1、Designation: D 6342 98 (Reapproved 2003)Standard 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 oforigina
2、l adoption or, in 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. Scope1.1 This standard covers a practice for the determination ofhydroxyl
3、numbers of polyols 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 a
4、lso described. Finally, 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 t
5、hesafety concerns, 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 Do
6、cuments2.1 ASTM Standards: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 Practice for General Technique
7、s of Infrared Quanti-tative AnalysisE 222 Hydroxyl Groups Using Acetic Anhydride Acetyla-tionE 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, Multiva
8、riate, QuantitativeAnalysisE 1899 Hydroxyl Groups by Toluenesulfonyl Isocyanate3. Terminology3.1 DefinitionsTerminology used in this practice followsthat defined in Terminology D 883. For terminology related tomolecular spectroscopy methods, refer to Terminology E 131.For terms relating to multivari
9、ate 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. Summary of Practice4.1 Multivariate mathematics is applied to correlate the NIRabsorbance values for
10、 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 estimate of theirhydroxyl numbers.4.2 Multilinear regression (MLR), principal componentsregression (PC
11、R), 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 of the calibration model. Outliers may includehigh leverage samples and samples whose hydroxyl number
12、sare 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 statisti-cally compared to the reference hydroxyl number for this set totest for agreement of the mo
13、del with the reference method.4.5 Statistical expressions are given for calculating theprecision and bias of the NIR method relative to the referencemethod.1This practice is under the jurisdiction of ASTM Committee D20 on Plastics andis the direct responsibility of Subcommittee D20.22 on Cellular Ma
14、terialsPlasticsand Elastomers.Current edition approved November 1, 2003. Published December 2003.Originally approved in 1998. Last previous edition approved in 1998 as D 6342 -98.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org.
15、 For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5. Significance and Use5.1 General Utility:5.1.1 It is necessary to
16、 know the hydroxyl number ofpolyols in order to formulate polyurethane systems.5.1.2 This practice is suitable for research, quality 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 calibra
17、tion procedure isstarted. Chemical structure, interferences, any nonlinearities,the effect of temperature, and the interaction of 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 w
18、hich can not beadequately controlled.5.2.2 Calibrations are generally considered valid only forthe specific NIR instrument used to generate the calibration.Using different instruments (even when made by the samemanufacturer) for calibration and analysis can seriously affectthe accuracy and precision
19、 of the measured hydroxyl number.Procedures used for transferring calibrations between instru-ments are problematic and should be utilized with cautionfollowing the guidelines in Section 16. These proceduresgenerally require a completely new validation and statisticalanalysis of errors on the new in
20、strument.5.2.3 The analytical results are statistically valid only for therange of hydroxyl numbers used in the calibration. Extrapola-tion to lower or higher hydroxyl values can increase the errorsand degrade precision. Likewise, the analytical results are onlyvalid for the same chemical compositio
21、n as used for thecalibration set. A significant change in composition or contami-nants can also affect the results. Outlier detection, 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 co
22、mplete description of all applicabletypes of NIR instrumentation is beyond the scope of thisstandard. Only a general outline is given 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 s
23、ources forNIR instruments. Most of the detectors used for NIR aresolid-state semiconductors. PbS, PbSe, and InGaAs detectorsare most commonly used.6.3 Light DispersionSpectrophotometers can be classifiedbased on the procedure by which the instrument accomplisheswavelength selection.6.3.1 Monochromat
24、or InstrumentGrating monochromatorinstruments, often called “dispersive” instruments, are com-monly used in the laboratory and for process 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.
25、3.2 Filter-Wheel InstrumentIn this type of NIR instru-ment, one or several narrow band filters are mounted on a turretwheel so that the individual wavelengths are presented to asingle detector sequentially.6.3.3 Acoustic Optic Tunable Filter (AOTF) InstrumentThe AOTF is a continuous variant of the f
26、ixed-filter photometerwith no moving optical parts for wavelength selection. Abirefringent TeO2crystal is used in a noncollinear configura-tion in which acoustic and optical waves move through thecrystal at different angles. Variations in the acoustic frequencycauses the crystal lattice spacing to c
27、hange. That in turn causesthe crystal to act as a variable transmission diffraction gratingfor one wavelength. The main advantage of 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 Di
28、ode (LED) InstrumentEach wave-length band is produced by a different diode. The majoradvantages of the system are its small size and 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
29、 into two beams whose relative pathsare varied by use of a moving optical element. The beams arerecombined to produce an interference pattern that contains allof the wavelengths of interest. The interference pattern ismathematically converted into spectral data using Fouriertransform. FT interferome
30、ter optics provide complete spectrawith very high wavelength resolution. FT signal averaging alsoprovides higher signal-to-noise ratios 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
31、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 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:6.4.3.1 Transmission
32、 ProbeTransmission probes com-bined with optic fibers are ideal for analyzing clear liquids,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 fib
33、er bundle and variable pathlength probefor sample measurements. Radiation from the source 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
34、reflection occurs when an absorbing mediumFIG. 1 Schematic of a Near-IR SystemD 6342 98 (2003)2(the sample) is in close contact with the surface of a crystalmaterial of higher refractive index. At an optimized angle, theNIR beam reflects internally along the crystal faces, penetrat-ing a few microns
35、 into the sample surface, where selectiveabsorption occurs. The resulting spectrum is very close to theconventional transmission spectrum for the sample. There aremany designs of ATR plates and rods for specific applications.Single or multiple reflection units are available. ATR samplingaccessories
36、are available for the laboratory and, in the form offiber optic probes, can be used for on-line analysis. This is anadvantage when handling viscous liquids and highly absorbingmaterials.6.5 SoftwareThe ideal software should have the followingcapabilities:6.5.1 The capability to record all sample ide
37、ntification 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 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
38、 data, and toaverage spectra,6.5.5 The capability to perform transformations of log l/Roptical data into derivatives, or other forms of mathematicaltreatment, and to reverse the transformation,6.5.6 The capability to compute multiple linear regression(MLR), principal component regression (PCR), and
39、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 data forsubsequent examination and interpretation,6.5.8 The capability to enable the operator to evaluate thecalibration model by computing the standar
40、d error of validation(SEV), coefficient of regression, and the root mean squaredeviation (RMSD), and to display various plots,6.5.9 The capability to perform cross-validation automati-cally,6.5.10 The capability to identify an outlier(s), and6.5.11 The capability to develop and save regressionequati
41、ons and analyze a sample to calculate a hydroxylnumber.6.6 Software PackagesMost NIR instruments providenecessary software for collecting and modeling data. Severalnon-instrumental companies also supply chemometric softwarepackages that can be used to analyze NIR data.7. Near-IR Spectral Measurement
42、s7.1 NIR spectral measurements are based on Beers law,namely, the absorbance of a homogeneous sample containingand absorbing substance is linearly proportional 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 t
43、heTransmittance (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 For most types of instrumentation, the radiant powerincident on the sample cannot be measured directly. Instead, areference (background) measurement of
44、 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 ratio, T, is calculated. The backgroundmeasurement may be conducted in a variety of ways dependingon the application and instrumentation. The sample and
45、itsholder may be physically removed from the light beam and abackground measurement made on the “empty beam”. Thesample holder (cell) may be emptied, and a backgroundmeasurement may be taken for the empty cell. The cell may befilled with a material that has minimal absorption in thespectral range of
46、 interest, and the background measurementmay be taken. Alternatively, the light beam may be split oralternately passed through the sample and through an emptybeam, and empty cell, or a background material in the cell.7.1.3 The particular background referencing scheme that isused may vary among instr
47、uments, and among applications.The same sample background referencing scheme must beemployed for the measurement of all spectra of calibrationsamples, validation samples, and unknown samples to beanalyzed. Any differences between instrument conditions usedfor referencing and measurement should be mi
48、nimized.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 transmit in the region ofinterest). Alternatively, transfer pipes can continuously flowliquid through an optical cell in the instrument for c
49、ontinuousanalysis. 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 alsobe used, in which case only one fiber or bundle is needed. Theappropriate grade of optical fibers for use in the NIR rangeneeds to be
