Overview
ISO 20927:2019 specifies a standardized laboratory method to determine the aggregate size distribution (ASD) of precipitated silica using a disc centrifuge based on the principle of sedimentation. The procedure requires pre-treatment by strong ultrasonic de‑agglomeration in water, followed by sedimentation in a sucrose density gradient. The method supports characterization and inter-laboratory comparison of silica used as rubber compounding ingredients.
Key topics and requirements
- Scope & principle: Measurement of silica ASD by disc centrifuge photosedimentometry after ultrasonic dispersion; typical measurable range ~5 nm to 100 µm.
- Apparatus: Disc centrifuge (typical speeds up to 24 000 r/min, commonly 20 000–24 000 r/min for fine silica); probe‑type sonicator (recommended nominal power ≥ 200 W, probe ≈ ½"); cooling bath, precision balance, syringes, rolled‑rim bottles.
- Sample preparation: Example procedure - 0.60 g ± 0.05 g silica in 20 ml de‑ionized water (30 g/L); grind granular silica before dispersion to avoid flotation.
- Ultrasonic calibration: Calibrate/verify ultrasonic energy input using the IRM 100 silica standard (ASTM D5900) to control probe ageing and ensure reproducible de‑agglomeration.
- Density gradient: Sucrose gradient prepared from 8 % to 24 % w/w in 10 levels (1.8 ml per level) or by mixing two solutions (24 % and 8 %) per provided table; cap with 0.5 ml dodecane; allow stabilization (recommended ≥ 30 min).
- Calibration standard: Use a narrow‑distribution PVC latex standard (~220 nm ± 20 nm) to set parameters used in Stokes‑law calculations and to correct for instrument and gradient variables.
- Quality & precision: Routine alignment checks with standards (IRM 100/PVC) and monitoring of baseline/peak stability are recommended; safety precautions and precision data are included in the standard.
Applications and users
- Who uses it: QC laboratories, R&D and product development teams in the rubber and elastomers industry, silica manufacturers, instrument laboratories performing particle sizing.
- Why it matters: ASD of precipitated silica influences filler performance in rubber compounding (processing, reinforcement, dynamic properties). Standardizing dispersion, centrifuge conditions, and calibration enables reliable comparison of ASD results across batches and labs.
- Practical outcomes: Enables material specification, supplier qualification, troubleshooting of compounding performance, and consistent QC protocols.
Related standards
- ASTM D5900 referenced for the IRM 100 silica standard (used here as an example calibration material).
- ISO 20927 complements other particle sizing and filler characterization methods; consult laboratory best practices and instrument-specific manuals for implementation details.
Keywords: ISO 20927:2019, precipitated silica, aggregate size distribution, disc centrifuge, ultrasonic dispersion, sucrose gradient, IRM 100, PVC calibration, rubber compounding.