ASTM D6645-01

Standard Test Method for Methyl (Comonomer) Content in Polyethylene by Infrared Spectrophotometry

SIGNIFICANCE AND USE
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 may have a significant effect on the final properties of products made from these resins.
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/m3. High-pressure low-density polyethylenes (LDPE) and terpolymers are excluded.
1.2 The values stated in SI units, based on IEEE/ASTM S1-10, 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 and health practices and determine the applicability of regulatory limitations prior to use.Note 1--There is no similar or equivalent ISO standard.

General Information

• Status: Historical
• Publication Date: 09-Mar-2001
• ICS: 19.100 - Non-destructive testing
• ICS: 83.080.20 - Thermoplastic materials
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.70 - Analytical Methods

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information.
Designation:D6645–01
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 ASTM Test Methods
E932 Practice for Describing and Measuring Performance
1.1 This test method covers the determination of methyl
of Dispersive Infrared Spectrometers
groups (that is, comonomer content) in polyethylenes by
E1421 Practice for Describing and Measuring Performance
infrared spectrophotometry. The test method is applicable to
of Fourier Transform Mid-Infrared (FT-MIR) Spectrom-
copolymers of ethylene with 1-butene, 1-hexene, or 1-octene
eters: Level Zero and Level One Tests
having densities above 900 kg/m . High-pressure low-density
IEEE/ASTM SI-10 Standard for Use of the International
polyethylenes (LDPE) and terpolymers are excluded.
SystemofUnits(SI):TheModernSystem(ReplacesASTM
1.2 The values stated in SI units, based on IEEE/ASTM S1-
E 380 and ANSI/IEEE Standard 268-1992)
10IEEE/ASTM SI-10, are to be regarded as the standard.
1.3 This standard does not purport to address all of the
3. Terminology
safety concerns, if any, associated with its use. It is the
3.1 Terminology—The units, symbols, and abbreviations
responsibility of the user of this standard to establish appro-
used in this test method appear in Terminology E131 or
priate safety and health practices and determine the applica-
IEEE/ASTM SI-10.
bility of regulatory limitations prior to use.
3.2 comonomer—a-olefin monomer. In this test method,
NOTE 1—There is no similar or equivalent ISO standard.
comonomer refers to 1-butene, 1-hexene, and 1-octene only.
2. Referenced Documents
4. Summary of Test Method
-1 -1
2.1 ASTM Standards:
4.1 The band located between 1377 cm and 1379 cm is
D792 Test Methods for Density and Specific Gravity (Rela-
due to a deformation vibration of the –CH group. Bands at
-1
tive Density) of Plastics by Displacement
approximately772cm (branchmethylenerockingmode),895
D1505 Test Method for Density of Plastics by the Density- -1 -1
cm (methyl rocking mode), and 785 cm (branch methylene
Gradient Technique
rocking mode) are characteristic of ethyl (that is, butene
D1898 Practice for Sampling of Plastics
copolymer), butyl (that is, hexene copolymer), and hexyl (that
D2238 Test Methods for Absorbance of Polyethylene Due 3
is, octene copolymer) branches, respectively.
Ø1
to Methyl Groups at 1378 cm
4.2 This test method determines the methyl (that is,
D3124 Test Method for Vinylidene Unsaturation in Poly-
comonomer) content of a polyethylene copolymer based on the
ethylene by Infrared Spectrophotometry -1
IR absorbance at 1378 cm from a pressed plaque. The
D5576 Practice for Determination of Structural Features in
comonomer type has to be known and a calibration curve has
Polyolefins and Polyolefin Copolymers by Infrared Spec-
to be available prior to the analysis. If the comonomer is not
-1 -1
trophotometry (FT-IR)
known a priori, the presence of bands at 772 cm , 895 cm ,
-1
E131 Terminology Relating to Molecular Spectroscopy
and 785 cm can be used to identify ethyl (minimum of 1
E168 Practices for General Techniques of Infrared Quanti-
branchper1000carbons),butyl(minimumofabout5branches
tative Analysis
per 1000 carbons), and hexyl (minimum of about 5 branches
E177 Practice for Use of the Terms Precision and Bias in
per 1000 carbons) branches, respectively.Amore sensitive and
less ambiguous identification is obtained by C13 NMR spec-
troscopy. The latter technique is also used as a reference
This test method is under the jurisdiction ofASTM Committee D20 on Plastics
technique to provide polymer standards for the generation of
and is the direct responsibility of Subcommittee D20.70 on Analytical Methods.
Current edition approved March 10, 2001. Published June 2001. DOI: 10.1520/ calibration curves.
D6645-01.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Blitz, J. P., and McFadden, D. C., “The Characterization of Short Chain
Standards volume information, refer to the standard’s Document Summary page on Branching in Polyethylene Using Fourier Transform Infrared Spectroscopy,” J.
the ASTM website. Appl. Pol. Sci., 51, 13 (1994).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6645–01
NOTE 2—For comonomer identification, it is recommended, for maxi-
7.2 Compression Molding Press, with platens capable of
mum sensitivity, to view the second derivative of the IR spectrum.
being heated to 180°C.
7.3 Two Metal Plates, 150 by 150 mm or larger, of 0.5-mm
4.3 The method is calibrated by plotting absorbance at 1378
-1
thickness with smooth surfaces.
cm per unit area of the methylene combination band at 2019
-1
7.4 Brass Shims, approximately 75 by 75 mm, of 0.3 mm
cm (thatis,internalthicknesscorrectionapproach)orperunit
thickness with an aperture in the center at least 25 by 38 mm.
of spectral cross-section (that is, the reciprocal of the product
7.5 Micrometer (optional), with thimble graduations of
ofplaquethicknessanddensity)versusnumberofbranchesper
0.001 mm.
1000 carbons as determined by C13 NMR spectroscopy.
7.6 Film Mounts,withaperturesatleast6by27mm,tohold
Although both approaches give equivalent results, the one
the specimens in the infrared spectrophotometer.
using internal thickness correction is recommended in this test
method since it is considerably simpler to execute.
8. Materials
5. Significance and Use
8.1 Polyethylene Terephthalate, Aluminum Foil or Matte
5.1 This method determines the number of branches (that is, Finished Teflon-Fibreglass Sheets.
comonomer content) in copolymers of ethylene with 1-butene,
9. Hazards
1-hexene or 1-octene. This information can be correlated with
physicalpropertiessuchasmeltingpoint,density,andstiffness,
9.1 Caution must be used during plaque preparation to
all of which depend on the degree of crystallinity of the
handle the hot platens with appropriate gloves for hand
polymer. Differences in the comonomer content thus may have
protection.
a significant effect on the final properties of products made
from these resins. 10. Procedure
10.1 Preparation of Polymer Plaque:
6. Interferences
10.1.1 Preheat the press to about 50°C above the melting
6.1 A conformational CH wagging absorbance at 1368
2 point of the polymer.
-1 -1
cm overlapsthemethylabsorbanceat1378cm ,butdoesnot
10.1.2 Place a 0.3-mm thick brass shim on the sheet
cause significant interference in this test method since its
material chosen (see 8.1) which in turn covers a metal plate.
intensity is not significantly affected by the comonomer con-
NOTE 3—When using aluminum foil, place the dull side next to the
tent, but rather by the plaque thickness. The result of not
polymer to give the sample film some texture, thereby reducing fringe
correcting for this overlap is a positive ordinate intercept for
effects in the infrared spectrum.
the calibration curve (see 10.4). Another conformational CH
...