Overview
EN ISO 787-21:2017 (CEN adoption of ISO 787-21:1979) defines a general test method for comparing the heat stability of pigments using a stoving medium. The procedure is intended primarily for comparative testing against an agreed reference sample and can also be used to determine the heat resistance of individual pigments. The method supports consistent, repeatable assessment of thermal behavior for pigments and extenders used in coatings and related applications.
Key Topics
- Scope and purpose: Comparison of heat stability by specifying agreed heating temperatures and times; optional determination of heat resistance.
- Sampling: Representative sampling in accordance with ISO 842 is required before testing.
- Preparation: Prepare dispersions of both the test pigment and the agreed reference in the same stoving medium; dilute to an agreed consistency.
- Application: Apply dispersions to cleaned, lightly abraded metal test panels (for example 150 mm × 100 mm) to a wet film thickness of 75–120 µm.
- Preconditioning: Allow panels to stand at 23 ± 2 °C and about 50 ± 5% relative humidity for 30 minutes, then cut into strips at least 30 mm wide and label.
- Stoving: Stove one strip of each panel at the minimum temperature required to fully cure the film; stove other strips at the higher agreed temperatures and times using a well-ventilated oven capable of stable control.
- Comparison and assessment: After cooling, compare colour change of test and reference panels by diffuse daylight or artificial daylight following ISO 3668. If required, repeat the comparison after 48 hours.
- Reporting: Record identification, reference to the standard, agreed test conditions (temperatures, times, medium, pigment concentration), any deviations, whether natural or artificial daylight was used, and the final verdict (colour change less than, equal to, or greater than the agreed sample) and test date.
Applications
- Quality control and batch acceptance of pigments used in high-temperature coatings.
- Comparative selection of pigments for heat-resistant coatings, enamels, and stoving processes.
- Formulation development and R&D to evaluate pigment performance under specified curing regimes.
- Contractual testing where an agreed reference pigment is used as a benchmark.
Related Standards
- ISO 842 - Raw materials for paints and varnishes: sampling (referenced for representative sampling).
- ISO 3668 - Visual comparison of the colour of paints and varnishes (used for diffuse daylight comparison).
This standard provides a practical, repeatable framework for thermal comparison of pigments using a stoving medium. It is intended to be cross-referenced by specific pigment specifications where applicable, with any necessary modifications noted to reflect product-specific behaviour.
