TiO2 Tinting Strength and the Reynolds Method

Tinting strength is the practical measure of TiO2's hiding power — how much white-pigment "muscle" you get per kg of pigment. It's the single most important commercial property for paint and ink formulators because it directly determines TiO2 loading and therefore paint cost.

The Reynolds method: The Reynolds tinting strength test (named after the British scientist who developed it) measures the lightening effect of a TiO2 sample on a standard blue or black tint. The test compares the lightness shift caused by the test TiO2 against a reference TiO2 standard.

Test procedure (ASTM D332 / ISO 787-24): 1. A standard amount of black or blue pigment (typically carbon black) is dispersed in a binder 2. A standard amount of test TiO2 is added to the mixture 3. The mixture is applied as a drawdown 4. The L* (lightness) is measured 5. The same procedure is repeated with the reference TiO2 standard 6. Tinting strength is calculated as a ratio: Reynolds = (L*_test - L*_blank) / (L*_reference - L*_blank) × 1850

The 1850 normalization is historical convention — derived from the original Reynolds reference standard.

Typical Reynolds values for TiO2: - Premium chloride rutile (e.g., SEMITI 706): 1900+ - Universal chloride rutile (e.g., SEMITI 996, 902): 1850 - Sulfate-process rutile (e.g., SEMITI 298): 1750–1800 - Anatase (e.g., SEMITI A100): 1700

Practical implications: A grade with Reynolds 1900 delivers ~3% more hiding power per kg than a grade with Reynolds 1850. In a paint formulation at 20% TiO2 loading, this means: - You can reduce TiO2 by 3% (to 19.4%) at iso-hiding - Or you can keep TiO2 at 20% and accept marginally higher hiding (which mostly goes to waste) - For a paint plant doing 5000 t/year, the 3% TiO2 reduction translates to ~150 t TiO2 savings = ~$300k/year at current prices

Why tinting strength varies: - Particle size distribution (narrower = higher Reynolds) - Surface treatment (affects optical spacing in dried film) - Crystal form (rutile higher than anatase due to refractive index) - Process route (chloride higher than sulfate due to narrower PSD) - Production conditions (oxidizer temperature affects crystal perfection)

Comparing across brands: Reynolds numbers reported by different producers are theoretically directly comparable, but in practice small calibration differences (which reference standard is used, exact dispersion conditions) cause 1–3% variations. For meaningful comparison, run side-by-side test in your own lab against your reference paint formulation.

Alternative metrics: Some buyers prefer "Tinting Strength %" relative to a reference rather than Reynolds number. The two are interchangeable — Reynolds 1850 ≈ 100% tinting strength vs typical Ti-Pure R-902 reference.

Selection guidance: - For premium architectural and industrial paint, choose Reynolds ≥ 1900 (SEMITI 706, 706W) - For mid-tier and universal applications, Reynolds 1850 is sufficient (SEMITI 996, 902, 2310) - For economy applications, Reynolds 1700–1800 acceptable (SEMITI 298, A100)