ISO 787-15:2019

Overview

ISO 787-15:2019 - General methods of test for pigments and extenders - Part 15 - specifies a comparative method to assess the resistance to light (light fastness) of coloured pigments of similar types. The standard defines how to prepare side-by-side test specimens using an agreed reference pigment and a test sample, and describes two exposure methods: Method A (natural light under glass) and Method B (direct artificial light, e.g. xenon arc). Results are evaluated by comparing colour change between the sample and the reference.

Key topics and technical requirements

• Test principle: Disperse test pigment and agreed reference pigment in the same binder, apply as adjacent films on a common substrate, expose both under identical conditions, and compare colour change.
• Specimen preparation: Apply wet films of 50–100 µm (or mass-tone prints ≈1.5 µm), dry 24 h in diffuse daylight; retain a dark control specimen.
• Substrates: White, non‑absorbent high‑gloss coated aluminium, rigid cardboard, or paper for mass‑tone prints.
• Method A (natural exposure under glass):
  • Use an exposure cabinet with a single clear glass cover (2–3 mm) that transmits ≈90% at 360 nm and throughout visible wavelengths.
  • Position cabinet to receive direct sunlight; slope toward the equator (approximate latitude angle) or specify if different.
• Method B (artificial exposure):
  • Use a suitable weathering cabinet (xenon arc or equivalent) with controlled spectral irradiance:
    • Total irradiance (300–800 nm): (550 ± 55) W/m²
    • Irradiance (300–400 nm): (50 ± 15) W/m²
    • Wavelengths < 320 nm limited to ≤ 0.5 W/m²
  • Maintain black panel temperature at (50 ± 5) °C; no water spray.
• Assessment: Compare change using a grey scale for assessing change in colour (ISO 105‑A02). The document recommends making comparisons when the agreed reference pigment reaches grey scale grades around 4 and 3.
• Reporting: Include details of reference pigment, binder, dispersion method, exposure method and angle (if applicable), apparatus and results.

Applications and who uses it

ISO 787-15:2019 is used by:

• Pigment manufacturers for product development and quality control (batch-to-batch comparison).
• Paint, inks and coatings formulators to qualify pigments for end-use lightfastness performance.
• Test laboratories conducting comparative weathering tests (natural vs accelerated).
• Procurement/specification writers and compliance teams who need standardized test evidence for material selection and supplier claims.

Practical uses include supplier comparison, R&D to improve light fastness, and contractual verification of pigment performance.

Related standards

• ISO 105‑A02 - Grey scale for assessing change in colour
• ISO 15528 - Sampling for paints, varnishes and raw materials
• ISO 18451‑1 - Terminology for pigments, dyestuffs and extenders
• ISO 4892‑1 / ISO 4892‑2 - Methods and apparatus for artificial weathering
• CIE Publication No. 20 - Simulated solar radiation spectral recommendations

Keywords: ISO 787-15:2019, resistance to light, light fastness, coloured pigments, xenon arc, natural light exposure, pigments testing, paint industry, grey scale ISO 105-A02.