofer diffraction with Mie correction have found wide use in the industry during the last decade. They can make fast and very reproducible measurements while being able to determine particle sizes between

0. 05 and 1500 pm, although it seems that the lower range cannot be reached with all substances. An advantage is that these instruments can also be used with fluidized samples and can determine the particle size distribution of dry powders.

While using a dilute suspension in a pump-through cell there is the possibility to determine even particles far from the main distribution, present only in minor amounts, with high precision. This makes it possible to catch the particles several times, resulting in good reproducibility of the measurement result.

As stated above these instruments need the complex index of refraction at all the wavelengths used by the instrument. Most types apply only a single wavelength but there are also instruments available which make use of four different wavelengths. While this can be advantageous with respect to the resolution of the instrument it should be ascertained that the index of refraction is known with sufficient certainty for the materials to be measured.

Furthermore it should be proven in every case and for every instrument that the results are independent ofthe remnant error in this and every other input parameter. One of the most important topics regarding the particle size distribution is sample preparation and dispersion. The dispersion process typically generates most of the particles determined in the measurement.

The dispersion procedure must reflect the conditions that the particles are sub­jected to in the application considered for the substances. In practice the task is often not to determine the mean particle size of a pigment, but to analyze problems occurring in an application.

As pigments are most often used in paints and varnishes a dispersion with a high energetic input should be used if not otherwise stated. This can conceal effects occurring at lower energy levels as, for example, more agglomerates can be broken up. A common feature of both instrument types mentioned above is the use of a very dilute solution. The exact concentration is dependent on the specific type of instrument used, as well as on the material and particle size to be measured, but is nearly always below 1% (weight).

Although the effect of the concentration of suspension on the results of the mea­surement of pigments has never been proven, the development of techniques able to cope with concentrations closer to the applications if of interest. These would make it possible, for example, to determine the particle size distribution in a dispersion paint or in a reaction vessel where a pigment is produced by the precipitation process. A measurement technique having no problems, in principle, with high concentration dispersions is the scattering of ultrasonic waves. Nevertheless the instruments on the market have up to now failed to realize the great expectations of this technique.

Sieve Analysis. The sieving residue can be determined by two methods:

1. Wet Sieving by Hand. In the utilization of pigments, it is important to know the content of pigment particles that are appreciably larger than the mean par­ticle size. This material can consist of coarse impurities, pigment aggregates (agglomerates), or large primary particles. The dried pigment is washed with water through a sieve of the appropriate mesh size, and the retained material is determined gravimetrically after drying. For standards, see Table 1.1 (“Residue on sieve: By water”).

2. Wet Sieving by a Mechanical Flushing Procedure. The sieve residue is the portion of coarse particles that cannot be washed through a specified test sieve with water. The result depends on the mesh size of the sieve. For standards, see Table 1.1 (“Residue on sieve: Mechanical method”). Apparatus: Mocker’s apparatus.

Additionally, wet sieving down to a mesh size of 5 gm can be realized by applying special sieves of pure nickel membranes. The material is fluidized in an ultrasonic bath.

The specific surface is usually understood to mean the area per unit mass of the solid material, but it is sometimes useful to relate the surface area to the volume of the solid (see Section 1.2.1). The specific surface area can only be determined indirectly owing to the small size of the pigment particles:

1. Gas Adsorption by the Brunauer, Emmett, and Teller (BET) Method. The specific surface area of porous or finely divided solids is measured. The method is limited to solids that do not react with the gas used (e. g., while the gas is adsorbed), and nonmicroporous materials. For standards, see Table 1.1 (“Specific Surface, BET Method” and “N2 Adsorption”).

2. Carman’s Gas Permeability Method. A gas or a wetting liquid is made to flow through the porous material in a tube by applying vacuum or pressure. The pressure drop or flow rate is measured. For pigments, a modified procedure is used in which mainly nonlaminar flow takes place [1.16]. For standards, see Table 1.1 (“Specific surface: Permeability techniques”).

1.2.2.5