Overview
ISO/TR 20659-2:2024 - Rheological test methods - Fundamentals and interlaboratory comparisons - Part 2: Determination of the time-dependent structural change (thixotropy) is a Technical Report from ISO that describes how to determine thixotropy (the reversible, time-dependent decrease of shear viscosity at constant shear rate or shear stress) and how to perform interlaboratory comparisons for these measurements. The report focuses on measurement techniques, standardized measuring profiles, instrument verification and comparative testing to ensure reproducible, comparable rheological data for complex fluids such as paints, coatings and suspensions.
Key topics and technical requirements
- Measuring technique and conditions
- Detailed specification of measuring profiles (shear rate, shear stress, shear strain, durations, number of points).
- Control of boundary conditions: temperature, pressure, sample history, evaporation, sample filling and waiting time.
- Use of appropriate measuring geometries and instruments (rotational viscometers, rheometers with air bearings); absolute geometries per ISO 3219-2 are required for geometry-independent results.
- Verification and reference checks
- Verification with Newtonian reference samples across the expected viscosity range (multiple reference fluids if viscosity spans decades).
- Monitoring torque range, torque resolution and rotational speed limits of instruments.
- Measurement methods covered
- Flow curves with evaluation of the hysteresis area (rotational tests).
- Step tests with recovery (rotational tests controlled by shear rate or alternating shear stress/rate).
- Combined oscillatory and rotational tests (controlled shear strain and shear rate).
- Example measuring programmes (Method A - linear ramp; Method B - logarithmic ramp) with prescribed segments and measuring points.
- Interlaboratory comparison framework
- Guidance on preliminary tests, comparative testing programme, data evaluation and reporting to establish reproducibility and repeatability across laboratories.
Practical applications and users
ISO/TR 20659-2:2024 is directly applicable to industries and labs that need reliable characterization of time-dependent rheological behavior:
- Paints and coatings manufacturers (levelling, sagging, wet film thickness, pigment orientation)
- Additive and thixotropic agent suppliers (effectiveness evaluation)
- Quality control and R&D laboratories performing rheological testing
- Rheometer and viscometer manufacturers (instrument verification and compliance)
- Standards bodies and interlaboratory study organizers
Practical benefits include improved product formulation (pumpability, application behaviour), consistent QC across facilities, and validated methods for regulatory or customer specifications.
Related standards
- ISO 3219-1 - Rheology vocabulary and symbols for rotational and oscillatory rheometry (normative reference).
- ISO 3219-2 (absolute measurement geometries) and other parts of the ISO 20659 series for complementary guidance on rheological testing and interlaboratory comparisons.
Keywords: ISO/TR 20659-2:2024, thixotropy, rheological test methods, interlaboratory comparison, flow curves, hysteresis area, rheometer, paints and varnishes, thixotropic agents.