1、Designation: D 6342 08Standard Practice forPolyurethane Raw Materials: Determining Hydroxyl Numberof Polyols by Near Infrared (NIR) Spectroscopy1This standard is issued under the fixed designation D 6342; the number immediately following the designation indicates the year oforiginal adoption or, in
2、the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This standard covers a practice for the determination ofhydroxyl numbers of polyol
3、s using NIR spectroscopy.1.2 Definitions, terms, and calibration techniques are de-scribed. Procedures for selecting samples, and collecting andtreating data for developing NIR calibrations are outlined.Criteria for building, evaluating, and validating the NIRcalibration model are also described. Fi
4、nally, the procedure forsample handling, data gathering and evaluation are described.1.3 The implementation of this standard requires that theNIR spectrometer has been installed in compliance with themanufacturers specifications.1.4 This standard does not purport to address all of thesafety concerns
5、, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.NOTE 1There is no equivalent or similar ISO standard.2. Referenced Documents2.1 ASTM S
6、tandards:2D 883 Terminology Relating to PlasticsD 4274 Test Methods for Testing Polyurethane Raw Mate-rials: Determination of Hydroxyl Numbers of PolyolsD 4855 Practice for Comparing Test MethodsE 131 Terminology Relating to Molecular SpectroscopyE 168 Practices for General Techniques of Infrared Qu
7、anti-tative AnalysisE 222 Test Methods for Hydroxyl Groups Using AceticAnhydride AcetylationE 275 Practice for Describing and Measuring Performanceof Ultraviolet, Visible, and Near-Infrared Spectrophotom-etersE 456 Terminology Relating to Quality and StatisticsE 1655 Practices for Infrared Multivari
8、ate QuantitativeAnalysisE 1899 Test Method for Hydroxyl Groups Using Reactionwith p-Toluenesulfonyl Isocyanate (TSI) and Potentiomet-ric Titration with Tetrabutylammonium Hydroxide3. Terminology3.1 DefinitionsTerminology used in this practice followsthat defined in Terminology D 883. For terminology
9、 related tomolecular spectroscopy methods, refer to Terminology E 131.For terms relating to multivariate analysis, refer to PracticeE 1655.3.2 Definitions of Terms Specific to This Standard:3.2.1 hydroxyl numberthe milligrams of potassium hy-droxide equivalent to the hydroxyl content of1gofsample.4.
10、 Summary of Practice4.1 Multivariate mathematics is applied to correlate the NIRabsorbance values for a set of calibration samples to therespective reference hydroxyl number for each sample. Theresultant multivariate calibration model is then applied to theanalysis of unknown samples to provide an e
11、stimate of theirhydroxyl numbers.4.2 Multilinear regression (MLR), principal componentsregression (PCR), and partial least squares regression (PLS)are the mathematical techniques used for the development ofthe calibration model.4.3 Statistical tests are used to detect outliers during thedevelopment
12、of the calibration model. Outliers can includehigh leverage samples and samples whose hydroxyl numbersare inconsistent with the model.4.4 Validation of the calibration model is performed byusing the model to analyze a set of validation samples. Thehydroxyl number estimates for the validation set are
13、 statisti-cally compared to the reference hydroxyl number for this set totest for agreement of the model with the reference method.1This practice is under the jurisdiction ofASTM Committee D20 on Plastics andis the direct responsibility of Subcommittee D20.22 on Cellular Materials - Plasticsand Elas
14、tomers.Current edition approved March 1, 2008. Published April 2008. Originallyapproved in 1998. Last previous edition approved in 2003 as D 6342 - 98(2003).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of AS
15、TMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1*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.4.5 Statistical express
16、ions are given for calculating theprecision and bias of the NIR method relative to the referencemethod.5. Significance and Use5.1 General Utility:5.1.1 It is necessary to know the hydroxyl number ofpolyols in order to formulate polyurethane systems.5.1.2 This practice is suitable for research, quali
17、ty control,specification testing, and process control.5.2 Limitations:5.2.1 Factors affecting the NIR spectra of the analytepolyols need to be determined before a calibration procedure isstarted. Chemical structure, interferences, any nonlinearities,the effect of temperature, and the interaction of
18、the analyte withother sample components such as catalyst, water and otherpolyols needs to be understood in order to properly selectsamples that will model those effects which can not beadequately controlled.5.2.2 Calibrations are generally considered valid only forthe specific NIR instrument used to
19、 generate the calibration.Using different instruments (even when made by the samemanufacturer) for calibration and analysis can seriously affectthe accuracy and precision of the measured hydroxyl number.Procedures used for transferring calibrations between instru-ments are problematic and are to be
20、utilized with cautionfollowing the guidelines in Section 16. These proceduresgenerally require a completely new validation and statisticalanalysis of errors on the new instrument.5.2.3 The analytical results are statistically valid only for therange of hydroxyl numbers used in the calibration. Extra
21、pola-tion to lower or higher hydroxyl values can increase the errorsand degrade precision. Likewise, the analytical results are onlyvalid for the same chemical composition as used for thecalibration set.Asignificant change in composition or contami-nants can also affect the results. Outlier detectio
22、n, as discussedin Practices E 1655, is a tool that can be used to detect thepossibility of problems such as those mentioned above.6. Instrumentation6.1 IntroductionA complete description of all applicabletypes of NIR instrumentation is beyond the scope of thisstandard. Only a general outline is give
23、n here. A diagram of atypical NIR spectrometer is shown in Fig. 1.6.2 Light Source and DetectorTungsten-halogen lampswith quartz envelopes usually serve as the energy sources forNIR instruments. Most of the detectors used for NIR aresolid-state semiconductors. PbS, PbSe, and InGaAs detectorsare most
24、 commonly used.6.3 Light DispersionSpectrophotometers can be classifiedbased on the procedure by which the instrument accomplisheswavelength selection.6.3.1 Monochromator InstrumentGrating monochromatorinstruments, often called “dispersive” instruments, are com-monly used in the laboratory and for p
25、rocess applications. In ahalographic grating system, the grating is rotated so that onlya narrow band of wavelengths is transmitted to a singledetector at given time.6.3.2 Filter-Wheel InstrumentIn this type of NIR instru-ment, one or several narrow band filters are mounted on a turretwheel so that
26、the individual wavelengths are presented to asingle detector sequentially.6.3.3 Acoustic Optic Tunable Filter (AOTF) InstrumentTheAOTF is a continuous variant of the fixed-filter photometerwith no moving optical parts for wavelength selection. Abirefringent TeO2crystal is used in a noncollinear conf
27、igura-tion in which acoustic and optical waves move through thecrystal at different angles. Variations in the acoustic frequencycauses the crystal lattice spacing to change. That in turn causesthe crystal to act as a variable transmission diffraction gratingfor one wavelength. The main advantage of
28、using AOTFinstruments is the speed. A wavelength or an assembly ofwavelengths can be changed hundreds of times per secondunder computer control.6.3.4 Light-Emitting Diode (LED) InstrumentEach wave-length band is produced by a different diode. The majoradvantages of the system are its small size and
29、compactness,stability of construction with no moving parts, and low powerconsumption.6.3.5 Fourier Transfer (FT) InstrumentIn FT-NIR instru-ments, the light is divided into two beams whose relative pathsare varied by use of a moving optical element. The beams arerecombined to produce an interference
30、 pattern that contains allof the wavelengths of interest. The interference pattern ismathematically converted into spectral data using Fouriertransform. FT interferometer optics provide complete spectrawith very high wavelength resolution. FT signal averaging alsoprovides higher signal-to-noise rati
31、os in general than can beachieved with other types of instruments.6.4 Sampling SystemDepending upon the applications,several different sampling systems can be used in the labora-tory or for on-line instruments, or both.6.4.1 CuvetteQuartz or glass cuvettes with fixed or ad-justable path lengths can
32、be used in the laboratory.6.4.2 Flow-Through CellThis type cell can be used forcontinuous or intermittent monitoring of liquid sample.6.4.3 Probes:FIG. 1 Schematic of a Near-IR SystemD63420826.4.3.1 Transmission ProbeTransmission probes com-bined with optic fibers are ideal for analyzing clear liqui
33、ds,slurries, suspensions, and other high viscosity samples. Lowabsorptivity in the NIR region permits sampling pathlengths ofup to 10 cm.6.4.3.2 Immersion ProbeThe immersion system uses abi-directional optic fiber bundle and variable pathlength probefor sample measurements. Radiation from the source
34、 is trans-mitted to the sample by the inner ring of fibers, and diffusetransmitted radiation is collected by the outer ring of fibers fordetection.6.4.3.3 Attenuated Total Reflection (ATR) ProbeAttenuated total reflection occurs when an absorbing medium(the sample) is in close contact with the surfa
35、ce of a crystalmaterial of higher refractive index. At an optimized angle, theNIR beam reflects internally along the crystal faces, penetrat-ing a few microns into the sample surface, where selectiveabsorption occurs. The resulting spectrum is very close to theconventional transmission spectrum for
36、the sample. There aremany designs of ATR plates and rods for specific applications.Single or multiple reflection units are available. ATR samplingaccessories are available for the laboratory and, in the form offiber optic probes, can be used for on-line analysis. This is anadvantage when handling vi
37、scous liquids and highly absorbingmaterials.6.5 SoftwareThe ideal software has the following capa-bilities:6.5.1 The capability to record all sample identification andspectral data accurately and to access the reference data,6.5.2 The capability to record the date and time of day thatall spectra and
38、 files were recorded or created,6.5.3 The capability to move or copy spectra, or both, fromfile to file,6.5.4 The capability to add or subtract spectral data, and toaverage spectra,6.5.5 The capability to perform transformations of log l/Roptical data into derivatives, or other forms of mathematical
39、treatment, and to reverse the transformation,6.5.6 The capability to compute multiple linear regression(MLR), principal component regression (PCR), and partialleast squares regression (PLS),6.5.7 The capability to store PCR or PLS loading, weights,scores or other desirable data, and to display these
40、 data forsubsequent examination and interpretation,6.5.8 The capability to enable the operator to evaluate thecalibration model by computing the standard error of validation(SEV), coefficient of regression, and the root mean squaredeviation (RMSD), and to display various plots,6.5.9 The capability t
41、o perform cross-validation automati-cally,6.5.10 The capability to identify an outlier(s), and6.5.11 The capability to develop and save regressionequations and analyze a sample to calculate a hydroxylnumber.6.6 Software PackagesMost NIR instruments providenecessary software for collecting and modeli
42、ng data. Severalnon-instrumental companies also supply chemometric softwarepackages that can be used to analyze NIR data.7. Near-IR Spectral Measurements7.1 NIR spectral measurements are based on Beers law,namely, the absorbance of a homogeneous sample containingand absorbing substance is linearly p
43、roportional to the concen-tration of the absorbing species. The absorbance of a sample isdefined as the logarithm to the base ten of the reciprocal of theTransmittance (T):A 5 log101/T! (1)where:T = the ratio of radiant power transmitted by the sample tothe radiant power incident on the sample.7.1.1
44、 For most types of instrumentation, the radiant powerincident on the sample cannot be measured directly. Instead, areference (background) measurement of the radiant power ismade without the sample being present in the light beam.7.1.2 A measurement is then conducted with the samplepresent, and the r
45、atio, T, is calculated. The backgroundmeasurement can be conducted in a variety of ways dependingon the application and instrumentation. The sample and itsholder can be physically removed from the light beam and abackground measurement made on the “empty beam”. Thesample holder (cell) can be emptied
46、, and a backgroundmeasurement taken for the empty cell. The cell can be filledwith a material that has minimal absorption in the spectralrange of interest, and the background measurement taken.Alternatively, the light beam may be split or alternately passedthrough the sample and through an empty bea
47、m, and emptycell, or a background material in the cell.7.1.3 The particular background referencing scheme that isused can vary among instruments, and among applications. Thesame sample background referencing scheme must be em-ployed for the measurement of all spectra of calibrationsamples, validatio
48、n samples, and unknown samples to beanalyzed. Any differences between instrument conditions usedfor referencing and measurement are to be minimized.7.2 Traditionally, a sample is manually brought to theinstrument and placed in a suitable optical container (a cell,vial, or cuvette with windows that t
49、ransmit in the region ofinterest). Alternatively, transfer pipes can continuously flowliquid through an optical cell in the instrument for continuousanalysis. With optical fibers, the sample can be analyzedremotely from the instrument. Light is sent to the samplethrough an optical fiber or fibers and returned to the instrumentby means of another fiber or group of fibers. Instruments havebeen developed that use a single fiber to transmit and receivethe light, as well as use bundles of fibers for this purpose.Detectors and light sources external to the instrument can
