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D2621-87 – Standard Test Method Technical Guide
This test method covers the qualitative characterization or identification of separated paint vehicle solids using infrared spectroscopy within the limitations of the technique. It is part of ASTM D2621-87, reaffirmed in 2024.
📐 Scope and Significance
1.1 This test method is intended for the identification of vehicle solids from solvent-reducible paints by infrared spectroscopy. It is useful for characterizing unknown or competitive coatings, complaint investigations, and in-process control.
1.2 The standard does not purport to address all safety concerns; users should establish appropriate safety, health, and environmental practices.
⚙️ Apparatus and Procedure
6.1 Spectrophotometer: A recording double-beam infrared spectrophotometer with a wavelength range from at least 2.5 µm to 15 µm and spectral resolution of at least 0.04 µm over that range (see Practice E275).
6.2 Demountable Cell Mount with NaCl window.
6.3 Vacuum Drying Oven: thermostatically controlled to operate at 60 °C ± 2 °C.
6.4 Oven: Gravity or forced draft, capable of maintaining 105 °C to 110 °C.
7.1 Place the vehicle separated according to Practice D2372 on a NaCl window and spread to form a uniform film. Adjust thickness so that when recorded, the transmittance of the strongest band falls between 5 % and 15 %. Dry the film at 105 °C to 110 °C for 15 min and cool in a desiccator. Inspect for bubbles, wrinkles, or contamination; recast if defective. For easily oxidizable substances like tung, oiticica, or linseed oils, dry at 60 °C ± 2 °C in a vacuum oven for 1 h. For low-volatility solvents, extend drying time as needed.
| 🟦 Component | 📏 Specification |
|---|---|
| Spectrophotometer | Wavelength range: 2.5 µm to 15 µm; Resolution: ≥ 0.04 µm |
| Vacuum Drying Oven | 60 °C ± 2 °C |
| Gravity/Forced Draft Oven | 105 °C to 110 °C |
Tip: For accurate identification, ensure film thickness results in the strongest band transmittance between 5% and 15%. Avoid defects like bubbles or contamination for reliable spectra.
📊 Key Considerations for Analysis
4.1 Infrared spectra are prepared from dried films of isolated paint vehicles. Vehicle types are identified by comparing spectra to a reference collection.
5.1 This qualitative characterization is valuable for identifying unknown coatings, competitive analysis, complaint investigation, and in-process control.
For substances prone to oxidation or with low-volatility solvents, adhere to specified drying conditions to maintain sample integrity.
Warning: Easily oxidizable substances like tung, oiticica, or linseed oils require vacuum drying at 60 °C ± 2 °C to prevent degradation. Extended drying may be needed for solvents like cyclohexanone or isophorone.
❓ Frequently Asked Questions
🔍 What is the purpose of this test method?
This test method provides qualitative characterization of paint vehicle solids via infrared spectroscopy, aiding in the identification of unknown or competitive coatings.
💡 How should the film be prepared for analysis?
The vehicle is spread on a NaCl window to form a uniform film. Thickness must ensure the strongest infrared band transmittance is between 5% and 15% for optimal spectra.
⚡ What drying conditions are required for oxidizable substances?
For substances like tung, oiticica, or linseed oils, dry the film at 60 °C ± 2 °C in a vacuum oven for 1 hour to avoid oxidation.
📌 Why is film thickness critical?
Film thickness directly affects transmittance; too thick or too thin can obscure key bands. Proper thickness ensures the strongest band falls within the 5% to 15% transmittance range for accurate identification.