Overview
ISO 787-25:2019 specifies a colorimetric test method for comparing the colour, in full‑shade (optically hiding) systems, of white, black and coloured pigments against an agreed reference pigment. The standard describes preparation of a defined test medium, dispersion and specimen preparation, colour measurement using spectrometers or tristimulus colorimeters, and expression of results (e.g., relative hue, lightness, chroma and total colour difference).
Keywords: ISO 787-25, pigments, extenders, colour comparison, colorimetric method, full-shade systems, pigment testing, spectrometer, tristimulus colorimeter.
Key topics and technical requirements
- Scope and principle
- Compares the colour of a test pigment and an agreed reference in a single‑pigment (mass tone/full shade) system using tristimulus measurements and colourimetry (ISO 18314-1; ISO 11664-4 for derived attributes).
- Test medium
- Preferred binder: an alkyd resin composed of 63% linseed oil, 23% phthalic anhydride, 14% trimethylol propane with specified acid value, viscosity and hydroxyl value.
- Fumed silica (to prevent flocculation and control flow) blended at a recommended ratio; preferred mix: 97 parts alkyd : 3 parts fumed silica, dispersed on a triple‑roll mill.
- Fumed silica requirements include BET surface area 175–225 m2/g and pH of a 4% aqueous dispersion 3.6–4.5.
- Sample preparation and apparatus
- Use of an automatic muller (ground‑glass plates), triple‑roll mill for pre‑dispersion, film applicator, suitable non‑fluorescent substrates and stencils/glass covers.
- Measurement and instrument requirements
- Spectrometer or tristimulus colorimeter per ISO 18314‑1.
- Special provisions for black pigments: high instrument accuracy, zero adjustment using a light‑trap black standard and repeatability requirement where standard deviation σ of consecutive tristimulus measurements must be **