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D5670-95 – Standard Test Method Technical Guide
📐 Specimen Preparation and Standards
ASTM D5670-95 (Reapproved 2023) establishes a standardized test method for determining the percentage of residual unsaturation—specifically the double bonds in unhydrogenated butadiene—in hydrogenated nitrile rubber (HNBR) using infrared spectrophotometry. As stated in the scope (Section 1.2), this method is applicable to all grades of raw hydrogenated nitrile rubber. The test relies on the infrared examination of rubber films cast from solution, requiring experienced personnel for accurate preparation and analysis.
The test specimen undergoes a rigorous two-part purification process described in Section 3.1. First, the raw rubber is purified by precipitation with methanol from a solution in methyl ethyl ketone (MEK), or by Soxhlet extraction of the solid rubber with methanol. This critical step removes additives that could interfere with the infrared spectrum. The purified rubber is then re-dissolved in MEK and cast as a thin, uniform film onto a potassium bromide (KBr) disc for spectral analysis.
⚠️ Safety Note: Per Section 1.6, this standard does not purport to address all safety concerns. It is the responsibility of the user to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Personnel should exercise caution when handling solvents like MEK and methanol, as well as preparing KBr discs.
| 🟦 Element | 📏 Specification | 📐 Purpose |
|---|---|---|
| Standard Designation | D5670 – 95 (Reapproved 2023) | Defines the method for residual unsaturation in HNBR |
| Unit of Frequency | Wavenumbers (cm⁻¹) | Standard unit in infrared spectrophotometry (Section 1.4) |
| ASTM E932 | Dispersive IR Spectrometers | Describes performance testing for applicable equipment |
| ASTM E1421 | FT-MIR Spectrometers | Covers Level Zero and Level One performance tests |
⚙️ Test Procedure and Equipment Selection
The core of the test method involves obtaining the infrared spectrum of the purified cast film. Section 1.3 emphasizes that the equipment must be operated according to the manufacturer’s directions for optimum performance, and that no detailed operating instructions for the spectrophotometers themselves are included in this standard. Performance verification relies on Practices E932 for dispersive instruments and E1421 for Fourier Transform Infrared (FT-MIR) spectrophotometers.
The analyst identifies and measures the corrected absorbance of specific absorption bands using the baseline method. The three key bands are attributed to Acrylonitrile (ACN), Butadiene (BD), and Hydrogenated Butadiene (HBD). According to Section 3.4, the percentage of residual unsaturation is then calculated with the aid of established absorbance factors from the referenced literature.
| 🎯 Component | ⚡ Measured Parameter | 📐 Significance in Calculation |
|---|---|---|
| Acrylonitrile (ACN) | Corrected Absorbance | Provides a compositional reference for the base polymer |
| Butadiene (BD) | Corrected Absorbance (Unhydrogenated) | Directly measures the residual double bonds |
| Hydrogenated Butadiene (HBD) | Corrected Absorbance | Accounts for the saturated portion of the butadiene units |
| Residual Unsaturation | Calculated Percentage (%) | Quantifies the remaining double bonds post-hydrogenation |
The significance of this test is highlighted in Section 4.1, which notes that HNBR rubbers are available at different unsaturation levels. Highly saturated grades offer optimum resistance to aging, making the accurate quantification of residual double bonds critical for material quality control and application suitability.
💡 Tip for Analysts: The baseline method is highly sensitive to the quality of the cast film. Ensure the film is free of pinholes and has consistent thickness. Proper baseline selection across the ACN, BD, and HBD absorption bands is essential for obtaining reliable corrected absorbance values and accurate residual unsaturation calculations.
📊 Key Measured Properties and Calculations
The fundamental output of ASTM D5670 is the percentage of residual unsaturation. This value is derived from the corrected absorbances of the ACN, BD, and HBD spectral bands, compensating for any overlap using factors from the established literature. This single calculated number defines the saturation level of the HNBR, directly correlating to the material’s thermal and chemical resistance properties as outlined in the Significance and Use section.
❓ Frequently Asked Questions
🔍 What is the primary scope of ASTM D5670?
The scope covers the determination of the percentage of residual unsaturation (double bonds in unhydrogenated butadiene) in hydrogenated nitrile rubber (HNBR) using infrared examination of films cast from solution.
💡 Why is the rubber specimen purified before the analysis?
Purity is essential to avoid interference with the infrared spectrum. The standard specifies purification via precipitation from MEK solution with methanol or by Soxhlet extraction with methanol to remove additives from the raw rubber.
⚡ What types of infrared spectrophotometers are permitted by this test method?
Both Fourier Transform Infrared Spectrophotometers (FT-IR) and Dispersive Infrared Spectrophotometers are permitted. Their performance must conform to Practices E1421 and E932, respectively.
📌 What frequency unit is used instead of Hertz in this standard?
As stated in Section 1.4, it is customary in infrared spectrophotometry to use wavenumbers (cm⁻¹) rather than Hertz (Hz), and this unit is employed throughout the test method.