ASTM D6342-98(2003)
(Practice)Standard Practice for Polyurethane Raw Materials: Determining Hydroxyl Number of Polyols by Near Infrared (NIR) Spectroscopy
Standard Practice for Polyurethane Raw Materials: Determining Hydroxyl Number of Polyols by Near Infrared (NIR) Spectroscopy
SIGNIFICANCE AND USE
General Utility:
5.1.1 It is necessary to know the hydroxyl number of polyols in order to formulate polyurethane systems.
5.1.2 This practice is suitable for research, quality control, specification testing, and process control.
Limitations:
5.2.1 Factors affecting the NIR spectra of the analyte polyols need to be determined before a calibration procedure is started. Chemical structure, interferences, any nonlinearities, the effect of temperature, and the interaction of the analyte with other sample components such as catalyst, water and other polyols needs to be understood in order to properly select samples that will model those effects which can not be adequately controlled.
5.2.2 Calibrations are generally considered valid only for the specific NIR instrument used to generate the calibration. Using different instruments (even when made by the same manufacturer) for calibration and analysis can seriously affect the accuracy and precision of the measured hydroxyl number. Procedures used for transferring calibrations between instruments are problematic and should be utilized with caution following the guidelines in Section 16. These procedures generally require a completely new validation and statistical analysis of errors on the new instrument.
5.2.3 The analytical results are statistically valid only for the range of hydroxyl numbers used in the calibration. Extrapolation to lower or higher hydroxyl values can increase the errors and degrade precision. Likewise, the analytical results are only valid for the same chemical composition as used for the calibration set. A significant change in composition or contaminants can also affect the results. Outlier detection, as discussed in Practices E 1655, is a tool that can be used to detect the possibility of problems such as those mentioned above.
SCOPE
1.1 This standard covers a practice for the determination of hydroxyl numbers of polyols using NIR spectroscopy.
1.2 Definitions, terms, and calibration techniques are described. Procedures for selecting samples, and collecting and treating data for developing NIR calibrations are outlined. Criteria for building, evaluating, and validating the NIR calibration model are also described. Finally, the procedure for sample handling, data gathering and evaluation are described.
1.3 The implemenation of this standard requires that the NIR spectrometer has been installed in compliance with the manufacturer's specifications.
1.4 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 and health practices and determine the applicability of regulatory limitations prior to use.
Note 1-There is no equivalent or similar ISO standard.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D6342–98 (Reapproved 2003)
Standard Practice for
Polyurethane Raw Materials: Determining Hydroxyl Number
of Polyols by Near Infrared (NIR) Spectroscopy
This standard is issued under the fixed designation D 6342; 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 275 Practice for Describing and Measuring Performance
of Ultraviolet, Visible, and Near Infrared Spectrophotom-
1.1 This standard covers a practice for the determination of
eters
hydroxyl numbers of polyols using NIR spectroscopy.
E 456 Terminology Relating to Quality and Statistics
1.2 Definitions, terms, and calibration techniques are de-
E 1655 Practices for Infrared, Multivariate, Quantitative
scribed. Procedures for selecting samples, and collecting and
Analysis
treating data for developing NIR calibrations are outlined.
E 1899 Hydroxyl Groups by Toluenesulfonyl Isocyanate
Criteria for building, evaluating, and validating the NIR
calibration model are also described. Finally, the procedure for
3. Terminology
sample handling, data gathering and evaluation are described.
3.1 Definitions—Terminology used in this practice follows
1.3 The implementation of this standard requires that the
that defined in Terminology D 883. For terminology related to
NIR spectrometer has been installed in compliance with the
molecular spectroscopy methods, refer to Terminology E 131.
manufacturer’s specifications.
For terms relating to multivariate analysis, refer to Practice
1.4 This standard does not purport to address all of the
E 1655.
safety concerns, if any, associated with its use. It is the
3.2 Definitions of Terms Specific to This Standard:
responsibility of the user of this standard to establish appro-
3.2.1 hydroxyl number—the milligrams of potassium hy-
priate safety and health practices and determine the applica-
droxide equivalent to the hydroxyl content of1gof sample.
bility of regulatory limitations prior to use.
NOTE 1—There is no equivalent or similar ISO standard. 4. Summary of Practice
4.1 Multivariate mathematics is applied to correlate the NIR
2. Referenced Documents
absorbance values for a set of calibration samples to the
2.1 ASTM Standards:
respective reference hydroxyl number for each sample. The
D 883 Terminology Relating to Plastics
resultant multivariate calibration model is then applied to the
D 4274 Test Methods for Testing Polyurethane Raw Mate-
analysis of unknown samples to provide an estimate of their
rials: Determination of Hydroxyl Numbers of Polyols
hydroxyl numbers.
D 4855 Practice for Comparing Test Methods
4.2 Multilinear regression (MLR), principal components
E 131 Terminology Relating to Molecular Spectroscopy
regression (PCR), and partial least squares regression (PLS)
E 168 Practice for General Techniques of Infrared Quanti-
are the mathematical techniques used for the development of
tative Analysis
the calibration model.
E 222 Hydroxyl Groups Using Acetic Anhydride Acetyla-
4.3 Statistical tests are used to detect outliers during the
tion
development of the calibration model. Outliers may include
high leverage samples and samples whose hydroxyl numbers
are inconsistent with the model.
This practice is under the jurisdiction ofASTM Committee D20 on Plastics and
4.4 Validation of the calibration model is performed by
is the direct responsibility of Subcommittee D20.22 on Cellular Materials—Plastics
using the model to analyze a set of validation samples. The
and Elastomers.
hydroxyl number estimates for the validation set are statisti-
Current edition approved November 1, 2003. Published December 2003.
cally compared to the reference hydroxyl number for this set to
Originally approved in 1998. Last previous edition approved in 1998 as D 6342 -
98.
test for agreement of the model with the reference method.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
4.5 Statistical expressions are given for calculating the
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
precision and bias of the NIR method relative to the reference
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. method.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6342–98 (2003)
5. Significance and Use 6.3.1 Monochromator Instrument—Gratingmonochromator
instruments, often called “dispersive” instruments, are com-
5.1 General Utility:
monly used in the laboratory and for process applications. In a
5.1.1 It is necessary to know the hydroxyl number of
halographic grating system, the grating is rotated so that only
polyols in order to formulate polyurethane systems.
a narrow band of wavelengths is transmitted to a single
5.1.2 This practice is suitable for research, quality control,
detector at given time.
specification testing, and process control.
6.3.2 Filter-Wheel Instrument—In this type of NIR instru-
5.2 Limitations:
ment,oneorseveralnarrowbandfiltersaremountedonaturret
5.2.1 Factors affecting the NIR spectra of the analyte
wheel so that the individual wavelengths are presented to a
polyols need to be determined before a calibration procedure is
single detector sequentially.
started. Chemical structure, interferences, any nonlinearities,
6.3.3 Acoustic Optic Tunable Filter (AOTF) Instrument—
theeffectoftemperature,andtheinteractionoftheanalytewith
TheAOTFisacontinuousvariantofthefixed-filterphotometer
other sample components such as catalyst, water and other
with no moving optical parts for wavelength selection. A
polyols needs to be understood in order to properly select
birefringent TeO crystal is used in a noncollinear configura-
samples that will model those effects which can not be 2
tion in which acoustic and optical waves move through the
adequately controlled.
crystal at different angles. Variations in the acoustic frequency
5.2.2 Calibrations are generally considered valid only for
causes the crystal lattice spacing to change.That in turn causes
the specific NIR instrument used to generate the calibration.
the crystal to act as a variable transmission diffraction grating
Using different instruments (even when made by the same
for one wavelength. The main advantage of using AOTF
manufacturer) for calibration and analysis can seriously affect
instruments is the speed. A wavelength or an assembly of
the accuracy and precision of the measured hydroxyl number.
wavelengths can be changed hundreds of times per second
Procedures used for transferring calibrations between instru-
under computer control.
ments are problematic and should be utilized with caution
6.3.4 Light-Emitting Diode (LED) Instrument—Each wave-
following the guidelines in Section 16. These procedures
generally require a completely new validation and statistical length band is produced by a different diode. The major
advantages of the system are its small size and compactness,
analysis of errors on the new instrument.
5.2.3 Theanalyticalresultsarestatisticallyvalidonlyforthe stability of construction with no moving parts, and low power
consumption.
range of hydroxyl numbers used in the calibration. Extrapola-
tion to lower or higher hydroxyl values can increase the errors
6.3.5 Fourier Transfer (FT) Instrument—In FT-NIR instru-
and degrade precision. Likewise, the analytical results are only ments, the light is divided into two beams whose relative paths
valid for the same chemical composition as used for the
are varied by use of a moving optical element. The beams are
calibrationset.Asignificantchangeincompositionorcontami- recombined to produce an interference pattern that contains all
nants can also affect the results. Outlier detection, as discussed
of the wavelengths of interest. The interference pattern is
in Practices E 1655, is a tool that can be used to detect the mathematically converted into spectral data using Fourier
possibility of problems such as those mentioned above. transform. FT interferometer optics provide complete spectra
with very high wavelength resolution. FTsignal averaging also
6. Instrumentation
provides higher signal-to-noise ratios in general than can be
achieved with other types of instruments.
6.1 Introduction—A complete description of all applicable
6.4 Sampling System—Depending upon the applications,
types of NIR instrumentation is beyond the scope of this
standard. Only a general outline is given here. A diagram of a several different sampling systems can be used in the labora-
tory or for on-line instruments, or both.
typical NIR spectrometer is shown in Fig. 1.
6.2 Light Source and Detector—Tungsten-halogen lamps
6.4.1 Cuvette—Quartz or glass cuvettes with fixed or ad-
with quartz envelopes usually serve as the energy sources for justable path lengths can be used in the laboratory.
NIR instruments. Most of the detectors used for NIR are
6.4.2 Flow-Through Cell—This type cell can be used for
solid-state semiconductors. PbS, PbSe, and InGaAs detectors
continuous or intermittent monitoring of liquid sample.
are most commonly used.
6.4.3 Probes:
6.3 Light Dispersion—Spectrophotometers can be classified
6.4.3.1 Transmission Probe—Transmission probes com-
based on the procedure by which the instrument accomplishes
bined with optic fibers are ideal for analyzing clear liquids,
wavelength selection.
slurries, suspensions, and other high viscosity samples. Low
absorptivity in the NIR region permits sampling pathlengths of
up to 10 cm.
6.4.3.2 Immersion Probe—The immersion system uses a
bi-directional optic fiber bundle and variable pathlength probe
for sample measurements. Radiation from the source is trans-
mitted to the sample by the inner ring of fibers, and diffuse
transmitted radiation is collected by the outer ring of fibers for
detection.
6.4.3.3 Attenuated Total Reflection (ATR) Probe—
FIG. 1 Schematic of a Near-IR System Attenuated total reflection occurs when an absorbing medium
D6342–98 (2003)
(the sample) is in close contact with the surface of a crystal 7.1.1 For most types of instrumentation, the radiant power
material of higher refractive index. At an optimized angle, the incident on the sample cannot be measured directly. Instead, a
NIR beam reflects internally along the crystal faces, penetrat-
reference (background) measurement of the radiant power is
ing a few microns into the sample surface, where selective made without the sample being present in the light beam.
absorption occurs. The resulting spectrum is very close to the
7.1.2 A measurement is then conducted with the sample
conventional transmission spectrum for the sample. There are
present, and the ratio, T, is calculated. The background
many designs ofATR plates and rods for specific applications.
measurementmaybeconductedinavarietyofwaysdepending
Single or multiple reflection units are available.ATR sampling
on the application and instrumentation. The sample and its
accessories are available for the laboratory and, in the form of
holder may be physically removed from the light beam and a
fiber optic probes, can be used for on-line analysis. This is an
background measurement made on the “empty beam”. The
advantage when handling viscous liquids and highly absorbing
sample holder (cell) may be emptied, and a background
materials.
measurement may be taken for the empty cell. The cell may be
6.5 Software—Theidealsoftwareshouldhavethefollowing
filled with a material that has minimal absorption in the
capabilities:
spectral range of interest, and the background measurement
6.5.1 The capability to record all sample identification and
may be taken. Alternatively, the light beam may be split or
spectral data accurately and to access the reference data,
alternately passed through the sample and through an empty
6.5.2 The capability to record the date and time of day that
beam, and empty cell, or a background material in the cell.
all spectra and files were recorded or created,
7.1.3 The particular background referencing scheme that is
6.5.3 The capability to move or copy spectra, or both, from
used may vary among instruments, and among applications.
file to file,
The same sample background referencing scheme must be
6.5.4 The capability to add or subtract spectral data, and to
employed for the measurement of all spectra of calibration
average spectra,
samples, validation samples, and unknown samples to be
6.5.5 The capability to perform transformations of log l/R
analyzed. Any differences between instrument conditions used
optical data into derivatives, or other forms of mathematical
for referencing and measurement should be minimized.
treatment, and to reverse the transformation,
7.2 Traditionally, a sample is manually brought to the
6.5.6 The capability to compute multiple linear regression
instrument and placed in a suitable optical container (a cell,
(MLR), principal component regression (PCR), and partial
vial, or cuvette with windows that transmit in the region of
least squares regression (PLS),
interest). Alternatively, transfer pipes can continuously flow
6.5.7 The capability to store PCR or PLS loading, weights,
liquid through an optical cell in the instrument for continuous
scores or other desirable data, and to display these data for
analysis. With optical fibers, the sample can be analyzed
subsequent examination and interpretation,
remotely from the instrument. Light is sent to the sample
6.5.8 The capability to enable the operator to evaluate the
through an optical fiber or fibers and returned to the instrument
calibrationmodelbycomputingthestandarderrorofvalidation
by means of another fiber or group of fibers. Instruments have
(SEV), coefficient of regression, and the root mean square
deviation (RMSD), and to display various plots, been developed that use a single fiber to transmit and receive
the light, as well as use bundles of fibers for this purpose.
6.5.9 The capability to perform cross-validation automati-
cally, Detectors and light sources external to the instrument can also
6.5.10 The capability to identify an outlier(s), and be used, in which case only one fiber or bundle is needed. The
appropriate grade of optical fibers for use in the NIR range
6.5.11 The capability to develop and save regression
equations and analyze a sample to calculate a hydroxyl needstobespecified.Generally,thesearefiberswithlowwater
number. content (Low-OH). Total fiber length should not exceed
6.6 Software Packages—Most NIR instruments provide manufacturer’s recommendations.
necessary software for collecting and modeling data. Severa
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.