Silicic acid, aluminum magnesium salt

Particle size distribution (Granulometry)

Description of key information

5.87 µm (mean particle size)

Additional information

The particle size distribution of a silica aerosol is mainly influenced by the shear forces acting upon the starting material.

Under normal handling and use of commercial products, the silicas tend to form agglomerates of high mass median aerodynamic diameters that are not respirable (MMAD >> 10 µm; for SMAS about 200 to 400 µm). The respirable fraction (=< 10 µm) comprises of less than 1 wt% (Barthel 1998/Stintz 2001; Stintz 2003).

Under experimental conditions and in measurements with destructive techniques (e.g. dispersion), the MMADs may range significantly below 10 µm, the respirable fractions acounting for more than 70 % (Grace, 2009).

Particle Size, Definition and Dynamic Interactions (modified after ECETOC 2006):

With respect to particle size, the German standard DIN 53206 (1972) distinguishes between primary particles, aggregates and agglomerates.

- Primary particles are recognisable by electron (transmission, scanning) microscopy as sub-units of pyrogenic and precipitated SAS or individual particles in SAS sols. For SAS gels, primary particles are not visible: nucleation and condensation give rise to further particle growth. Primary particles in the case of pyrogenic or precipitated SAS do not exist in isolation.

- Aggregates are assemblies of primary particles which are grown together face-to-face in the form of chains or clusters. The aggregates are formed by the collision of primary particles during particle growth and/or by the further deposition of silica onto these aggregates.

SAS aggregates represent the smallest, stable, non-dispersible particle units of three-dimensional structure, with a size ranging from 100 to 1,000 nm for pyrogenic and precipitated SASs. These aggregates can be found at infinite dilution or after blending in a polymer matrix, e.g. composites. For SAS gels, aggregates form macroscopic structures. SAS sols consist of primary particles and aggregates only in a fluid like water, organic solvent or a polymeric matrix but typically agglomerate irreversibly under drying.

- Agglomerates are assemblies of aggregates, held together by strong physical adhesion forces.

By adjusting certain process parameters, the mean particle size, particle size distribution and degree of aggregation and/or

agglomeration can be varied over relatively broad ranges. However, the smallest particles in precipitated and pyrogenic SAS remain the corresponding aggregates, not the primary particles.

Shear forces cause a breakdown of agglomerates as well as dilution tends to prevent the  formation of larger agglomerates due to the larger interparticle distances and low collision frequencies. Under the technical conditions of inhalation exposure of experimental animals, in general high shear forces dominate that form aerosols of small particle sizes with a high fraction of respirable particles. This is in marked contrast to the largely stressless settling conditions that prevail under normal handling and use of these materials.