ASTM D6474-20

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Designation: D6474 − 19 D6474 − 20
Standard Test Method for
Determining Molecular Weight Distribution and Molecular
Weight Averages of Polyolefins by High Temperature Gel
Permeation Chromatography
This standard is issued under the fixed designation D6474; 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*
1.1 This test method covers the determination of molecular weight distributions and molecular weight averages of linear
polyolefins by high temperature gel permeation chromatography (GPC). This test method uses commercially available polystyrene
standards and equipment and is applicable to polyethylenes (excluding high pressure low density polyethylene–LDPE) and
polyethylene (LDPE) and ultra-high molecular weight polyethylene (UHMWPE)) and polypropylenes soluble in 1,2,4-
trichlorobenzene (TCB) at 140°C. This test method is not absolute and requires calibration.
NOTE 1—Size exclusion chromatography (SEC) often is used as an alternative name for gel permeation chromatography (GPC).
NOTE 2—Specific methods and capabilities of users may vary with differences in columns, instrumentation, applications software, and practices
between laboratories.
NOTE 3—One general method is outlined herein; alternative analytical practices can be followed and are attached in notes where appropriate.
NOTE 4—There is no known ISO equivalent to this standard.
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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
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:
D883 Terminology Relating to Plastics
D3016 Practice for Use of Liquid Exclusion Chromatography Terms and Relationships
D5296 Test Method for Molecular Weight Averages and Molecular Weight Distribution of Polystyrene by High Performance
Size-Exclusion Chromatography
E685 Practice for Testing Fixed-Wavelength Photometric Detectors Used in Liquid Chromatography
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
IEEE/ASTM S1-10 Standard for Use of the International System of Units (SI): The Modern System (replaces ASTM E 380 and
ANSI/IEEE Standard 268-1992)
3. Terminology
3.1 Definitions—Definitions of terms applying to plastics appear in Terminology D883.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 polyolefin, n—used in this context, refers to PE (except LDPE) and PP thermoplastics.
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommitttee D20.70 on Analytical Methods.
Current edition approved Nov. 1, 2019April 1, 2020. Published December 2019April 2020. Originally approved in 1999. Last previous edition approved in 20122019 as
D6474–12.–19. DOI: 10.1520/D6474-19.10.1520/D6474-20.
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
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6474 − 20
4. Summary of Test Method
4.1 In this test method, a polyolefin sample is dissolved in a solvent and injected onto a chromatographic column(s) packed with
a substrate, which separates the molecules according to their size in solution. The separated molecules are detected and recorded
as they elute from the column according to size by a concentration sensitive detector. Through calibration, retention times are
converted to molecular weights. Molecular weight averages and molecular weight distributions are determined from the calibration
curve and detector response.
5. Significance and Use
5.1 This test method measures the molecular weight distribution and molecular weight averages of polyethylene (except
LDPE)LDPE and UHMWPE) and polypropylene resins. Differences in molecular weight and molecular weight distribution
significantly affect physical properties, such as morphology, strength, melt flow etc., and as a result, the final properties of products
made from these resins.
6. Interferences
6.1 A major interference is the presence of insoluble, highly entangled, high molecular weight material that may be linear or
cross-linked. A successful outcome of the test requires that the sample be dissolved completely prior to the chromatographic
separation. The presence of the above-described material often precludes the necessary dissolution step.
6.2 A mismatch in the antioxidant level in the dissolved sample and that of the TCB eluent (see 8.1).
6.3 The accuracy of the molecular weight results decreases, that is, they become increasingly underestimated, as the α-olefin
comonomer content increases in linear low density polyethylene (LLDPE). For example, the results for an octene copolymer
containing 30 branches per 1000 carbon atoms will be about 6 % low.
7. Apparatus
7.1 Essential Components—The essential components of the instrumentation are a solvent reservoir, a pump, a solvent degasser,
a sample injection system, packed columns, and a concentration sensitive detector.
NOTE 5—Complete high temperature GPC units with a maximum operating temperature of 210°C are commercially available.
7.2 Solvent Reservoir—The solvent reservoir shall hold sufficient TCB to ensure consistency of composition for a number of
runs. The TCB is protected from exposure to water in the air and the reservoir material shall be inert to the solvent.
7.3 Pump—The principal requirement of the pump is production of a relatively constant flow, with minimum pulsations, of
solvent through the columns. In general, the rate is adjustable between 0.1 and 5.0 cm /min and back pressures are not to exceed
limits specified by the column manufacturer. Flow rate precision shall be at least 60.3 % as measured under the conditions and
time interval for a typical analysis.
7.4 Sample Injection System—The purpose of the injection system is to introduce the solution containing the sample into the
flow stream as a sharply defined zone. Either a six-port valve with an attached sample loop or a variable volume injector can be
used for this purpose in conjunction with an autosampler. Requirements include minimal contribution to band spreading, injector
ability to operate at the back pressure generated by the columns, repeatability of injection volume, and no carryover.
7.5 Columns—Stainless steel columns with uniform and highly polished inside walls are recommended for high temperature
GPC. Columns with lengths ranging from 20 to 50 cm with fittings, frits, and connectors designed to minimize dead volume and
mixing are recommended. Generally, the packing materials, typically styrene divinylbenzene copolymers, have narrow particle size
distributions in the 3 to 20 μm range. Packing materials are available in a variety of shapes and pore sizes. Columns are packed
with particles of relatively uniform pore size or with a “mixed bed” of particles to produce a broad range of pore sizes. If a set
of columns with uniform pore size is used, it is recommended that the columns be connected in order of increasing pore size
towards the low pressure detector side.
NOTE 6—Packed high temperature GPC columns are available from a number of manufacturers.
7.6 Detector—The detector provides a continuous measure of the concentration of solute eluting from the column(s). The
detector shall be sufficiently sensitive and respond linearly to the solute concentration, independently of molecular weight, and
shall be of low internal volume so as not to distort the concentration gradient during elution. For this test method, the detector cell
volume should be 30 μL or less. The most commonly used concentration detectors for high temperature GPC are refractive index
or infrared. The former has moderate sensitivity and general utility. When testing detector performance, follow the recommen-
dations of the instrument manufacturer.
NOTE 7—For polyolefins, the refractive index (dn/dc) increment is essentially constant above molecular weights of 5 000 g/mol. The response of the
components below 5 000 g/mol should be corrected prior to the molecular weight calculations using a pre-established dn/dc molecular weight calibration.
The principal disadvantage of the differential refractometer is that the temperature within the detector cell shall be controlled to within 0.0001°C.
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7.7 Tubing and Fittings—All tubing between the sample injector and the detector must be no greater than 0.25 mm (0.01 in.)
internal diameter and rated for pressures up to 42 MPa. Connecting column tubing must be kept as short as possible and all fittings
and connectors shall have low dead volumes to prevent mixing.
7.8 Data Acquisition/Handling System—Means shall be provided for determining chromatographic peak heights or integrated
area segments at prescribed time intervals and for handling and reporting data. This is best accomplished using a computer with
appropriate software.
NOTE 8—Data acquisition and handling systems for high temperature GPC have not been standardized. However, a number of different manufacturers
provide GPC specific computer software.
8. Reagents and Materials
8.1 Solvent—1,2,4-trichlorobenzene (TCB) is recommended as the solvent for this test method. With a refractive index detector,
the solvent shall have a refractive index different than that of the polyolefins analyzed. Solvent purity and consistency shall be
considered when choosing a solvent. For example, unless freshly distilled, and subsequently, kept in a glass container under an
inert gas, TCB will react with water to form hydrochloric acid that will attack tubing walls and degrade column packing. The TCB
reservoir must be protected against exposure to moisture, or be replaced frequently with fresh solvent, or both. An antioxidant, such
as 2,6-di-tert-butyl-4-methylphenol (BHT) must be added to the solvent reservoir at the same concentration, that is, about 250
mg/L, as in the solvent used to dissolve the polymer to minimize any interference due to an antioxidant mismatch peak.
NOTE 9—Several laboratories are successfully recycling TCB using partial vacuum distillation.
NOTE 10—Any solvent that has a boiling point higher than the operating temperature, is considered a good solvent for polyolefins, and is compatible
with the GPC components, can be used
8.2 Polymer Standards—Narrow MWD (M /M < 1.1) polystyrene standards of known molecular weight (available from
W n
several suppliers) are used for calibration.
8.3 Other Chemicals—Low MW compounds, such as toluene or hexadecane, that are used for determining plate count, shall be
of high purity.
9. Hazards
9.1 Solvents used in this test method are toxic, or highly flammable, or both. The user is advised to consult literature and follow
recommended procedures pertaining to the safe handling of solvents.
10. Sampling
10.1 Whenever possible, grinding is used to ensure a representative sample is analyzed.
11. Preparation of Apparatus
11.1 Flow Rate—A flow rate of 1 6 0.1 cm /min is suggested. It is recommended that the retention time of the air peak or that
of an added low molecular weight flow rate marker, such as toluene, be used to ascertain a flow rate constant to within 0.3 %.
11.2 Detector—Detector performance must be checked regularly for any deterioration in signal-to-noise ratio. The calibration
mixtures can be used for this purpose.
12. Preparation of Solutions
12.1 Polymer Samples:
12.1.1 Weigh the polymer samples directly into GPC autosampler vials or into larger heat resistant vials having a cap lined with
solvent resistant material.
12.1.2 Add antioxidant containing, that is, about 250 mg /L solvent, preferably siphoned from the solvent reservoir, to give a
polymer concentration of between 0.05 and 0.2 weight % (see 12.3).
12.1.3 Cap the vials and heat the solutions to about 150°C for 3 to 6 h to completely dissolve the samples.
NOTE 11—For polypropylene and some high MW polyethylenes, the samples may have to be heated to between 160 and 180°C for complete
dissolution.
NOTE 12—Magnetic stirring, frequent manual agitation, or a slow rotational arrangement inside an oven is recommended to aid dissolution. Excessive
temperatures, prolonged dissolution times, and ultrasonic devices may cause the polymer to degrade.
NOTE 13—Filtration of hot polymer solutions to remove or identify the presence of nonvisible gels or other undissolved material is not recommended
due to the difficulty involved in filtering at 150°C. Unless performed very carefully, sample losses may occur due to a drop in temperature during the
filtration. Also, most commercial instruments are or can easily be outfitted with an inline pre-column filter.
12.2 Polymer Standards—Prepare 3 to 5 “cocktails”, that is, mixtures containing 3 to 4 baseline resolved standards, containing
a total of at least 12 narrow MWD polystyrene standards, as well as some hydrocarbon and polyolefin standards, if available, to
give individual concentrations of 0.01 to 0.03 weight % (the high molecular weight standards being the more dilute. The standards
must be dissolved at room temperature (up to three days dissolution time) or at 150°C, as described in 12.1, for a few hours.
Analyze the standards within a month of their preparation. Stabilized polystyrene solutions have been shown to be stable at room
temperature for several months.
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12.3 Test for Sample Solution Suitability—The mass of the polymer injected is typically between 0.05 and 0.5 mg depending
on the expected breadth of the molecular weight distribution. Smaller samples are used when the molecular weight distribution is
narrower, or the molecular weight is higher, or both. This test method assumes that the mass of the injected polymer is low enough
for the hydrodynamic volume of the polymer and the chromatographic separation not to be mass dependent. If the injected sample
mass is too high, the peak elution volume and the shape of the chromatogram may be affected and lead to erroneous MW values.
If in doubt, it is advisable to rerun an unknown sample or standard at one half its original concentration to ensure that its elution
profile is repeatable. When a change is observed, the analysis must be repeated with a lower sample concentration.
13. Performance Requirements
13.1 Plate Count Number—The plate count number (N) is a dimensionless quantity related to column efficiency and provides
an indication of the extent of band broadening. Follow recommendations of the column manufacturer when initially evaluating
columns. The plate count num
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