Overview
ISO 8781-1:1990 defines a standardized laboratory method for assessing the dispersion characteristics of coloured pigments by measuring changes in tinting strength during staged dispersion. The method applies to pigments dispersed according to one of the procedures in ISO 8780 and is intended mainly for comparative tests (for example, a test pigment versus an agreed reference pigment). Tinting strength is quantified as a K/S value (per ISO 787-24) of reduced pastes made by mixing incremental mill-base samples with a white pigment paste.
Key Topics and Requirements
- Scope: Coloured pigments dispersed in an agreed binder system using ISO 8780 dispersion methods.
- Core principle: Prepare pigment dispersions in stages, take samples after each stage, mix each sample with a white pigment paste to form a reduced paste, and measure tinting strength (K/S).
- Measurement: Photometric or spectrometric measurement of reflectance (400–700 nm) and calculation of K/S values following ISO 787-24.
- Result expression: Calculate percentage increase in tinting strength (IS) between two agreed dispersion stages using IS = ((K/S)2 – (K/S)1) / (K/S)1 × 100. Optionally plot K/S vs. work input (or reciprocals) to determine development curves and ultimate tinting strength.
- Apparatus & materials: Balance, film applicator, automatic muller (for high-viscosity binders), spectrometer or photometer, agreed white pigment (typically TiO2 grade R2 per ISO 591 unless otherwise agreed).
- Supplementary information (Annex A): Must specify dispersion method (ISO 8780 part), white pigment type, reduction ratio, film application method, data presentation and any reference pigment.
- Interpretation: Guidance on significance - e.g., IS between 0–20 is not significant unless >7; in ranges 50–100 a difference >12 is considered significant. Graphical anomalies (peaks or troughs) may indicate overgrinding, flocculation or recrystallization.
Applications and Users
- Quality control laboratories comparing batch dispersibility of pigments.
- Pigment and paint manufacturers for product development and supplier qualification.
- Coatings, inks and plastics formulators selecting pigments for ease of milling and color development.
- R&D teams troubleshooting dispersion issues (e.g., overgrinding or flocculation) and optimizing milling parameters.
Related Standards
- ISO 8780 (Parts 1–6): methods of dispersion (oscillatory shaking, high-speed impeller, bead mill, automatic muller, triple-roll mill, etc.)
- ISO 787-24: determination of relative tinting strength (photometric methods)
- ISO 591: titanium dioxide pigments - specifications
- ISO 105-A01: general principles of colour fastness testing
Keywords: ISO 8781-1, tinting strength, dispersion characteristics, coloured pigments, K/S value, reduced paste, pigment dispersibility, ISO 8780, ISO 787-24, titanium dioxide.