ASTM D2238-92(2004)
(Test Method)Standard Test Methods for Absorbance of Polyethylene Due to Methyl Groups at 1378 cm
Standard Test Methods for Absorbance of Polyethylene Due to Methyl Groups at 1378 cm
SIGNIFICANCE AND USE
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.
SCOPE
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.
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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: D2238 − 92(Reapproved 2004)
Standard Test Methods for
Absorbance of Polyethylene Due to Methyl Groups at 1378
−1
cm
This standard is issued under the fixed designation D2238; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Materials for Wire and Cable
D1505Test Method for Density of Plastics by the Density-
1.1 These test methods cover measurement by infrared
−1 Gradient Technique
absorption spectrophotometry of the 1378.4-cm (7.255-µm)
2 D1898Practice for Sampling of Plastics (Withdrawn 1998)
band in polyethylene due to methyl groups. (1, 2, 4-7) Two
E131Terminology Relating to Molecular Spectroscopy
test methods are covered:
E168Practices for General Techniques of Infrared Quanti-
1.1.1 Test Method A uses compensation with a standard
tative Analysis
sample film or wedge of known methyl content.
E177Practice for Use of the Terms Precision and Bias in
1.1.2 Test Method B uses compensation with a wedge of
ASTM Test Methods
polymethyleneorapolyethyleneofknownlowmethylcontent.
IEEE/ASTM SI-10Standard for Use of the International
1.2 These test methods are applicable to polyethylenes of
System of Units (SI): The Modern Metric System
TypesI(density910to925kg/m ),II(density926to940),and
III (density 941 to 965).
3. Terminology
NOTE 1—For determination of density, see Specifications D1248. 3.1 Definitions of Terms Specific to This Standard:
NOTE 2—In cases of Type III polyethylene with densities greater than
3.1.1 absorbance—the logarithm to the base 10 of the
0.950 g/cm , different results are obtained with the two test methods.
reciprocal of the internal transmittance:
1.3 The values stated in SI units are to be regarded as the
A 5LOG 1/T 52LOG T (1)
~ !
10 10
standard.Thevaluesgiveninbracketsareforinformationonly.
3.2 Units, symbols, and abbreviations used in this test
1.4 This standard does not purport to address all of the
method appear in Terminology E131 or Practice IEEE/ASTM
safety concerns, if any, associated with its use. It is the
SI-10.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
4. Significance and Use
bility of regulatory limitations prior to use. Specific hazards
4.1 When interpreted with the aid of appropriate calibration
statements are given in Section 7.
data,eithertestmethodcanbeusedtocomparethetotalmethyl
NOTE 3—There is no similar or equivalent ISO standard covering the
contents of polyethylenes made by similar processes. Such
subject matter of these test methods.
informationcanbeinterpretedintermsofspecificalkylgroups
with the aid of data on infrared absorption at certain other
2. Referenced Documents
wavelengths (3).
2.1 ASTM Standards:
NOTE 4—The bias of determination of the concentration of total alkyl
D618Practice for Conditioning Plastics for Testing
groups depends on knowing the concentrations of methyl and ethyl
D1248Specification for Polyethylene Plastics Extrusion
branches present, since these branches have anomalously high absorptivi-
−1
ties per group at 1378 cm (7.25 µm).
1 4.2 Knowledge of total methyl groups in polyethylene,
These test methods are under the jurisdiction of ASTM Committee D20 on
Plastics and are the direct responsibility of Subcommittee D20.70 on Analytical
whencombinedwithdataonmolecularweightandonreactive
Methods (Section D20.70.08).
end groups such as vinyl, can lead to assignment of end-group
CurrenteditionapprovedJuly1,2004.PublishedJuly2004.Originallyapproved
structuresandcanshedlightuponpolymerizationmechanisms.
in 1964. Last previous edition approved in 1999 as D2238-92 (1999). DOI:
10.1520/D2238-92R04.
4.3 Data on total methyl groups in polyethylene can be
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
correlatedqualitativelywithcertainpolymerpropertiessuchas
these test methods.
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 last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D2238 − 92 (2004)
melting point, density, stiffness, and other mechanical proper- the manufacturer’s specifications for the new instrument.
ties that are closely dependent on the degree of crystallinity of Resolution should be checked to assure conformance with 6.1.
the polymer. The linearity of the photometric system should be measured;
linearityshouldnotdeviatefromabsolutebymorethan4%of
4.4 These test methods are especially suitable for research.
thetransmittancerangeofinterest.Frequencyorwavelengthin
They have not been tested for use in manufacturing control.
−1
the1430to1250-cm (7to8-µm)regionshouldbecalibrated.
5. Interferences
NOTE 5—For wavelength calibration, it is helpful to record the
spectrum of water vapor upon the spectra of the samples.
5.1 Compensation minimizes interference from methylene
−1 −1
groupabsorptionbandsat1368cm (7.31µm)and1352cm
10. Calibration and Standardization
−1
(7.39 µm) with the 1378-cm (7.255 µm) methyl deformation
10.1 Check the instrument for resolution and wavelength
band.
accuracy by checking against known wavelengths and absor-
5.2 In Test MethodAresidual absorption is often present at
−1
bance for methyl absorbance bands in the 2851.4 cm (3.507
−1
1352 cm after compensation, but this band is believed not to
µm) range.
contribute appreciable interference in the measurement of the
−1
methyl peak at 1378.4 cm in samples with very low methyl
11. Conditioning
content.
11.1 Conditioning—Condition the test specimens at 23 6
2°C[73.4 63.6°F]and50 65%relativehumidityfornotless
6. Apparatus
than 40h prior to test in accordance with Procedure A of
6.1 Infrared Spectrophotometer,doublebeam,withrocksalt
Practice D618, for those tests where conditioning is required.
prism,andspectralresolutionasdefinedbyConditionCinPart
In cases of disagreement, the tolerances shall be 61°C
III (Spectral Resolution) of the Proposed Methods for Evalu-
[61.8°F] and 62% relative humidity.
5,6
ation of Spectrophotometers, or
11.2 Test Conditions—Conduct tests in the standard labora-
6.2 Fourier Transform Instrument, capable of a spectral
tory atmosphere of 23 6 2°C [73.4 6 3.6°F] and 50 65%
−1
resolution of at least 2.0 cm .
relative humidity, unless otherwise specified in the test meth-
6.3 Compression-Molding Press, small, with platens ca-
ods or in this specification. In cases of disagreements, the
pable of being heated to 170°C.
tolerances shall be 61°C [61.8°F] and 62% relative humid-
ity.
6.4 Metal Plates,approximately150by150by0.5mmwith
smooth surfaces.
TEST METHOD A—MEASUREMENT OF THE
−1
6.5 Brass Shims,approximately75by75mmorlargerwith
ABSORBANCE AT 1378 cm (7.25 µm) BY A
an aperture in the center at least 25 by 38 mm in a series of at
STANDARD
least five thicknesses from 0.1 to 0.5 mm.
SAMPLE COMPENSATION METHOD
6.6 Micrometer Calipers,withthimblegraduationsof0.001
12. Materials
mm.
12.1 Aluminum Foil.
6.7 Mounts,forfilmspecimenswithapertureatleast6by27
12.2 Crushed Ice.
mm.
12.3 Reference Wedge or Films, prepared as described in
7. Hazards
13.2.1.
7.1 Caution must be used during molding to handle the hot
13. Calibration and Standardization For Test Method A
platens and molds with appropriate gloves for hand protection.
13.1 Calibration of Reference Polymer by a Self-
8. Sampling
Compensation Method—Mold a 0.5 mm film of annealed
8.1 The polyethylene shall be sampled in accordance with
high-density polyethylene, as well as a series of thinner,
Practice D1898.
shock-cooled films of the same polymer over a range of
thickness from 0.1 to 0.4 mm (Note 6). Measure a series of
9. Preparation of Apparatus
difference spectra, with the annealed film in the sample beam
9.1 The precision obtained using this test method depends
of the spectrophotometer and each shock-cooled film, in turn,
very markedly upon the condition of the spectrophotometer.
inthereferencebeam.FromagraphofabsorptivityoftheCH
−1
Instrumentperformanceshouldbeatleastequaltothatcitedin
bandmaximumatabout1378cm (7.25µm)asafunctionof
−1
absorptivity at 1304 cm (7.67 µm), obtain a corrected value
−1
of absorptivity at 1378 cm (7.25 µm) as well as the slope of
Proceedings, ASTM, Vol 58, 1958, p. 472.
The Perkin-Elmer Model 21 Spectrophotometer, Beckman IR-4 and IR-7, the graph.
Hilger H-800, and Grubb-Parsons GS-2A Spectrophotometers are capable of this
NOTE 6—The polyethylene used for preparation of reference films
degree of resolution and have been found satisfactory for this purpose.
should have very low methyl group content, preferably less than 0.3 for
Hydraulic presses that have been found satisfactory for this purpose are made
each 100 carbon atoms. Essentially linear Type III polyethylene with
by Pasadena Hydraulics, Inc., Pasadena, CA, and Fred S. Carver, Inc., Summit, NJ.
Brown and Sharpe micrometer No. 223 RS has been found satisfactory for this density approximately 0.96 g/cm has been found satisfactory for this
purpose. purpose (Note 1).
D2238 − 92 (2004)
13.2 Procedure: tometertoachievetheresolutionspecifiedin8.1(Note7,Note
−1
13.2.1 From the reference polyethylene, mold three or four 8).Setthespeedat0.1µm/minor17cm /min.Setthegainto
shock-cooledfilmsabout0.5mminthicknessandanumberof produce overshoot of 1 division (1% of full scale) at 1368
−1
films with thicknesses varying from 0.1 to 0.4 mm. The films cm (7.31 µm).Adjust the electrical balance so that there is no
shallbesmoothandfreeofvoids.Theshock-cooledfilmsmay
drift.Adjustthebalancecontrolsothatnopartofthespectrum
−1
be prepared in the following way: Place the desired brass shim between1430and1250cm (7and8µm)hasmorethan90%
on the aluminum foil on top of one of the metal plates. Place
transmittance. Adjust the 0% transmittance. Adjust the ab-
−1
sufficientpolymerintheapertureoftheshimtofillcompletely scissa scale to at least 75 mm/200 cm (1.1 µm).
this aperture after pressing. Cover the preparation with a
−1
NOTE 7—The 1378-cm (7.25-µm) methyl band has a half-width less
secondaluminumfoilandmetalplate.Heatthepressto170°C.
−1
than 6 cm (0.03 µm) and is thus very sensitive to slit width in prism
Insertthemoldassemblybetweenthepressplatens.Preheatfor
instruments. Close control of slit width is essential for precise measure-
15s,thenapplypressureslowlyuntilafter30sthepressurehas
ments.
reached 30 000 lb. Hold the preparation at this pressure for an
NOTE 8—For the Perkin-Elmer Model 21 Spectrophotometer, the
additional 30 s. Release the pressure, grasp the assembly with following settings are generally satisfactory: Response 1 (electrical and
mechanical), Suppression 2, Resolution 3 (960 program), electrical
pliers,andquicklyplungeitintoabucketcontainingaslurryof
balance to give no drift. For the Beckman Model IR-4 or IR-7 Spectro-
iceandwater.Carefullyremovethefilmanddryitwithacloth
photometer, slits 0.40 mm, gain to give single beam/double beam ratio
or tissue.
−1
about 1 to 1368 cm (7.31 µm), Response 2 s.
13.2.2 Anneal several 0.5-mm shock-cooled films by a
−1
13.2.6 Run the spectrum from 1430 to 1250 cm (7 to 8
suitablepressoroventechniquetoobtainanincreaseindensity
µm) (see Fig. 1).
at 23°C of at least 0.020 g/cm .
−1
13.2.7 Return to 1430 cm (7 µm) at full speed without
13.2.3 Mount each film on a suitable holder. Measure the
thickness in millimeters at three places in the aperture and releasing the drum or paper lock. Record a spectrum of water
vapor on the paper below the polyethylene trace.
recordtheaveragethicknessonthesampleholder.Measurethe
densityofsmallclippingsmadeclosetobutnotintheaperture
13.2.8 Repeat 13.2.5, 13.2.6, and 13.2.7 for the remaining
of the holder. Measure the density according to Test Method
shock-cooled films.
D1505.
13.3 Calculations:
13.2.4 Scan the spectrum between 11 and 13 µm and reject
13.3.1 Draw a base line to each curve from 1396 to 1330
any film showing interference fringes.
−1
−1
cm (7.17 to 7.52 µm). Draw a second base line from 1330 to
13.2.5 Measurespectraintherangefrom1430to1250cm
−1
−1
1270 cm (7.52 to 7.87 µm) on each spectrum.
(7 to 8 µm) and record the absorbance of the 1368-cm
(7.31-µm) band on each sample as follows: Place an annealed 13.3.2 Measuretheabsorbanceofthemethylbandat1378.4
−1 −1
sample in the sample beam of the spectrophotometer. Place a cm (7.255 µm). Measure the absorbance at 1304 cm (7.67
shock-cooled film in the reference beam. Set the spectropho- µm).
FIG. 1 Example of Self-Compensation Spectrum of Type III Polyethylene (Method A)
D2238 − 92 (2004)
NOTE 9—Type I polyethylene may show weak bands between 7.3 and
Record R and the corresponding A (due to methyl
8.0 µm which interfere slightly in this measurement.
groups) on each reference film mount.
13.3.3 Calculate the following quantities for each pair of
13.4 Factors:
films:
13.4.1 Factors, f , in methyl groups for each 100 carbon
α 5 A / d t 2 d t (2)
~ !
1378 s s r r
atoms, in grams per square centimeter, are used to convert
absorptivity data to methyl groups for each 100 carbon atoms
β 5 A / d t 2 d t (3)
~ !
1304 s s r r
by the following relationship:
where:
Methylgroups calculatedasmethylinalkylgroups.C 5 (5)
~ !
−1 3
A = absorbance at 1378 cm (7.25 µm),
−1
A = absorbance at 1304 cm (7.67 µm),
f 3K`
1378 ~.C3! 1378
d = density of sample film, g/cm ,
s
d = density of reference film, g/cm ,
13.4.2 Theconversionfactors, f ,mustbedeterminedfor
r
t = thickness of sample film, cm, and
s each spectrophotometer.
t = thickness of reference film, cm.
r
NOTE 12—In the calculations, the Beer-Lambert law is assumed to
−1
13.3.4 Plot the quantity α as ordinate against β as abscissa
apply, that is, the absorptivity at 1378 cm (7.25 µm) is proportional to
on graph paper for each pair of films.
the concentration of methyl groups.
13.3.5 Draw the best straight line through the points on the NOTE 13—The conversion factor, f , has been derived from mea-
surementsofhomopolymersof1-olefins.Inthelaboratoryoforigin,using
graph (see Fig. 2). The intercept on the ordinate is the
a Perkin-Elmer Model 21 Infrared Spectrophotometer, f has been
1378(>C3)
absorptivity, K` , in square centimeters per gram, due to the
determined to be equal to 0.110. In the absence of suitable polyme
...
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