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. Understanding how tinting strength is measured, reported, and compared across grades is essential for intelligent grade selection and for evaluating supplier-reported numbers with appropriate skepticism.

The Reynolds method: how tinting strength is measured

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, producing a dimensionless number that expresses relative scattering efficiency.

Test procedure (ASTM D332 / ISO 787-24): 1. A standard amount of black or blue pigment (typically carbon black at a defined mass loading) is dispersed in a binder vehicle (typically linseed oil or a defined alkyd) 2. A standard mass of the test TiO2 is added to the mixture 3. The mixture is applied as a uniform drawdown on black-and-white contrast paper or Leneta chart 4. L* (lightness) of the dried drawdown is measured by spectrophotometer 5. The same procedure is performed with the reference TiO2 standard 6. Tinting strength is calculated as: Reynolds = (L*_test − L*_blank) / (L*_reference − L*_blank) × 1850

The 1850 normalization is historical convention — derived from the original Reynolds reference standard (a rutile TiO2 typical of the 1960s). A Reynolds number of 1850 means "equal to the reference"; 1900 means "5.4% stronger than the reference"; 1800 means "2.7% weaker."

The test is sensitive to dispersion quality — poorly dispersed TiO2 in the test mixture will show lower Reynolds than a well-dispersed sample of the same grade. This means that test conditions (dispersant type, mixing energy, temperature) must be controlled tightly for reproducible results. Labs using different dispersants or mixing procedures can report Reynolds numbers 2–5% apart for identical materials.

Typical Reynolds values by TiO2 grade

The ~200-point spread between premium chloride rutile (SEMITI 706) and anatase (SEMITI A100) is commercially significant: to achieve the same hiding in a paint formula, a formulator switching from SEMITI 706 to SEMITI A100 would need to increase TiO2 loading by approximately 11–12%. That TiO2 volume increase must be weighed against the lower unit cost of anatase.

What drives tinting strength variation

Four physical factors control tinting strength, in approximate order of importance:

1. Particle size distribution (PSD): The dominant driver. Tinting strength peaks when D50 is in the 0.22–0.28 μm range — near the optimal Mie scattering diameter for visible light. Chloride-process TiO2 achieves narrower PSD (span 0.7–0.9) with more particles concentrated near this optimum. Sulfate-process TiO2 has broader PSD (span 1.0–1.3) with more particles outside the optimal window, wasting scattering potential. This PSD difference is the single largest contributor to the chloride/sulfate Reynolds gap.

2. Crystal form and refractive index: Rutile (n = 2.73–2.75) scatters more efficiently than anatase (n = 2.50–2.55) due to higher refractive index contrast with the binder. At the same D50 and PSD, rutile delivers approximately 10% higher Reynolds than anatase. This is a fixed physics difference that cannot be overcome by process optimization — it is inherent to crystal structure.

3. Surface treatment: Well-designed organic surface treatment (polyol, polysiloxane, or amine-based) improves particle spacing in the dried film, reducing crowding losses. A grade with optimal spacing treatment can show 3–5% higher in-formulation hiding versus one with poor spacing, even at identical CoA Reynolds. SEMITI 706W is specifically engineered for maximum optical spacing efficiency in waterborne latex systems.

4. Production conditions: In chloride-process TiO2, the oxidation temperature (1,400–1,500°C) and residence time in the oxidizer determine crystal perfection and PSD. Poorly controlled batches from any producer show lower Reynolds. Premium producers (LB Group, Chemours, Tronox) run tighter process controls that deliver more consistent batch-to-batch Reynolds. SEMITI 706 and 706W are produced from LB Group's premium chloride line with enhanced PSD control.

Practical implications for paint cost

The economic value of a higher Reynolds grade depends entirely on the price premium and the TiO2 loading in the formula. A simple break-even analysis:

Example: SEMITI 902 at Reynolds 1855, price $2.30/kg vs SEMITI 706 at Reynolds 1920, price $2.85/kg. - TiO2 saving from 706: (1920 − 1855) / 1920 = 3.4% less TiO2 at iso-hiding - Formula at 20% TiO2 loading, paint cost $8.00/kg: TiO2 costs 20% × $2.30 = $0.46/kg paint with 902 - With 706 at 3.4% reduced loading: 19.3% × $2.85 = $0.55/kg paint — more expensive despite less TiO2 - The premium grade costs more here. The math only works in the buyer's favor when the premium grade's price is close enough to the standard that the efficiency gain exceeds the unit cost delta.

For a paint plant doing 5,000 t/year at 20% TiO2 loading consuming 1,000 t TiO2/year: a 3.4% Reynolds advantage saves 34 t TiO2/year. At $2.30/kg that is $78,200 in TiO2 volume savings. If the premium grade costs $0.55/kg more per kg, switching to 706 costs an additional $0.55 × 966 t × 1,000 = $531,000 — far more than the saving. Run the specific numbers for your formula before assuming a higher Reynolds grade pays back.

The Reynolds payback is most compelling when comparing within the same process route (e.g., SEMITI 902 vs 706, both chloride) where the price gap is smaller, or when switching away from a very high-cost Western grade to a Chinese chloride grade of similar Reynolds at meaningfully lower price.

Comparing Reynolds numbers across brands

Reynolds numbers reported by different producers are theoretically directly comparable since the formula is standardized, but in practice several sources of variation make cross-supplier comparison unreliable without in-house verification:

• Reference standard selection: different producers use different internal reference standards even when nominally referencing the same historical baseline. A 1% calibration difference propagates directly to reported Reynolds.
• Dispersion conditions: the test's sensitivity to dispersion means labs using different dispersants or mixing equipment report different numbers for the same material.
• Binder system: some producers report Reynolds in linseed oil, others in alkyd or acrylic. The binder affects wetting and spacing, shifting results 2–3%.
• Measurement geometry: spectrophotometer geometry (d/8° vs 45°/0°) affects L* reading and therefore Reynolds calculation.

Practical guidance: always run a side-by-side lab test of any new TiO2 grade against your current reference in your own formulation, in your own lab. Use your standard drawdown procedure. Do not rely solely on supplier-reported Reynolds when making grade selection decisions.

Alternative metrics: Some buyers and producers express tinting strength as "% vs reference" rather than Reynolds number. The two are interchangeable: Reynolds 1850 = 100%, Reynolds 1900 = 102.7%, Reynolds 1800 = 97.3%. The same inter-lab variability caveats apply to percent metrics.

SEMITI grade selection by Reynolds target

• Reynolds ≥ 1900 target: premium architectural gloss and semi-gloss, industrial coatings, automotive refinish — choose SEMITI 706 or 706W (706W specifically for waterborne latex systems)
• Reynolds 1840–1880 target: universal architectural and industrial applications, standard plastics — SEMITI 996, 902, 826D, 2310
• Reynolds 1750–1800 acceptable: economy interior flat paint, primers, ceiling white, rubber — SEMITI 298 (sulfate rutile, lower cost)
• Reynolds 1680–1720 acceptable: fiber delustering, white rubber, some paper — SEMITI A100 or A200 (anatase). Note: for fiber delustering, tinting strength is secondary to particle size, purity, and fiber-break resistance; do not select anatase primarily on Reynolds.