ASTM D2238 Standard Test Methods for Absorbance of Polyethylene Due to Methyl Groups at 1378 cm−1

Get Certified ASTM D2238 Testing for Reliable Polyethylene Characterization

Polyethylene structure and branching can influence important material properties. Infrared measurements of the methyl-group band at 1378 cm⁻¹ provide information that researchers can use to compare polyethylene materials produced by similar processes. Controlled compensation and calibration procedures help laboratories obtain comparable infrared measurements. ASTM D2238 testing can support polymer research, material characterization, and comparative studies. When combined with other molecular and material data, the results can help researchers investigate polyethylene structure and qualitative relationships between methyl content and selected material properties.

• Measures the infrared absorption band near 1378 cm⁻¹ associated with methyl groups in polyethylene.
• Uses a standard sample film with known methyl content for Test Method A.
• Uses a wedge of polymethylene or polyethylene with known low methyl content for Test Method B.
• Applies to Type I polyethylene with densities from 0.910 to 0.925 g/cm³.
• Applies to Type II polyethylene with densities from 0.926 to 0.940 g/cm³.
• Applies to Type III polyethylene with densities from 0.941 to 0.965 g/cm³.
• Allows comparison of total methyl content in polyethylene materials made by similar processes when appropriate calibration data are used.
• Supports research-oriented characterization of polyethylene.
• Notes that Test Methods A and B can produce different results for Type III polyethylene with densities greater than 0.950 g/cm³.

What are the Uses of ASTM D2238 Testing?

ASTM D2238 testing helps laboratories compare the total methyl content of polyethylene materials produced by similar processes. Researchers can also combine the results with other molecular and material information to investigate polyethylene structure and selected properties. This standard-

• Allows comparison of total methyl content in suitable polyethylene materials.
• Provides information that can contribute to studies of polymerization mechanisms.
• Supports interpretation of end-group structures when combined with molecular-weight and reactive-end-group data.
• Supports qualitative evaluation of relationships between methyl-group content and crystallinity-related properties.
• Allows qualitative comparisons with melting point, density, stiffness, and certain mechanical properties.
• Supports research and development of polyethylene materials.
• Provides infrared characterization data for comparative laboratory studies.

Which Materials Can Be Tested Under ASTM D2238?

ASTM D2238 applies to polyethylene Types I, II, and III within specified density ranges. Type I covers polyethylene with densities from 0.910 to 0.925 g/cm³. Type II covers densities from 0.926 to 0.940 g/cm³. Type III covers densities from 0.941 to 0.965 g/cm³. Laboratories should identify the polyethylene type and density range before selecting and applying the appropriate test method. For Type III polyethylene with a density greater than 0.950 g/cm³, ASTM notes that the two test methods can produce different results.

Why is ASTM D2238 Important?

ASTM D2238 provides a standardized infrared approach for evaluating the methyl-group absorption band in polyethylene. When laboratories use appropriate calibration data, the measurements can help compare the total methyl content of polyethylene materials made by similar processes. Methyl-group information can also help interpret polyethylene molecular structure when researchers combine it with molecular-weight and reactive-end-group data. ASTM further identifies qualitative relationships between total methyl-group data and properties such as melting point, density, stiffness, and other mechanical properties associated with crystallinity.

ASTM D2238 Testing Process and Data Collection

The analyst first identifies the polyethylene material and selects the appropriate ASTM D2238 test method. Technicians then prepare and condition the polyethylene films as required and verify the infrared instrument before testing. For Test Method A, technicians use reference films with known methyl content and apply the specified compensation procedure using measurements near the 1378 cm⁻¹ methyl band and 1304 cm⁻¹ region, along with film thickness and density information, to establish the calibration relationship. For Test Method B, technicians use a compensation wedge made of polymethylene or polyethylene with known low methyl content and evaluate the corrected infrared measurements according to the specified calibration approach. Laboratories should also consider ASTM’s warning for Type III polyethylene with densities greater than 0.950 g/cm³ because the two test methods may produce different results.

Common Challenges and Troubleshooting

The spectrophotometer’s condition and performance, wavelength or frequency calibration, spectral resolution, and specimen quality all affect the accuracy of ASTM D2238 measurements. Typical problems are interference from the 1378 cm-1 methyl band, instrument performance and wavelength accuracy, film thickness variation, and specimen quality. Engineers should ensure that the instrument is accurate, calibrate the frequency or wavelength, accurately measure the film thickness, and make a smooth film with good interference fringes. Standardization of specimen preparation, conditioning, instrument verification, and compensation procedures assist the laboratory in minimizing the variability in measurement.

• The analyst records the infrared absorption band near 1378 cm⁻¹, which is associated with methyl-group absorption in polyethylene.
• The laboratory reports methyl-group content as a concentration or calculated value based on the applicable calibration procedure, while the measured absorption is expressed as an absorbance at 1378 cm⁻¹.
• The analyst considers the effects of nearby methylene absorption bands, particularly those near 1368 cm⁻¹ and 1352 cm⁻¹, and applies the specified compensation procedure when required.
• Engineers compare results only among applicable polyethylene materials and density ranges, and consider differences between Test Methods A and B when testing Type III polyethylene with densities above 0.950 g/cm³.
• Engineers can use the methyl-group results, along with density, melting point, stiffness, mechanical properties, molecular weight, and reactive end-group information, to support interpretation of polyethylene structure and crystallinity.

FAQs

Why is ASTM D2238 important?

ASTM D2238 provides infrared absorption spectrophotometric test methods for evaluating the 1378 cm⁻¹ methyl-group band in polyethylene. With appropriate calibration data, the methods can be used to compare total methyl content in polyethylene materials made by similar processes.

What is ASTM D2238 used for?

ASTM D2238 is used to measure the infrared absorption associated with methyl groups in polyethylene. The results can support research into polyethylene structure, polymerization mechanisms, and qualitative relationships between methyl content and selected material properties.

What is measured during ASTM D2238 testing?

The test methods measure the infrared absorption of the 1378 cm⁻¹ band in polyethylene that is associated with methyl groups. Compensation techniques address interference from nearby methylene absorption bands.

What types of polyethylene can be tested under ASTM D2238?

ASTM D2238 applies to Type I, Type II, and Type III polyethylene within specified density ranges. Type I covers 0.910 to 0.925 g/cm³, Type II covers 0.926 to 0.940 g/cm³, and Type III covers 0.941 to 0.965 g/cm³.

What instruments are used for ASTM D2238?

ASTM D2238 permits a double-beam infrared spectrophotometer with a NaCl prism and the specified spectral resolution or a Fourier Transform instrument capable of a spectral resolution of at least 2.0 cm⁻¹. The test also requires equipment for compression molding, film-thickness measurement, and specimen mounting.

Can ASTM D2238 be used for research samples?

Yes. ASTM states that these test methods are especially suitable for research. However, the standard also states that the methods have not been tested for manufacturing control.

Link to ASTM D2238