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D6131-17 – Standard Test Method Technical Guide
📐 Scope and Significance of the Test Method
ASTM D6131-17 (Reapproved 2023) provides a standardized procedure for determining the relative tint undertone—specifically the blueness or yellowness—of titanium dioxide (TiO₂) pigments. This characteristic is directly linked to the effective particle size of the pigment, making it a critical parameter for predicting the visual tone of tinted coatings. The method involves comparing a test pigment against a reference pigment agreed upon by the parties involved.
The standard leaves the choice of dispersion vehicle and the mechanical method of preparing the dispersion to the user. However, it explicitly warns that variations in these practices can significantly increase result variance. Laboratories are urged to fix these parameters internally to reduce uncertainty.
⚠️ Important Note on Variance: This test method evaluates the relative tint undertone. The results are strictly comparative against the chosen reference pigment and are highly dependent on the replication of the dispersion protocol. Interlaboratory comparisons are rarely directly transferable.
⚙️ Apparatus and Test Procedure
The test pigment and reference pigment are dispersed identically in a liquid medium suitable for the user’s intended end-use application. The resulting dispersions are applied as a drawdown on standardized black and white sealed drawdown charts. The reflectance of these films is then measured instrumentally.
Specific apparatus requirements include an electronic balance with a readability of 0.01 g and a resolution of 0.02 g. The instrumental color measurement must be performed in accordance with relevant ASTM practices, including Practice E308 for computing CIE system colors and applicable test methods for spectrophotometry or tristimulus colorimetry (E1331, E1347, E1349).
| 🟦 Instrument / Equipment | 📐 Specification / Requirement | ⚡ Key Practice/Standard |
|---|---|---|
| Analytical Balance | Readability: 0.01 g; Resolution: 0.02 g | Weighing of pigment and vehicle components |
| Drawdown Cards | Sealed, black and white charts | Substrate for reflectance measurement |
| Spectrophotometer / Colorimeter | Hemispherical (E1331) or Bidirectional Geometry (E1349) | Practice E1164, Practice E308, Test Method E1347 |
📊 Key Measured Properties and Calculation
The instrumental readings obtained from the drawdowns are used to calculate the Yellowness Index. This single numerical value directly correlates to the blue (negative yellowness) or yellow (positive yellowness) undertone of the dispersed pigment. The test pigment’s value is compared against the reference pigment’s value to determine the relative tint undertone.
💡 Tip for Consistent Results: To minimize uncertainty, always use the same dispersion equipment, vehicle type, and mixing protocol within your laboratory. The standard specifically notes that fixing these parameters leads to “reduced uncertainty of the results within that laboratory”.
| 🎯 Property Measured | 🎨 Input Data | 📏 Outcome / Significance |
|---|---|---|
| Reflectance Factor | Readings from black & white drawdown cards | Raw data for color calculation |
| Yellowness Index (YI) | Calculated per CIE system (Practice E308) | Quantifies blue (low/negative YI) or yellow (high/positive YI) undertone |
| Relative Tint Undertone | Difference in YI between test and reference pigment | Measure of effective particle size and tone |
❓ Frequently Asked Questions
🔍 What does the tint undertone indicate about a titanium dioxide pigment?
The tint undertone (blue or yellow) is directly related to the effective particle size of the pigment. A bluer undertone typically indicates a smaller average particle size, while a yellower undertone suggests a larger particle size. This significantly impacts the opacity and brightness of the final tinted coating.
💡 Am I required to use a specific dispersion vehicle or method?
No. The standard intentionally leaves the vehicle, dispersion equipment, and method up to the user to match their intended end-use. However, because these variables heavily influence results, the test pigment and reference pigment must be treated identically. For best repeatability, these parameters should be fixed within a single laboratory.
⚡ How is the final result expressed for this test method?
The result is expressed as the Yellowness Index, calculated from spectral reflectance data. The relative tint undertone is determined by comparing the Yellowness Index of the test pigment against the Yellowness Index of the agreed-upon reference pigment, establishing whether the test sample is bluer or yellower.
📌 Which ASTM color measurement standards are referenced in D6131-17?
The standard specifically references Practice E308 for computing colors using the CIE system, Practice E1164 for obtaining spectrometric data, and Test Methods E1331 (Hemispherical Geometry), E1347 (Tristimulus Colorimetry), and E1349 (Bidirectional Geometry) for reflectance and color measurement.