ASTM D6645-18

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Designation: D6645 − 01 (Reapproved 2010) D6645 − 18
Standard Test Method for
Methyl (Comonomer) Content in Polyethylene by Infrared
Spectrophotometry
This standard is issued under the fixed designation D6645; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 This test method covers the determination of methyl groups (that is, comonomer content) in polyethylenes by infrared
spectrophotometry. The test method is applicable to copolymers of ethylene with 1-butene, 1-hexene, or 1-octene having densities
above 900 kg/m . High-pressure low-density polyethylenes (LDPE) and terpolymers are excluded.
1.2 The values stated in SI units, based on IEEE/ASTM SI-10, are to be regarded as the standard.
1.3 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
NOTE 1—There is no known ISO equivalent to this standard.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D792 Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
D1505 Test Method for Density of Plastics by the Density-Gradient Technique
D1898 Practice for Sampling of Plastics (Withdrawn 1998)
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D2238 Test Methods for Absorbance of Polyethylene Due to Methyl Groups at 1378 cm
D3124 Test Method for Vinylidene Unsaturation in Polyethylene by Infrared Spectrophotometry
D5576 Practice for Determination of Structural Features in Polyolefins and Polyolefin Copolymers by Infrared Spectrophotom-
etry (FT-IR)
E131 Terminology Relating to Molecular Spectroscopy
E168 Practices for General Techniques of Infrared Quantitative Analysis
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E932 Practice for Describing and Measuring Performance of Dispersive Infrared Spectrometers
E1421 Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FT-MIR) Spectrometers: Level
Zero and Level One Tests
IEEE/ASTM SI-10 Standard for Use of the International System of Units (SI): The Modern System
3. Terminology
3.1 Terminology—The units, symbols, and abbreviations used in this test method appear in Terminology E131 or IEEE/
ASTM SI-10.
3.2 comonomer—α-olefin monomer. In this test method, comonomer refers to 1-butene, 1-hexene, andor 1-octene only.
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.70 on Analytical Methods.
Current edition approved Jan. 1, 2010Oct. 1, 2018. Published January 2010October 2018. Originally approved in 2001. Last previous edition approved in 20012010 as
D6645 - 01.D6645 - 01(2010). DOI: 10.1520/D6645-01R10.10.1520/D6645-18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
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D6645 − 18
4. Summary of Test Method
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4.1 The band located between 1377 cm and 1379 cm is due to a deformation vibration of the –CH group. Bands at
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approximately 772 cm (branch methylene rocking mode), 895 cm (methyl rocking mode), and 785 cm (branch methylene
rocking mode) are characteristic of an ethyl branch (that is, butene copolymer), a butyl branch (that is, hexene copolymer), and
a hexyl branch (that is, octene copolymer) branches, copolymer), respectively.
4.2 This test method determines the methyl group (that is, comonomer) content of a polyethylene copolymer based on the IR
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absorbance at 1378 cm from a pressed plaque. The comonomer type has to be known and a calibration curve has to be available
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prior to the analysis. If the comonomer is not known a priori, the presence of bands at 772 cm , 895 cm , and 785 cm can be
used to identify ethyl (minimum of 1 branch per 1000 carbons), butyl (minimum of about 5 branches per 1000 carbons), and hexyl
(minimum of about 5 branches per 1000 carbons) branches, respectively. A more sensitive and less ambiguous identification is
obtained by C13 NMR spectroscopy. The latter technique is also used as a reference technique to provide polymer standards for
the generation of IR calibration curves.
NOTE 2—For comonomer identification,maximum sensitivity, it is recommended, for maximum sensitivity, recommended to view the second derivative
of the IR spectrum.spectrum to identify the comonomer type.
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4.3 The method is calibrated by plotting absorbance at 1378 cm per unit area of the methylene combination band at 2019 cm
(that is, internal thickness correction approach) or per unit of spectral cross-section (that is, the reciprocal of the product of plaque
thickness and density) versus number of branches per 1000 carbons as determined by C13 NMR spectroscopy. Although both
approaches give equivalent results, the one using internal thickness correction is recommended in this test method since it is
considerably simpler to execute.
5. Significance and Use
5.1 This method determines the number of branches (that is, comonomer content) in copolymers of ethylene with 1-butene,
1-hexene or 1-octene. This information can be correlated with physical properties such as melting point, density, and stiffness, all
of which depend on the degree of crystallinity of the polymer. Differences in the comonomer content thus maycan have a
significant effect on the final properties of products made from these resins.
6. Interferences
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6.1 A conformational CH wagging absorbance at 1368 cm overlaps the methyl absorbance at 1378 cm , but does not cause
significant interference in this test method since its intensity is not significantly affected by the comonomer content, but rather by
the plaque thickness. The result of not correcting for this overlap is a positive ordinate intercept for the calibration curve (see 10.4).
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Another conformational CH wagging absorbance at 1352 cm does not significantly overlap the 1378 cm absorbance.
6.2 The presence of most pigments will interfere with this method.
6.3 The presence of low molecular weight hydrocarbons will produce high results in this method due to absorbance by their end
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methyl groups at 1378 cm .
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6.4 The secondary antioxidant Irgafos 168 shows an absorbance at 768 cm which interferes with the identification of low
levels (that is, typically less than 5 branches per 1000 carbons or less) of ethyl branches.
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6.5 Vinylidene groups absorb at 888 cm and thus maycan interfere with a conclusive identification of a hexene copolymer
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from its 895 cm resonance, depending on the relative intensities of the two peaks.
7. Apparatus
7.1 Infrared Spectrophotometer, either double beam or a Fourier transform (FTIR).
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7.1.1 Dispersive Infrared Spectrophotometer, capable of achieving a spectral bandwidth of 4 cm (see Practice E932). The
instrument shouldmust be capable of scale expansion along the wavenumber axis.
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7.1.2 Fourier Transform Infrared Spectrometer, capable of 4 cm resolution (see Practice E1421). The instrument shouldmus
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