ASTM D2238-92(1999)
(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
SCOPE
1.1 These test methods cover measurement by infrared absorption spectrophotometry of the 1378.4-cm -1 (7.255[mu]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/m ), II (density 926 to 940), and III (density 941 to 965). Note 1-For determination of density, see Specifications D1248. Note 2-In cases of Type III polyethylene with densities greater than 0.950 g/cm , 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 parentheses are for information only.
1.4 This standard does not purport to address all of the safety problems, 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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Designation: D 2238 – 92 (Reapproved 1999)
Standard Test Methods for
Absorbance of Polyethylene Due to Methyl Groups at 1378
−1
cm
This standard is issued under the fixed designation D 2238; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope D 1248 Specification for Polyethylene Plastics Molding and
Extrusion Materials
1.1 These test methods cover measurement by infrared
−1
D 1505 Test Method for Density of Plastics by the Density-
absorption spectrophotometry of the 1378.4-cm (7.255-μm)
2 Gradient Technique
band in polyethylene due to methyl groups. (1, 2, 4-7) Two
D 1898 Practice for Sampling of Plastics
test methods are covered:
E 131 Terminology Relating to Molecular Spectroscopy
1.1.1 Test Method A uses compensation with a standard
E 168 Practices for General Techniques of Infrared Quanti-
sample film or wedge of known methyl content.
tative Analysis
1.1.2 Test Method B uses compensation with a wedge of
E 177 Practice for Use of the Terms Precision and Bias in
polymethylene or a polyethylene of known low methyl content.
ASTM Test Methods
1.2 These test methods are applicable to polyethylenes of
E 380 Practice for Use of the International System of Units
Types I (density 910 to 925 kg/m ), II (density 926 to 940), and
(SI)
III (density 941 to 965).
NOTE 1—For determination of density, see Specifications D 1248. 3. Terminology
NOTE 2—In cases of Type III polyethylene with densities greater than
3.1 Definitions of Terms Specific to This Standard:
0.950 g/cm , different results are obtained with the two test methods.
3.1.1 absorbance—the logarithm to the base 10 of the
1.3 The values stated in SI units are to be regarded as the
reciprocal of the internal transmittance:
standard. The values given in parentheses are for information
A 5 LOG ~1/T!52LOG T (1)
10 10
only.
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 E 131 or Practice E 380.
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
4. Significance and Use
priate safety and health practices and determine the applica-
4.1 When interpreted with the aid of appropriate calibration
bility of regulatory limitations prior to use. Specific hazards
data, either test method can be used to compare the total methyl
statements are given in Section 7.
contents of polyethylenes made by similar processes. Such
NOTE 3—There is no similar or equivalent ISO standard covering the
information can be interpreted in terms of specific alkyl groups
subject matter of these test methods.
with the aid of data on infrared absorption at certain other
wavelengths (3).
2. Referenced Documents
2.1 ASTM Standards: NOTE 4—The bias of determination of the concentration of total alkyl
groups depends on knowing the concentrations of methyl and ethyl
D 618 Practice for Conditioning Plastics and Electrical
branches present, since these branches have anomalously high absorptivi-
Insulating Materials for Testing
−1
ties per group at 1378 cm (7.25 μm).
4.2 Knowledge of total methyl groups in polyethylene,
These test methods are under the jurisdiction of ASTM Committee D-20 on
when combined with data on molecular weight and on reactive
Plastics and are the direct responsibility of Subcommittee D20.70 on Analytical
end groups such as vinyl, can lead to assignment of end-group
Methods (Section D20.70.08).
structures and can shed light upon polymerization mechanisms.
Current edition approved Dec. 15, 1992. Published February 1993. Originally
published as D 2238 – 64 T. Last previous edition D 2238 – 68 (1986).
The boldface numbers in parentheses refer to the list of references at the end of
these test methods. Annual Book of ASTM Standards, Vol 14.01.
3 5
Annual Book of ASTM Standards, Vol 08.01. Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 2238
4.3 Data on total methyl groups in polyethylene can be the manufacturer’s specifications for the new instrument.
correlated qualitatively with certain polymer properties such as Resolution should be checked to assure conformance with 6.1.
melting point, density, stiffness, and other mechanical proper- The linearity of the photometric system should be measured;
ties that are closely dependent on the degree of crystallinity of linearity should not deviate from absolute by more than 4 % of
the polymer. the transmittance range of interest. Frequency or wavelength in
−1
4.4 These test methods are especially suitable for research. the 1430 to 1250-cm (7 to 8-μm) region should be calibrated.
They have not been tested for use in manufacturing control.
NOTE 5—For wavelength calibration, it is helpful to record the spec-
trum of water vapor upon the spectra of the samples.
5. Interferences
10. Calibration and Standardization
5.1 Compensation minimizes interference from methylene
−1 −1
group absorption bands at 1368 cm (7.31 μm) and 1352 cm
10.1 Check the instrument for resolution and wavelength
−1
(7.39 μm) with the 1378-cm (7.255 μm) methyl deformation
accuracy by checking against known wavelengths and absor-
−1
band.
bance for methyl absorbance bands in the 2851.4 cm (3.507
5.2 In Test Method A residual absorption is often present at
μm) range.
−1
1352 cm after compensation, but this band is believed not to
contribute appreciable interference in the measurement of the
11. Conditioning
−1
methyl peak at 1378.4 cm in samples with very low methyl
11.1 Conditioning—Condition the test specimens at 23 6
content.
2°C (73.4 6 3.6°F) and 50 6 5 % relative humidity for not less
than 40 h prior to test in accordance with Procedure A of
6. Apparatus
Practice D 618, for those tests where conditioning is required.
6.1 Infrared Spectrophotometer, double beam, with rock
In cases of disagreement, the tolerances shall be 61°C
salt prism, and spectral resolution as defined by Condition C in
(61.8°F) and 62 % relative humidity.
Part III (Spectral Resolution) of the Proposed Methods for
11.2 Test Conditions—Conduct tests in the standard labora-
,
6 7
Evaluation of Spectrophotometers, or
tory atmosphere of 23 6 2°C (73.4 6 3.6°F) and 50 6 5%
6.2 Fourier Transform Instrument, capable of a spectral
relative humidity, unless otherwise specified in the test meth-
−1
resolution of at least 2.0 cm .
ods or in this specification. In cases of disagreements, the
6.3 Compression-Molding Press, small, with platens ca-
tolerances shall be 61°C (61.8°F) and 62 % relative humid-
pable of being heated to 170°C.
ity.
6.4 Metal Plates, approximately 150 by 150 by 0.5 mm with
smooth surfaces.
TEST METHOD A—MEASUREMENT OF THE
−1
6.5 Brass Shims, approximately 75 by 75 mm or larger with
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, with thimble graduations of 0.001
mm.
12. Materials
6.7 Mounts, for film specimens with aperture at least 6 by 27
12.1 Aluminum Foil.
mm.
12.2 Crushed Ice.
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
platens and molds with appropriate gloves for hand protection.
13. Calibration and Standardization For Test Method A
8. Sampling 13.1 Calibration of Reference Polymer by a Self-
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 D 1898.
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.
in the reference beam. From a graph of absorptivity of the CH
Instrument performance should be at least equal to that cited in
−1
band maximum at about 1378 cm (7.25 μm) as a function of
−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 graph.
The Perkin-Elmer Model 21 Spectrophotometer, Beckman IR-4 and IR-7,
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.
8 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.
density approximately 0.96 g/cm has been found satisfactory for this
Brown and Sharpe micrometer No. 223 RS has been found satisfactory for this
purpose. purpose (Note 1).
D 2238
13.2 Procedure: tometer to achieve the resolution specified in 8.1 (Note 7, Note
−1
13.2.1 From the reference polyethylene, mold three or four 8). Set the speed at 0.1 μm/min or 17 cm /min. Set the gain to
shock-cooled films about 0.5 mm in thickness and a number of produce overshoot of 1 division (1 % of full scale) at 1368 cm
films with thicknesses varying from 0.1 to 0.4 mm. The films −1 (7.31 μm). Adjust the electrical balance so that there is no
shall be smooth and free of voids. The shock-cooled films may drift. Adjust the balance control so that no part of the spectrum
−1
be prepared in the following way: Place the desired brass shim between 1430 and 1250 cm (7 and 8 μm) has more than 90%
on the aluminum foil on top of one of the metal plates. Place transmittance. Adjust the 0 % transmittance. Adjust the ab-
−1
sufficient polymer in the aperture of the shim to fill completely 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
second aluminum foil and metal plate. Heat the press to 170°C. −1
than 6 cm (0.03 μm) and is thus very sensitive to slit width in prism
Insert the mold assembly between the press platens. Preheat for
instruments. Close control of slit width is essential for precise measure-
15 s, then apply pressure slowly until after 30 s the pressure has ments.
NOTE 8—For the Perkin-Elmer Model 21 Spectrophotometer, the fol-
reached 30 000 lb. Hold the preparation at this pressure for an
lowing settings are generally satisfactory: Response 1 (electrical and
additional 30 s. Release the pressure, grasp the assembly with
mechanical), Suppression 2, Resolution 3 (960 program), electrical
pliers, and quickly plunge it into a bucket containing a slurry of
balance to give no drift. For the Beckman Model IR-4 or IR-7 Spectro-
ice and water. Carefully remove the film and dry it with a cloth
photometer, slits 0.40 mm, gain to give single beam/double beam ratio
−1
or tissue.
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
suitable press or oven technique to obtain an increase in density
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
releasing the drum or paper lock. Record a spectrum of water
thickness in millimeters at three places in the aperture and
vapor on the paper below the polyethylene trace.
record the average thickness on the sample holder. Measure the
13.2.8 Repeat 13.2.5, 13.2.6, and 13.2.7 for the remaining
density of small clippings made close to but not in the aperture
shock-cooled films.
of the holder. Measure the density according to Test Method
13.3 Calculations:
D 1505.
13.3.1 Draw a base line to each curve from 1396 to 1330
−1
13.2.4 Scan the spectrum between 11 and 13 μm and reject
cm (7.17 to 7.52 μm). Draw a second base line from 1330 to
any film showing interference fringes.
−1
−1 1270 cm (7.52 to 7.87 μm) on each spectrum.
13.2.5 Measure spectra in the range from 1430 to 1250 cm
13.3.2 Measure the absorbance of the methyl band at 1378.4
−1
(7 to 8 μm) and record the absorbance of the 1368-cm
−1 −1
cm (7.255 μm). Measure the absorbance at 1304 cm (7.67
(7.31-μm) band on each sample as follows: Place an annealed
μm).
sample in the sample beam of the spectrophotometer. Place a
shock-cooled film in the reference beam. Set the spectropho- NOTE 9—Type I polyethylene may show weak bands between 7.3 and
FIG. 1 Example of Self-Compensation Spectrum of Type III Polyethylene (Method A)
D 2238
8.0 μm which interfere slightly in this measurement.
Record R and the corresponding A (due to methyl
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
a5 A /~d t 2 d t ! (2)
1378 s s r r
atoms, in grams per square centimeter, are used to convert
b5 A /~d t 2 d t ! (3)
absorptivity data to methyl groups for each 100 carbon atoms
1304 s s r r
by the following relationship:
where:
−1
Methyl groups ~calculated as methyl in alkyl groups . C ! 5
A = absorbance at 1378 cm (7.25 μm),
−1
f 3 K8 (5)
1378 ~.C3! 1378
A = absorbance at 1304 cm (7.67 μm),
d = density of sample film, g/cm ,
s 13.4.2 The conversion factors, f , must be determined for
d = density of reference film, g/cm ,
r each spectrophotometer.
t = thickness of sample film, cm, and
s
NOTE 12—In the calculations, the Beer-Lambert law is assumed to
t = thickness of reference film, cm.
r
−1
apply, that is, the absorptivity at 1378 cm (7.25 μm) is proportional to
13.3.4 Plot the quantity a as ordinate against b as abscissa
the concentration of methyl groups.
on graph paper for each pair of films.
NOTE 13—The conversion factor, f , has been derived from mea-
13.3.5 Draw the best straight line through the points on the
surements of homopolymers of 1-olefins. In the laboratory of origin, using
graph (see Fig. 2). The intercept on the ordinate is the
a Perkin-Elmer Model 21 Infrared
...
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