Silicon dioxide

Particle size distribution (Granulometry)

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

particle size distribution (granulometry)

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2016-12-13

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with national standard methods

Qualifier:

according to guideline

Guideline:

ISO 13320 (Particle size analysis - Laser diffraction methods)

GLP compliance:

yes

Other quality assurance:

ISO/IEC 17025 (General requirements for the competence of testing and calibration laboratories)

Type of method:

Laser scattering/diffraction

Type of particle tested:

agglomerate

Type of distribution:

volumetric distribution

Specific details on test material used for the study:

Name of product: AEROSIL® OX 50
EC number: 231-545-4
CAS no.: 112945-52-5
LOT: 155111045 / UB 862812

Key result

Percentile:

D50

Mean:

0.33 µm

St. dev.:

0.122

Conclusions:

AEROSIL OX 50 has a D50 value of 0.33 µm using the method ISO 13320 via laser diffraction and based on a volumetric distribution

Reference 2

Endpoint:

particle size distribution (granulometry)

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2001

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with national standard methods

Qualifier:

according to guideline

Guideline:

ISO 13320 (Particle size analysis - Laser diffraction methods)

GLP compliance:

yes

Other quality assurance:

ISO/IEC 17025 (General requirements for the competence of testing and calibration laboratories)

Type of method:

Laser scattering/diffraction

Remarks:

GRADIS

Type of particle tested:

agglomerate

Type of distribution:

volumetric distribution

Key result

Percentile:

D50

Mean:

680 µm

St. dev.:

9.85

Conclusions:

HDK V15A has a D50 value of 680 µm using the method ISO 13320 via laser diffraction and based on a volumetric distribution

Description of key information

Low end D50 value is 0.33 µm for Aerosil OX 50, a pyrogenic form of SAS

High end D50 value is 680 µm for HDK V15A, also a pyrogenic form of SAS.

Similar to the BET, density also these PSD values (which belong here to agglomerates) represent an overall range of the various SAS products.

Additional information

Expert-Statement

Provided by Dr. Juergen Nolde

Synthetic Amorphous Silica (SAS)

Particle Size Distribution

Constituent Particles/Aggregates/Agglomerates

The SAS particle size structure of SAS for the dry forms (silica gel, precipitated and pyrogenic SAS) have to be distinguished into three bottom up particular systems constituent particles, aggregates and agglomerates while colloidal silica is a monodisperse system in rare cases poly-disperse consisting of the constituent particles only.

Typical ranges for the SAS particular systems:

· Constituent (primary) particles:

Size range 1 to 100 nm, with mostly spherical form. The constituent particles size can be influenced to a certain extent by the product / process conditions

· Aggregates:

Size range >100 nm to 3 to 5 mm, depending on the form of SAS. There are chemical bonds between the primary particles, they are fused together and have sintering necks. Unnatural high forces are needed to break them.

Silica Gel typically forms very large aggregates due to manufacturing process in which sodium silicate solution and the diluted mineral acid are forming a complete solid gel, which can develop depending on the aging time cross-links and through Oswald rippling very strong chemical bindings between the constituent particles.

Precipitated and pyrogenic SAS are forming smaller aggregates, typically between >100 nm to several even low two digit micrometers.

Colloidal SAS is either monodisperse or in rare cases polydisperse. The spherical monodisperse particles or the very small aggregates consisting of a few spherical particles only are typically below 100 nm and thus fall under the definition of a nanoparticle. Colloidal SAS grades are only stable in a solvent, typically water, and once dried the particles formed cannot be re-dispersed forming a comparable product in the meaning of particle size distribution.

· Dry powders typically form agglomerates while forming large loose structures of aggregates, with van der Waals and H-bridges bonds between the aggregate surfaces, which can be easily destroyed towards the forming aggregates by inducing low shear forces. Typical agglomerate size of SAS powders can reach several hundred micrometers. Large silica gel particles show no tendency to form agglomerates.

Major difference between the aggregates and agglomerates is the amount of energy needed to destroy them into smaller particular units. Larger aggregates are not reintroduced while powders in dry stage always form agglomerates i.e. while filled into packaging or during transport.

Typical particle size distribution on commercial SAS grades is provided on mass based. For commercially relevant are the d10, d50 and d90 values. The mass based particle size distribution is measured through standard industrial particle size measurement either in suspension or in air. The measurements are based on ISO standards, which have been adopted and modified by the producers of SAS within in-house standard operating procedures. Hence, in-house measurements and even round robin measurements between different independent analytical laborites and industry on the same samples show a certain variation of the results. These variations depending on the sample preparation, the instrument and software used for transferring the signals into particle size and counting.

Agglomerate and in case of silica gels aggregate size, which is typically measured for SAS products, cannot be sued as a distinguishing parameter to cluster SAS grades, as they are widely overlapping and application driven. In principle the same material can be produced in different aggregates and agglomerates sizes to a certain extend by i.e. milling processes.