Overview
ISO 6388:1989 - "Surface active agents - Determination of flow properties using a rotational viscometer" specifies a standardized method to characterize the flow properties (rheology, viscosity) of non‑solid surface active agents (surfactants) and products containing them. The method uses a rotational viscometer (co‑axial cylinders, cone‑and‑plate, double cone or combinations) to determine viscosity for Newtonian and non‑Newtonian test portions, including measurement of shear rate and apparent shear rates, hysteresis (thixotropy/anti‑thixotropy) and yield stress where applicable.
Key topics and requirements
- Scope: Characterization of flow properties of liquid surfactants, alone or in mixtures, and products consisting essentially of surfactants.
- Rheological phenomena covered: shear thinning, dilatancy, time‑dependent viscosity (thixotropy, rheopexy, anti‑thixotropy), plasticity and hysteresis loops.
- Apparatus: Specified rotational viscometers with defined geometries (co‑axial cylinders, cone and plate, double cone). Instruments must permit multiple rotation frequencies and, where required, geometric corrections (guard rings, conical bases).
- Performance & calibration: Instruments should be accurate to within ±2% of full scale for viscosity measurements. Viscosity measurement range typically covers a wide span (approximately 10^-4 Pa·s to 10^3 Pa·s). Calibration checks with Newtonian reference liquids are required.
- Temperature control: Thermostatically controlled bath to maintain test temperature (commonly 23 °C) with a tolerance of ±0.2 °C (noting stricter tolerances may be needed for precision work).
- Sampling & procedure: Test portion preparation and storage per ISO 607; remove air bubbles; standardize handling for time‑dependent products; perform measurements at several shear rates and report geometry, shear program and any corrections used.
- Reporting: Report apparent viscosity vs. shear rate, instrument geometry, shear program, yield stress (for plastic bodies) and any sample handling that could affect rheology.
Applications and who uses it
ISO 6388 is used in:
- Quality control and acceptance testing of surfactants, detergents, cosmetics, paints and emulsions.
- R&D and formulation labs to characterize flow behaviour, stability and shear sensitivity.
- Rheology labs and testing services validating instrument performance and producing comparable flow curves.
- Instrument manufacturers and validation teams for calibration and method compatibility.
Practical uses include selecting processing conditions, predicting pumpability and spreadability, detecting thixotropy or yield behavior, and ensuring batch‑to‑batch consistency.
Related standards
- ISO 3219 (plastics - definitions & apparatus reference)
- ISO 607 (sampling of surface active agents)
- ISO 862 (vocabulary - surface active agents)
- ISO 3104 / ISO 1652 (viscosity of Newtonian liquids)
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