ASTM D6474-99(2006)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:D6474 −99(Reapproved 2006)
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 Molecular Weight Distribution of Polystyrene by High
Performance Size-Exclusion Chromatography
1.1 This test method covers the determination of molecular
E685 Practice for Testing Fixed-Wavelength Photometric
weight distributions and molecular weight averages of linear
Detectors Used in Liquid Chromatography
polyolefins by high temperature gel permeation chromatogra-
E691 Practice for Conducting an Interlaboratory Study to
phy (GPC). This test method uses commercially available
Determine the Precision of a Test Method
polystyrene standards and equipment and is applicable to
IEEE/ASTM S1-10 Standard for Use of the International
polyethylenes (excluding high pressure low density polyethyl-
System of Units (SI): The Modern System (replaces
ene–LDPE) and polypropylenes soluble in 1,2,4-
ASTM E 380 and ANSI/IEEE Standard 268-1992)
trichlorobenzene (TCB) at 140°C. This test method is not
absolute and requires calibration.
3. Terminology
NOTE 1—Size exclusion chromatography (SEC) often is used as an
3.1 Definitions—Definitions of terms applying to plastics
alternative name for gel permeation chromatography (GPC).
appear in Terminology D883.
NOTE 2—Specific methods and capabilities of users may vary with
differences in columns, instrumentation, applications software, and prac-
3.2 Definitions of Terms Specific to This Standard:
tices between laboratories.
3.2.1 polyolefin, n—used in this context,referstoPE(except
NOTE 3—One general method is outlined herein; alternative analytical
LDPE) and PP thermoplastics.
practices can be followed and are attached in notes where appropriate.
NOTE 4—There is no similar or equivalent ISO standard.
4. Summary of Test Method
1.2 The values stated in SI units, based on IEEE/ASTM
4.1 In this test method, a polyolefin sample is dissolved in a
S1-10, are to be regarded as the standard.
solvent and injected onto a chromatographic column(s) packed
1.3 This standard does not purport to address all of the
withasolidsubstrate,whichseparatesthemoleculesaccording
safety concerns, if any, associated with its use. It is the
to their size in solution. The separated molecules are detected
responsibility of the user of this standard to establish appro-
and recorded as they elute from the column according to
priate safety and health practices and determine the applica-
concentration. Through calibration, retention times are con-
bility of regulatory limitations prior to use.
verted to molecular weights.Average molecular weight param-
eters and molecular weight distribution are determined from
2. Referenced Documents
the molecular weight concentration data.
2.1 ASTM Standards:
5. Significance and Use
D883 Terminology Relating to Plastics
D3016 Practice for Use of Liquid Exclusion Chromatogra-
5.1 This test method measures the molecular weight distri-
phy Terms and Relationships
bution and molecular weight averages of polyethylene (except
D5296 Test Method for Molecular Weight Averages and
LDPE) and polypropylene resins. Differences in molecular
weight and molecular weight distribution significantly affect
physical properties, such as morphology, strength, melt flow
This test method is under the jurisdiction ofASTM Committee D20 on Plastics
etc., and as a result, the final properties of products made from
and is the direct responsibility of Subcommitttee D20.70 on Analytical Methods.
these resins.
Current edition approved March 15, 2006. Published April 2006. Originally
approved in 1999. Last previous edition approved in 1999 as D6474–99 DOI:
6. Interferences
10.1520/D6474-99R06.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
6.1 Amajor interference is the presence of insoluble, highly
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
entangled,highmolecularweightmaterialthatmaybelinearor
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. cross-linked.Asuccessful outcome of the test requires that the
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6474−99 (2006)
sample be dissolved completely prior to the chromatographic 7.6 Detector—The detector provides a continuous measure
separation. The presence of the above-described material often of the concentration of solute eluting from the column(s). The
precludes the necessary dissolution step. detector shall be sufficiently sensitive and respond linearly to
the solute concentration, independently of molecular weight,
6.2 A mismatch in the antioxidant level in the dissolved
and shall be of low internal volume so as not to distort the
sample and that of the TCB eluent (see 8.1).
concentration gradient during elution. For this test method, the
6.3 The accuracy of the molecular weight results decreases,
detector cell volume should be 30 µL or less. The most
that is, they become increasingly underestimated, as the
commonly used concentration detectors for high temperature
α-olefin comonomer content increases in linear low density
GPC are refractive index or infrared. The former has moderate
polyethylene (LLDPE). For example, the results for an octene
sensitivity and general utility. When testing detector
copolymer containing 30 branches per 1000 carbon atoms will
performance, follow the recommendations of the instrument
be about 6 % low.
manufacturer.
NOTE 7—For polyolefins, the refractive index (dn/dc) increment is
7. Apparatus
essentially constant above molecular weights of 5 000 g/mol. The
7.1 Essential Components—The essential components of
response of the components below 5 000 g/mol should be corrected prior
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
7.2 Solvent Reservoir—The solvent reservoir shall hold
(0.01 in.) internal diameter and rated for pressures up to 42
sufficient TCB to ensure consistency of composition for a
MPa. Connecting column tubing should be kept as short as
number of runs. The TCB should be protected from exposure
possible and all fittings and connectors shall have low dead
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.
shouldnotexceedlimitsspecifiedbythecolumnmanufacturer.
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
8. Reagents and Materials
system is to introduce the solution containing the sample into
the flow stream as a sharply defined zone. Either a six-port
8.1 Solvent—1,2,4-trichlorobenzene(TCB)isrecommended
valve with an attached sample loop or a variable volume
as the solvent for this test method; however, any solvent that
injector can be used for this purpose in conjunction with an
has a boiling point higher than the operating temperature, is
autosampler. Requirements include minimal contribution to
considered a good solvent for polyolefins, and is compatible
band spreading, injector ability to operate at the back pressure
with the GPC components, may be used. With a refractive
generated by the columns, repeatability of injection volume,
index detector, the solvent shall have a refractive index
and no carryover.
different than that of the polyolefins analyzed. Solvent purity
and consistency shall be considered when choosing a solvent.
7.5 Columns—Stainless steel columns with uniform and
For example, unless freshly distilled, and subsequently, kept in
highly polished inside walls are recommended for high tem-
a glass container under an inert gas, TCB will react with water
perature GPC. Columns with lengths ranging from 20 to 50 cm
to form hydrochloric acid that will attack tubing walls and
with fittings, frits, and connectors designed to minimize dead
degrade column packing. The TCB reservoir should be pro-
volume and mixing are recommended. Generally, the packing
tected against exposure to moisture, or be replaced frequently
materials, typically styrene divinylbenzene copolymers, have
with fresh solvent, or both.An antioxidant, such as 2,6-di-tert-
narrow particle size distributions in the 3 to 20 µm range.
butyl-4-methylphenol (BHT) should be added to the solvent
Packing materials are available in a variety of shapes and pore
reservoir at the same concentration, that is, about 250 mg/L, as
sizes. Columns may be packed with particles of relatively
in the solvent used to dissolve the polymer to minimize any
uniformporesizeorwitha“mixedbed”ofparticlestoproduce
interference due to an antioxidant mismatch peak.
a broad range of pore sizes. If a set of columns with uniform
pore size is used, it is recommended that the columns be
NOTE 9—Several laboratories are successfully recycling TCB using
connected in order of increasing pore size towards the low partial vacuum distillation.
pressure detector side.
8.2 Polymer Standards—Narrow MWD (M /M < 1.1)
W n
polystyrene standards of known molecular weight (available
NOTE 6—Packed high temperature GPC columns are available from a
number of manufacturers. from several suppliers) are used for calibration.
D6474−99 (2006)
8.3 Other Chemicals—Low MW compounds, such as tolu- polystyrene solutions have been shown to be stable at room
ene or hexadecane, that are used for determining plate count, temperature for several months.
shall be of high purity.
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
recommendations of the column manufacturer when initially
regularly for any deterioration in signal-to-noise ratio. The
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:
12.1 Polymer Samples: Solvent: TCB
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
lined with solvent resistant material. Concentration: ;0.01 % w/v
Injection volume: 300 µL
12.1.2 Add antioxidant containing, that is, about 250 mg /L
For an approximately Guassian-shaped solute peak, the
solvent, preferably siphoned from the solvent reservoir, to give
following expression can be used to calculate the number of
apolymerconcentrationofbetween0.05and0.2weight%(see
plates (N)/m:
12.3).
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-
where:
pylene and some high MW polyethylenes, the samples may
L = total column length, m,
have to be heated to between 160 and 180°C for complete
V = peak elution volume, mL, or time, min, and
r
dissolution.
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
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