Rubber compounding ingredients — Precipitated silica — Determination of aggregate size distribution by disc centrifuge
This document specifies a general method for determining the aggregate size distribution (ASD) of silica by using a disc centrifuge according to the principle of sedimentation. As pre-stage the silica is de-agglomerated in water using strong ultrasonic power treatment. The method is used for precipitated silica.
Ingrédients de mélange du caoutchouc — Silice précipitée — Détermination de la distribution dimensionnelle par à disque centrifuge
Standards Content (Sample)
Rubber compounding ingredients —
Precipitated silica — Determination
of aggregate size distribution by disc
Ingrédients de mélange du caoutchouc — Silice précipitée —
Détermination de la distribution dimensionnelle par à disque
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1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Significance and use . 1
5 Apparatus . 2
6 Reagents and materials . 2
7 Calibration of the test equipment . 3
7.1 Calibration of the ultrasonic device . 3
7.2 Routine alignment check . 3
8 Operating instructions . 3
8.1 Preparation of the disc centrifuge . 3
8.2 Sample preparation . 4
8.3 Cleaning the centrifuge . 5
9 Procedure. 5
10 Evaluation and documentation of the test results . 6
11 Precision data . 7
12 Safety precautions . 8
Annex A (informative) Physical principles of measurement . 9
Annex B (informative) Precision data .11
© ISO 2019 – All rights reserved iii
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The determination of the aggregate size distribution (ASD) of silica by disc centrifuge
photosedimentometry can be used for characterizing and specifying these products. It is well accepted
that the aggregate size distribution of silica could have an influence on the performance of these
materials used in different applications. Therefore, a standardized procedure regarding the sampling
preparation and the testing of the aggregate size distribution seems to be necessary in order to
compare, discuss and interpret received results between the laboratories using this method.
See Annex A for physical principles of measurement.
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INTERNATIONAL STANDARD ISO 20927:2019(E)
Rubber compounding ingredients — Precipitated silica
— Determination of aggregate size distribution by disc
This document specifies a general method for determining the aggregate size distribution (ASD) of
silica by using a disc centrifuge according to the principle of sedimentation. As pre-stage the silica is
de-agglomerated in water using strong ultrasonic power treatment.
The method is used for precipitated silica.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
discrete, rigid colloidal entity that is the smallest dispersible unit in a suspension
Note 1 to entry: In comparison to carbon black the term silica aggregate is less defined and has to be seen
always in context with the silica treatment (i.e. ultrasonic power in a silica suspension in water). The references
apply to carbon black but are also broadly used by the rubber industry for precipitated silica.
4 Significance and use
A disc centrifuge is used for measuring the aggregate size distribution (ASD) of precipitated silica.
As a function of test time and rotational speed, aggregate sizes in the range of approximately 5 nm
to 100 µm can be analysed according to the principle of sedimentation. Firstly, the silica sample is
dispersed in an aqueous medium by using ultrasonic power treatment. Afterwards, the suspension is
transferred to the disc centrifuge and separated according to its aggregate size. The sedimentation is
accelerated by centrifugal forces generated by the rotation of the centrifuge. By using a density gradient
of sucrose solution the sedimentation can be stabilised. Over the course of the experiment, a separation
in different silica aggregate sizes is possible and can be evaluated.
For investigations between different laboratories, it is recommended to use the IRM 100 silica
standard according to ASTM D5900 (see Clause 7).
1) IRM 100 silica standard is an example of a suitable product available commercially. This information is given for
the convenience of users of this document and does not constitute an endorsement by ISO of this product. The IRM
100 silica standard can be provided from Balentine Enterprises, INC. (www .irmsilicastandard .com).
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The usual laboratory apparatus and, in particular, the following.
5.1 Disc centrifuge allowing a rotational speed of 24 000 r/min.
5.2 Probe-type sonicator typically with a nominal power of 200 W or more.
5.3 Ultrasonic device with a recommended probe size of ½”.
The chosen ultrasonic device shall guarantee that the values of the IRM 100 silica standard in dispersion
are achieved (see Clause 7).
5.4 Cooling bath, e.g. cryostat.
5.5 Precision balance, to ±0,01 g.
5.6 Disposable syringe, 1,0 ml and 2,0 ml with adequate needle size.
5.7 Rolled rim bottles with a recommended size of 35 ml, d = 30 mm, h = 65 mm.
When using the IRM 100 standard, the fixing of the dimension of the vessel is not necessary any longer.
However, an ejection of the suspension should be avoided.
6 Reagents and materials
Use only reagents of recognized analytical grade and only distilled water or water of equivalent purity.
6.1 Water, de-ionized.
6.2 Sucrose, powder, ≥99,7 %.
6.3 Dodecane, 99 %.
6.4 PVC calibration standard with a recommended size of approximately 220 nm ± 20 nm.
The determination of aggregate diameters from the Stokes equation relies on the measurement time
and the knowledge of four other experimental parameters (see Annex B). Since the accuracy of those
parameters strongly depends on other variables, such as the gradient preparation conditions, the
operating temperature in the disc or the stability of rotational velocity, a relevant approach generally
consists in setting the group of four parameters by prior calibration with a standard of known narrow
size distribution. A suitable standard for the characterization of precipitated silica is PVC suspended in
water, but any other standard is acceptable as long as its size distribution (peak and width) and density
is known accurately. A size around 200 nm is a suitable size for rapid calibration.
The PVC standard can be provided by the instrument manufacturer. PVC latex with a narrow particle
size distribution is used. However, slight variations from batch to batch are possible and have to be
considered. The value of a new batch is disclosed by the supplier.
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7 Calibration of the test equipment
7.1 Calibration of the ultrasonic device
Independent from the designated power of the ultrasonic generator under 5.2, the geometry of the
ultrasonic probe and the pre-set amplitude, the energy input into the silica suspension can be supposed
on a similar level when the documented parameters of the IRM 100 are in line with the detected results.
Additionally, the probe is underlying some significant ageing effects depending from the number of
tests but also test conditions such as amplitude, time, pulsed/un-pulsed treatment of the suspension.
These ageing effects can influence the ultrasonic power input into the sample and manipulate the
results. The comparison of results with former data or between different laboratories is impeded or
even impossible. By using the IRM 100 as calibration standard, the ageing effects can be detected very
early and precautions up to a replacement of the ultrasonic probe can be undertaken timely.
7.2 Routine alignment check
Before starting a test run, screen the disc centrifuge by using a silica standard. It is recommended to
revert to the international IRM 100 according to ASTM D5900.
Carry out the calibration procedure in accordance with Clauses 8 and 9.
By using the documented test parameters in Clause 9, the following results can be found.
— Mode, linear [nm]: most frequent aggregate size: 66,0 ± 2,8.
— Mean (dw) [nm]: average diameter by weight: 87,8 ± 3,9.
— D (25 oversize weight percentile) [nm]: 97,2 ± 3,3.
— D , median (50 weight percentile) [nm]: 76,4 ± 2,7.
— D (75 oversize weight percentile) [nm]: 60,7 ± 2,5.
— FWHM, linear [nm]: 55,1 ± 2,5.
The given ranges are calculated on a 95 % confidence level based on the total variation determined by
an analysis of variance.
This procedure makes sure that the production of the silica suspension has happened under the same
ultrasonic energy input, independently from the used device and the adjusted conditions (i.e. power of
the generator, ultrasonic time, suspension density, ageing status of the probe, etc). The ageing status of
the probe can be eliminated up to a certain level.
Hereby, it is possible to compare test results received by different laboratories or within one laboratory
(i.e. different operators).
8 Operating instructions
8.1 Preparation of the disc centrifuge
Turn on the disc centrifuge (5.1) and warm up the device as recommended by the supplier. Start
program and set the rotational speed of the disc to 20 000 r/min. When the target speed is reached, fill
the disc centrifuge with a density gradient of sucrose solutions.
The disc speed should be adjusted based on the size range of the sample to be measured. Typical values
are 20 000 r/min to 24 000 r/min for fine silica.
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The solutions of sucrose (6.2) in water consist of different concentrations. These solutions have to be
prepared. The concentrations of sucrose solutions, w , are between 8 % and 24 %. The density
gradient is structured in 10 levels:
— 24,0 % - 22,2 % - 20,4 % - 18,7 % - 16,9 % - 15,1 % - 13,3 % - 11,6 % - 9,8 % - 8,0 %
Inject 1,8 ml of sucrose solution per density level into the disc centrifuge, starting with the highest
Alternatively, it is possible to prepare only two sucrose solutions with the highest and lowest
concentration: respectively solution A = 24,0 % and solution B = 8,0 %. Mix them together in the
injection syringe (5.6) according to Table 1.
It is also possible to use automatic gradient builder (for example offered by CPS Instruments) for
preparing the solutions of sucrose.
1 1,8 ml solution A + 0,0 ml solution B