ASTM D6474-99

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D 6474 – 99
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 D 6474; 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 E 685 Practice for Testing Fixed-Wavelength Photometric
Detectors Used in Liquid Chromatography
1.1 This test method covers the determination of molecular
E 691 Practice for Conducting an Interlaboratory Study to
weight distributions and molecular weight averages of linear
Determine the Precision of a Test Method
polyolefins by high temperature gel permeation chromatogra-
IEEE/ASTM S1-10 Standard for Use of the International
phy (GPC). This test method uses commercially available
System of Units (SI): The Modern System (replacesASTM
polystyrene standards and equipment and is applicable to
E 380 and ANSI/IEEE Standard 268-1992)
polyethylenes (excluding high pressure low density polyethyl-
ene–LDPE) and polypropylenes soluble in 1,2,4-
3. Terminology
trichlorobenzene (TCB) at 140°C. This test method is not
3.1 Definitions—Definitions of terms applying to plastics
absolute and requires calibration.
appear in Terminology D 883.
NOTE 1—Size exclusion chromatography (SEC) often is used as an
3.2 Definition of Term Specific to This Standard:
alternative name for gel permeation chromatography (GPC).
3.2.1 polyolefin, n—used in this context, refers to PE
NOTE 2—Specific methods and capabilities of users may vary with
(except LDPE) and PP thermoplastics.
differences in columns, instrumentation, applications software, and prac-
tices between laboratories.
4. Summary of Test Method
NOTE 3—One general method is outlined herein; alternative analytical
practices can be followed and are attached in notes where appropriate. 4.1 In this test method, a polyolefin sample is dissolved in a
NOTE 4—There is no similar or equivalent ISO standard.
solvent and injected onto a chromatographic column(s) packed
with a solid substrate, which separates the molecules according
1.2 The values stated in SI units, based on IEEE/ASTM
to their size in solution. The separated molecules are detected
S1-10, are to be regarded as the standard.
and recorded as they elute from the column according to
1.3 This standard does not purport to address all of the
concentration. Through calibration, retention times are con-
safety concerns, if any, associated with its use. It is the
verted to molecular weights.Average molecular weight param-
responsibility of the user of this standard to establish appro-
eters and molecular weight distribution are determined from
priate safety and health practices and determine the applica-
the molecular weight concentration data.
bility of regulatory limitations prior to use.
5. Significance and Use
2. Referenced Documents
5.1 This test method measures the molecular weight distri-
2.1 ASTM Standards:
bution and molecular weight averages of polyethylene (except
D 883 Terminology Relating to Plastics
LDPE) and polypropylene resins. Differences in molecular
D 1898 Practice for Sampling of Plastics
weight and molecular weight distribution significantly affect
D 3016 Practice for Use of Liquid Exclusion Chromatogra-
physical properties, such as morphology, strength, melt flow
phy Terms and Relationships
etc., and as a result, the final properties of products made from
D 5296 Test Method for Molecular Weight Averages and
these resins.
Molecular Weight Distribution of Polystyrene by High
Performance Size-Exclusion Chromatography
6. Interferences
6.1 Amajor interference is the presence of insoluble, highly
entangled,highmolecularweightmaterialthatmaybelinearor
ThistestmethodisunderthejurisdictionofASTMCommitteeD-20onPlastics
cross-linked.Asuccessful outcome of the test requires that the
and is the direct responsibility of Subcommitttee D20.70 on Analytical Methods.
Current edition approved Nov. 10, 1999. Published February 2000.
Annual Book of ASTM Standards, Vol 08.01.
3 5
Annual Book of ASTM Standards, Vol 08.02. Annual Book of ASTM Standards, Vol 14.02.
4 6
Annual Book of ASTM Standards, Vol 08.03. Annual Book of ASTM Standards, Vol 14.04.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 6474
sample be dissolved completely prior to the chromatographic detector shall be sufficiently sensitive and respond linearly to
separation. The presence of the above-described material often the solute concentration, independently of molecular weight,
precludes the necessary dissolution step. and shall be of low internal volume so as not to distort the
6.2 A mismatch in the antioxidant level in the dissolved concentration gradient during elution. For this test method, the
sample and that of the TCB eluent (see 8.1). detector cell volume should be 30 µL or less. The most
6.3 The accuracy of the molecular weight results decreases, commonly used concentration detectors for high temperature
that is, they become increasingly underestimated, as the GPC are refractive index or infrared. The former has moderate
a-olefin comonomer content increases in linear low density sensitivity and general utility. When testing detector perfor-
polyethylene (LLDPE). For example, the results for an octene mance, follow the recommendations of the instrument manu-
copolymer containing 30 branches per 1000 carbon atoms will facturer.
be about 6 % low.
NOTE 7—For polyolefins, the refractive index (dn/dc) increment is
essentially constant above molecular weights of 5 000 g/mol. The
7. Apparatus
response of the components below 5 000 g/mol should be corrected prior
7.1 Essential Components—The essential components of
to the molecular weight calculations using a pre-established dn/dc
the instrumentation are a solvent reservoir, a pump, a solvent
molecular weight calibration. The principal disadvantage of the differen-
degasser, a sample injection system, packed columns, and a
tial refractometer is that the temperature within the detector cell shall be
solute mass detector. controlled to within 0.0001°C.
NOTE 5—Complete high temperature GPC units with a maximum 7.7 Tubing and Fittings—All tubing between the sample
operating temperature of 210°C are commercially available.
injector and the detector should be no greater than 0.25 mm
(0.01 in.) internal diameter and rated for pressures up to 42
7.2 Solvent Reservoir—The solvent reservoir shall hold
MPa. Connecting column tubing should be kept as short as
sufficient TCB to ensure consistency of composition for a
possible and all fittings and connectors shall have low dead
number of runs. The TCB should be protected from exposure
towaterintheairandthereservoirmaterialshallbeinerttothe volumes to prevent mixing.
solvent.
7.8 Data Acquisition/Handling System—Means shall be
7.3 Pump—The principal requirement of the pump is pro-
provided for determining chromatographic peak heights or
ductionofarelativelyconstantflow,withminimumpulsations,
integrated area segments at prescribed time intervals and for
of solvent through the columns. In general, the rate should be
handling and reporting data. This is best accomplished using a
adjustable between 0.1 and 5.0 cm /min and back pressures
computer with appropriate software.
shouldnotexceedlimits specified by the column manufacturer.
NOTE 8—Data acquisition and handling systems for high temperature
Flow rate precision shall be at least 60.3 % as measured under
GPC have not been standardized. However, a number of different
the conditions and time interval for a typical analysis.
manufacturers provide GPC specific computer software.
7.4 Sample Injection System—The purpose of the injection
system is to introduce the solution containing the sample into
8. Reagents and Materials
the flow stream as a sharply defined zone. Either a six-port
8.1 Solvent—1,2,4-trichlorobenzene (TCB) is recom-
valve with an attached sample loop or a variable volume
mended as the solvent for this test method; however, any
injector can be used for this purpose in conjunction with an
solvent that has a boiling point higher than the operating
autosampler. Requirements include minimal contribution to
temperature, is considered a good solvent for polyolefins, and
band spreading, injector ability to operate at the back pressure
is compatible with the GPC components, may be used. With a
generated by the columns, repeatability of injection volume,
refractive index detector, the solvent shall have a refractive
and no carryover.
index different than that of the polyolefins analyzed. Solvent
7.5 Columns—Stainless steel columns with uniform and
purity and consistency shall be considered when choosing a
highly polished inside walls are recommended for high tem-
solvent. For example, unless freshly distilled, and subse-
perature GPC. Columns with lengths ranging from 20 to 50 cm
quently, kept in a glass container under an inert gas, TCB will
with fittings, frits, and connectors designed to minimize dead
react with water to form hydrochloric acid that will attack
volume and mixing are recommended. Generally, the packing
tubing walls and degrade column packing. The TCB reservoir
materials, typically styrene divinylbenzene copolymers, have
should be protected against exposure to moisture, or be
narrow particle size distributions in the 3 to 20 µm range.
replaced frequently with fresh solvent, or both.An antioxidant,
Packing materials are available in a variety of shapes and pore
such as 2,6-di-tert-butyl-4-methylphenol (BHT) should be
sizes. Columns may be packed with particles of relatively
addedtothesolventreservoiratthesameconcentration,thatis,
uniformporesizeorwitha“mixedbed”ofparticlestoproduce
about 250 mg/L, as in the solvent used to dissolve the polymer
a broad range of pore sizes. If a set of columns with uniform
to minimize any interference due to an antioxidant mismatch
pore size is used, it is recommended that the columns be
peak.
connected in order of increasing pore size towards the low
pressure detector side.
NOTE 9—Several laboratories are successfully recycling TCB using
partial vacuum distillation.
NOTE 6—Packed high temperature GPC columns are available from a
number of manufacturers.
8.2 Polymer Standards—Narrow MWD (M /M < 1.1)
W n
7.6 Detector—The detector provides a continuous measure polystyrene standards of known molecular weight (available
of the concentration of solute eluting from the column(s). The from several suppliers) are used for calibration.
D 6474
8.3 Other Chemicals—Low MW compounds, such as tolu- within a month of their preparation. Stabilized polystyrene
ene or hexadecane, that are used for determining plate count, solutionshavebeenshowntobestableatroomtemperaturefor
shall be of high purity. several months.
12.3 Test for Sample Solution Suitability—The mass of the
polymer injected is typically between 0.05 and 0.5 mg depend-
9. Hazards
ing on the expected breadth of the molecular weight distribu-
9.1 Solvents used in this test method are toxic, or highly
tion. Smaller samples should be used when the molecular
flammable,orboth.Theuserisadvisedtoconsultliteratureand
weight distribution is narrower, or the molecular weight is
follow recommended procedures pertaining to the safe han-
higher, or both. This test method assumes that the mass of the
dling of solvents.
injected polymer is low enough for the hydrodynamic volume
of the polymer and the chromatographic separation not to be
10. Sampling
mass dependent. If the injected sample mass is too high, the
10.1 Whenever possible, grinding should be used to ensure
peak elution volume and the shape of the chromatogram may
a representative sample is analyzed.
be affected and lead to erroneous MW values. If in doubt, it is
advisable to rerun an unknown sample or standard at one half
11. Preparation of Apparatus
its original concentration to ensure that its elution profile is
11.1 Flow Rate—A flow rate of 1 6 0.1 cm /min is
repeatable. When a change is observed, the analysis should be
suggested. The flow rate should be checked regularly. It is
repeated with a lower sample concentration.
recommended that the retention time of the air peak or that of
13. Performance Requirements
an added low molecular weight flow rate marker, such as
toluene, be used to ascertain a flow rate constant to within
13.1 Plate Count Number—The plate count number (N)isa
0.3 %.
dimensionless quantity related to column efficiency and pro-
11.2 Detector—Detector performance should be checked
vides an indication of the extent of band broadening. Follow
regularly for any deterioration in signal-to-noise ratio. The
recommendations of the column manufacturer when initially
calibration mixtures can be used for this purpose.
evaluating columns. The plate count number also should be
determined under the same conditions as those used in this test
12. Preparation of Solutions
method. For example:
Solvent: TCB
12.1 Polymer Samples:
Temperature: 140°C
12.1.1 Weigh the polymer samples directly into GPC au-
Flow rate: 1 cm /min
tosampler vials or into larger heat resistant vials having a cap
Test solute: Hexadecane
Concentration: ;0.01 % w/v
lined with solvent resistant material.
Injection volume: 300 µL
12.1.2 Add antioxidant containing, that is, about 250 mg /L
solvent, preferably siphoned from the solvent reservoir, to give For an approximately Guassian-shaped solute peak, the
apolymerconcentrationofbetween0.05and0.2weight%(see following expression can be used to calculate the number of
12.3). plates (N)/m:
12.1.3 Cap the vials and heat the solutions to about 150°C
N/L 5 16 3 ~1/L! 3 ~V / W! (1)
r
for3to6hto completely dissolve the samples. For polypro-
pylene and some high MW polyethylenes, the samples may
where:
have to be heated to between 160 and 180°C for complete L = total column length, m,
dissolution. V = peak elution volume, mL, or time, min, and
r
W = peak width in units of volume (mL) or time (min) as
NOTE 10—Magnetic stirring, frequent manual agitation, or a slow
determined by measuring the distance between the
rotational arrangement inside an oven is recommended to aid dissolution.
baseline intercepts of lines drawn tangent to the peak
Excessive temperatures, prolonged dissolution times, and ultrasonic
inflection points.
devices may cause the polymer to degrade.
NOTE 11—Filtration of hot polymer solutions to remove or identify the 13.1.1 High temperature GPC columns are expected to
presence of non
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