ASTM D6579-00

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:D6579–00
Standard Practice for
Molecular Weight Averages and Molecular Weight
Distribution of Hydrocarbon and Terpene Resins by Size-
Exclusion Chromatography
This standard is issued under the fixed designation D 6579; 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 containing the same solvent used to prepare the resin solution.
The mobile phase transports the resin into and through a
1.1 This practice covers the determination of molecular
chromatography column (or set of columns connected in
weight (MW) averages and the molecular weight distribution
series) packed with a solid or semirigid, porous substrate that
(MWD) for soluble hydrocarbon and terpene resins by size-
separates the molecules according to their size in solution. A
exclusion chromatography (SEC). This technique is not abso-
detector monitors the eluate as a function of elution volume (or
lute; it requires calibration with standards of known molecular
time). Upon emerging from the column(s), the size-separated
weight. This practice is applicable for samples containing
molecules are detected and recorded according to their con-
molecular weight components that have elution volumes fall-
centration. Through calibration, the elution volumes (or times)
ing within the elution volume range defined by polystyrene
are converted to molecular weights, and various molecular
standards.
weight parameters for the sample are calculated from the
NOTE 1—The SEC is also known as gel permeation chromatography
molecular weight/concentration data.
(GPC).
4. Significance and Use
1.2 The SEC systems employ low-volume liquid chroma-
tography components and columns packed with relatively
4.1 The MWand MWD are fundamental characteristics of a
small (generally 3 to 20 µm) microporous particles. High-
resin. They may be used for a variety of correlations for
performance liquid chromatography instrumentation and auto-
fundamental studies, processing, or product applications. The
mated data handling systems for data acquisition and process-
MW and MWD values may also be used for manufacturing
ing are also required.
quality control of resins.
1.3 This standard does not purport to address all of the
4.2 Limitations—Comparison of SEC molecular weights
safety concerns, if any, associated with its use. It is the
should be made only if the data were obtained under identical
responsibility of the user of this standard to establish appro-
chromatographic conditions.
priate safety and health practices and determine the applica-
5. Apparatus
bility of regulatory limitations prior to use.
5.1 Solvent Reservoir—The solvent reservoir must hold
2. Referenced Documents
sufficient solvent to ensure consistency of composition for a
2.1 ASTM Standards:
number of runs or analyses. The reservoir should isolate the
D 3016 Practice for Use of Liquid Exclusion Chromatogra-
solvent from the atmosphere, permit control of the environ-
phy Terms and Relationships
ment in contact with the solvent, and be inert to the solvent
E 691 Practice for Conducting an Interlaboratory Study to
employed. Some means of agitation (for example, magnetic
Determine the Precision of a Test Method
stirring) is recommended to ensure uniform composition.
5.2 Solvent Pumping System—The principal requirement of
3. Summary of Practice
a pumping system is production of a relatively constant and
3.1 In this practice, a dilute solution of a hydrocarbon or
pulseless flow of solvent through the columns. In general, the
terpene resin sample is injected into a liquid mobile phase
rate of flow should be adjustable between 0.1 and 5.0 mL/min,
and back pressures should not exceed limits specified by the
column manufacturer. If the elution volume is not being
This practice is under the jurisdiction of ASTM Committee D01 on Paint and
measured directly or corrected for systematic changes, the
Related Coatings Materials, and Applications and is the direct responsibility of
Subcommittee D01.38 on Hydrocarbon Resins.
precision in the flow rate must be at least 60.3 % under the
Current edition approved Aug. 10, 2000. Published October 2000.
conditions and over the time interval required for running a
Annual Book of ASTM Standards, Vol. 08.02.
3 typical analysis.
Annual Book of ASTM Standards, Vols. 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6579
5.3 Sample Injector—The purpose of an injection system is 5.7 Recorder/Plotter (Optional)——Either a recording po-
to generate a sharply defined zone of solution containing the tentiometer with a full-scale response of at least2sora
sample when introducing the sample into the flow stream. A printing device connected to a data handling system may be
valve and loop assembly or any of a number of commercially used to plot the chromatographic data. Pen response and
signal-to-noise ratio should be chosen so that the concentration
available high-performance liquid chromatography automatic
injection systems can be used for this purpose. It is only signal is not appreciably perturbed.
5.8 Data Handling Systems—Means must be provided for
required that contribution to band spreading be minimal and
that the injector be able to operate at the back pressure determining chromatographic peak heights or integrated area
segments at prescribed intervals under the chromatogram and
generated by the columns.
for handling and reporting the data. This can best be accom-
5.4 Columns—Stainless steel columns with uniform and
plished by means of a computer or a real-time data acquisition
highly polished inside walls should be used. Columns with
system with either off-line or on-line data processing.
lengths ranging from 15 to 50 cm and special end fittings, frits,
and connectors designed to minimize dead volume and mixing
NOTE 4—Data acquisition and handling systems for SEC have not been
are recommended. Micro-particulate, semirigid organic gels, standardized. However, a number of manufacturers provide chromatogra-
phy data systems that include SEC software. Also, some users have
and rigid solid, porous packing materials are used for SEC.
developed their own specialized software.
Generally, the packing materials have narrow particle size
distributions, with particle sizes in the range from 3 to 20 µm.
5.9 Other Components (Optional)—Special solvent line
Packing materials are available in a variety of shapes and pore filters, pressure monitors, pulse dampers, flowmeters, thermo-
sizes. Columns may be packed with particles of relatively stated ovens, syphon counters, plotters, raw data storage
uniformporesizeorwitha“mixedbed”ofparticlestoproduce systems, software, and so forth, are often incorporated with the
a broad range of pore sizes for polymer separation. If a set of essential components previously listed.
columns is used, it is recommended that the columns be 5.10 The interrelationships of the components are shown
connected, starting from the injector outlet, from columns schematicallyinFig.1.Useofadegasserlocatedinthesolvent
reservoir or between the reservoir and pumping system is
having the smallest to those having the largest packing pore
size. recommended to remove air from the solvent.
5.11 Analytical Balance, readable to 60.0001 g.
NOTE 2—Select the number of columns and pore sizes based on the
molecular weight range of the resin being analyzed and on the degree of
6. Reagents and Materials
resolution required.
6.1 Low-MW Standards—Low-MW compounds, such as
5.5 Detectors—The purpose of the detector is to continu-
toluene, xylene, or o-dichlorobenzene, that are used for deter-
ously monitor the concentration of solute eluting from the
mining plate count, or as internal standards, must be of high
chromatographic column(s). Consequently, the detector must
purity.
be sufficiently sensitive and respond linearly to the solute
6.2 Polystyrene Standards—Unimodal, narrow, MW stan-
concentration. Additionally, the detector must not appreciably
dards that effectively bracket the desired range of the resins
distort the concentration gradient in the emerging stream. This
beingcharacterized.Selectionofaminimumofthreestandards
requirement imposes severe limitations on the volume of
per decade in molecular weight spanning the effective molecu-
solution available for detection. For example, use of detectors
lar weight range of the column set is recommended.
with cell volumes greater than 15 µL generally will not be
6.3 Solvent-Tetrahydrofuran (THF)—Stabilized, high pu-
acceptable for this practice. A differential refractometer has
rity. Depending on the detector used, ultraviolet (UV) grade
moderate sensitivity and general utility. The differential refrac-
THF may be required, however, caution should be used due to
tometer provides a signal proportional to the difference in
the formation of peroxides in unstabilized THF.
refractive index (RI) between the solvent and the column
–7 –8
eluate. The detector should respond to 10 to 10 RI unit and
7. Preparation of Apparatus
have a cell volume#10 µL. Other types of detectors may be
7.1 Assembly—The SEC system must be assembled as
used.
shown in Fig. 1 and readied for operation. For commercial
systems, follow the manufacturers’ guidelines and recommen-
NOTE 3—The principal disadvantage of the differential refractometer is
that precise control of temperature, pressure, and flow rate is required to dations for assembly and operation.
maintain a stable signal for an appropriate level of sensitivity. For
7.2 Temperature—Anoperatingtemperatureisnotspecified
–4
example, most organic liquids have a temperature coefficient of 10 RI
in this practice. However, precise control of the temperature of
units per K. Consequently, the temperature within the RI detector cell
the components (injection loop, column(s), detector, and con-
–4
must be controlled to within 10 °C.
necting tubing) is critical for controlling the reproducibility of
5.6 Tubing and Fittings—All tubing between the sample the SEC molecular weights and will significantly reduce
injector and the detector should be no greater than 0.25 mm baseline drift. The temperature of the previously mentioned
(0.010 in.) in internal diameter and of sufficient thickness for internal components during an analysis must be within 3°C of
use at pressures up to 42 MPa. Connecting column tubing their temperature at calibration.
should be kept as short as possible, and all fittings and 7.3 Flow Rate—Follow the column and instrument manu-
connectors must be designed to prevent mixing and have low facturers’ recommendations when selecting a flow rate and
dead volumes. starting the solvent pumping system. A flow rate of 1 6 0.1
D6579
FIG. 1 Schematic of a SEC System
mL/min is suggested, with the pumping system adjusted to low-MW materials such as toluene, xylene, or
deliver a relatively constant and pulseless flow of eluent from o-dichlorobenzene for determining the column plate count or
the detector outlet. Flow rate may be measured by determining
for use as an internal standard.
eitherthevolumeorweightofsolventelutedoverasufficiently
NOTE 6—Alternatively, the dissolved oxygen peak may be used as the
long period of time and under suitable conditions to guarantee
reference standard.
a precision of at least 60.3 %. Alternatively, an internal
standard or control may be used to monitor flow rate. Flow 8.3 Resin Samples—The typical concentration range for
rates must be determined during calibration and before or after resin solutions is from 0.2 to 1 mg/mL. Solutions are prepared
each analysis. as described in 8.1, dissolving with a minimum of agitation.
7.4 Detector—Detector settings should provide optimum Magnetic stirring devices or laboratory shakers may be used to
sensitivity for solute detection without causing undue baseline
aid dissolution, however, excessive shear, temperature or
noise or overloading of the output signal. ultrasonic devices may cause the polymer to degrade, and
7.5 Data Handling System—Users are advised to follow
therefore must not be used with this practice. It is a good
recommendations of their computer or data system manufac- practice to analyze the resin solutions within 24 h of their
turer for setting data acquisition and integration parameters.
preparation.
8.4 Filtration—It is recommended that all sample solutions
8. Preparation of Solutions
be filtered through membrane filters to remove any materials
8.1 Polystyrene Standards—The typical concentration
likely to obstruct the columns and other system components.
range for polystyrene standards is from 0.2 to 1 mg/mL.
Membrane filters with pore sizes in the range from 0.2 to 0.5
Prepare solutions by weighing the standard into a suitable
µm are recommended. (The membrane pore size must not
clean, dry, solvent-resistant stoppered flask or screw-capped
exceed 5 µm.) The filters must be inert to the solvent and not
vial, then add an appropriate amount of solvent from the
become clogged during filtration.
mobilephasereservoir.Dissolvethestandardatroomtempera-
NOTE 7—Filtration often reveals the presence of gel in solutions even
ture. Do not stir or filter the solutions. Mixtures of two or more
though the solutions appear clear to the eye, as is the case with many
narrow MWD polymer standards may be prepared in the same
microgels.Duringfiltration,gelparticlesarelikelytoplugtheporesofthe
flask. The standards selected for each solution should differ in
filter, noticeable by an excessive pressure needed for filtration. If such an
Mw values by a factor of 10 or greater. It is recommended that
obstruction occurs, the soluble portion of the resin may be partially
higher MW polymer standards (MW > 800 000 g/mol) be
removed during filtration, the obstructed membrane now acting as an
prepared as single, more dilute solutions to reduce problems
ultra-filtration device. In this case, the resin in the filtrate may no longer
relating to polymer size in solution and concentration during
be representative of the soluble portion of the sample. Therefore, if
calibration. extensive plugging of the membrane pores is indicated, the validity of the
chromatographic results is compromised.
NOTE 5—Low-molecular weight standards may be resolved into dis-
tinct oligomeric components on high-resolution column sets. In this case, 8.5 Test for Sample Solution Suitability—The mass of resin
see the information supplied by the manufacturer for assigning the
injected for a SEC analysis depends on the expected molecular
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