ASTM D2238-92(2004)

When interpreted with the aid of appropriate calibration data, either test method can be used to compare the total methyl contents of polyethylenes made by similar processes. Such information can be interpreted in terms of specific alkyl groups with the aid of data on infrared absorption at certain other wavelengths (3).

Note 4—The bias of determination of the concentration of total alkyl groups depends on knowing the concentrations of methyl and ethyl branches present, since these branches have anomalously high absorptivities per group at 1378 cm⁻¹ (7.25 μm).

Knowledge of total methyl groups in polyethylene, when combined with data on molecular weight and on reactive end groups such as vinyl, can lead to assignment of end-group structures and can shed light upon polymerization mechanisms.

Data on total methyl groups in polyethylene can be correlated qualitatively with certain polymer properties such as melting point, density, stiffness, and other mechanical properties that are closely dependent on the degree of crystallinity of the polymer.

These test methods are especially suitable for research. They have not been tested for use in manufacturing control.

1.1 These test methods cover measurement by infrared absorption spectrophotometry of the 1378.4-cm¹ (7.255-m) band in polyethylene due to methyl groups. (1, 2, 4-7) Two test methods are covered:

1.1.1 Test Method A uses compensation with a standard sample film or wedge of known methyl content.

1.1.2 Test Method B uses compensation with a wedge of polymethylene or a polyethylene of known low methyl content.

1.2 These test methods are applicable to polyethylenes of Types I (density 910 to 925 kg/m³), II (density 926 to 940), and III (density 941 to 965).

Note 1—For determination of density, see Specifications D 1248.

Note 2—In cases of Type III polyethylene with densities greater than 0.950 g/cm³, different results are obtained with the two test methods.

1.3 The values stated in SI units are to be regarded as the standard. The values given in brackets are for information only.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific hazards statements are given in Section 7.

Note 3—There is no similar or equivalent ISO standard covering the subject matter of these test methods.