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D2698-05 – Standard Test Method Technical Guide
ASTM D2698-05 (Reapproved 2022) defines a standard procedure for the rapid determination of pigment content in solvent-reducible paints using high-speed centrifugation. This method is widely adopted by paint producers and consumers for product acceptance and process control, providing a quicker alternative to traditional wash-out methods. The pigment content is derived from nonvolatile content determinations performed on both the total paint and the isolated liquid vehicle.
📐 Equipment, Scope, and Limitations
This test method is applicable to most solvent-reducible paints but is explicitly not suitable for formulations containing severely bleeding pigments or fine particulates such as carbon black, which are extremely difficult to fully separate by centrifugation. The central apparatus required is a laboratory centrifuge capable of delivering a relative centrifugal force of 32,000 g or higher. All nonvolatile determinations must be conducted according to Test Method D2369.
⚠️ Critical Limitation: This test method is not suitable for paints containing severely bleeding pigments or fine pigments such as carbon black, which are extremely difficult or impossible to separate completely via centrifugation. The user should verify applicability before proceeding.
⚙️ Test Procedure and Centrifugation Parameters
Thorough sample homogenization is critical for accuracy. The sample must be shaken for 10 minutes on a mechanical shaker, stirred with a paddle to loosen any caked pigment, and shaken again for 10 minutes. This cycle is repeated until no lumps remain. Sufficient mixed paint is then added to the centrifuge container to recover a minimum of 25 mL of clear vehicle after centrifugation. The bowl is revolved at 32,000 g or higher for at least 15 minutes, or until a clear supernatant is obtained. The separated vehicle must be transferred carefully into a jar and capped immediately to prevent evaporative loss, which would alter the nonvolatile measurement.
| ⏱️ Parameter | 📏 Specification / Requirement |
|---|---|
| Centrifuge Force | 32,000 g or higher |
| Centrifugation Time | 15 min minimum (or until clear vehicle is obtained) |
| Minimum Vehicle Recovery | 25 mL of clear separated vehicle |
| Nonvolatile Standard | Test Method D2369 |
💡 Sample Homogenization Tip: The standard specifies shaking the sample for 10 minutes on a mechanical shaker, stirring to loosen caked pigment, and shaking again for 10 minutes. Repeat this cycle until no lumps remain. This prevents uneven pigment distribution in the sample, which is the most common cause of poor reproducibility.
📊 Calculating the Pigment Content
The weight percent pigment (P) is calculated using the nonvolatile content of the original paint (A) and the nonvolatile content of the separated vehicle (B). This calculation mathematically isolates the pigment solids by subtracting the vehicle’s nonvolatile contribution from the total paint’s nonvolatile content. The formula provided in Section 7 of the standard is:
P = (A – B) × 100 / (100 – B)
Where:
- A = percent nonvolatile content of the original paint.
- B = percent nonvolatile content of the separated vehicle.
- P = weight percent pigment of the paint.
| 🔢 Variable | 📋 Description |
|---|---|
| A | % Nonvolatile of the original paint (determined by D2369) |
| B | % Nonvolatile of the separated vehicle (determined by D2369) |
| P | Calculated weight percent pigment in the paint |
❓ Frequently Asked Questions
🔍 What types of paints does this test method apply to?
This test method is specifically designed for solvent-reducible paints. It is generally applicable to most types but is not recommended for paints that contain severely bleeding pigments or very fine pigments like carbon black which resist separation by centrifugation.
⚡ What is the required centrifugal force for this test?
The standard mandates a centrifugal force of 32,000 g or higher. The sample must be spun for a minimum of 15 minutes, or until a visibly clear supernatant vehicle has been separated from the pigment solids.
💡 Why is the vehicle sample capped immediately after transfer?
Immediate capping of the jar containing the separated vehicle is critical to prevent the loss of solvent through evaporation. Any evaporative loss would change the nonvolatile content (variable B in the calculation), leading to an inaccurate determination of the pigment content.
📌 How is the weight percent pigment calculated in D2698?
The calculation is P = (A – B) × 100 / (100 – B). “A” is the percent nonvolatile of the total paint, and “B” is the percent nonvolatile of the separated vehicle. The results are derived from volatile content determinations performed under Test Method D2369.