1、Designation: D6342 12Standard Practice forPolyurethane Raw Materials: Determining Hydroxyl Numberof Polyols by Near Infrared (NIR) Spectroscopy1This standard is issued under the fixed designation D6342; the number immediately following the designation indicates the year oforiginal adoption or, in th
2、e case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 polyols u
3、sing 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. Final
4、ly, 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 The values stated in SI units are to be regarded asstandard. No other
5、units of measurement are included in thisstandard.1.5 This standard does not purport to address all of thesafety 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 r
6、egulatory limitations prior to use.NOTE 1This standard is equivalent ISO 15063.2. Referenced Documents2.1 ASTM Standards:2D883 Terminology Relating to PlasticsD4274 Test Methods for Testing Polyurethane Raw Materi-als: Determination of Hydroxyl Numbers of PolyolsD4855 Practice for Comparing Test Met
7、hods (Withdrawn2008)3E131 Terminology Relating to Molecular SpectroscopyE168 Practices for General Techniques of Infrared Quanti-tative AnalysisE222 Test Methods for Hydroxyl Groups Using AceticAnhydride AcetylationE275 Practice for Describing and Measuring Performance ofUltraviolet and Visible Spec
8、trophotometersE456 Terminology Relating to Quality and StatisticsE1655 Practices for Infrared Multivariate QuantitativeAnalysisE1899 Test Method for Hydroxyl Groups Using Reactionwith p-Toluenesulfonyl Isocyanate (TSI) and Potentiomet-ric Titration with Tetrabutylammonium Hydroxide2.2 ISO Standard:I
9、SO 15063 PlasticsPolyols for use in the production ofpolyurethanesDetermination of hydroxyl number byNIR spectroscopy3. Terminology3.1 DefinitionsTerminology used in this practice followsthat defined in Terminology D883. For terminology related tomolecular spectroscopy methods, refer to Terminology
10、E131.For terms relating to multivariate analysis, refer to PracticeE1655.3.2 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 cor
11、relate 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 estimate of theirhydroxyl numbers.4.2 Multilinear regression (MLR),
12、 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 of the calibration model. Outliers can include1This practice is un
13、der the jurisdiction ofASTM Committee D20 on Plastics andis the direct responsibility of Subcommittee D20.22 on Cellular Materials - Plasticsand Elastomers.Current edition approved Aug. 1, 2012. Published September 2012. Originallyapproved in 1998. Last previous edition approved in 2008 as D6342 - 0
14、8. DOI:10.1520/D6342-12.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The last approved version of this hi
15、storical standard is referenced onwww.astm.org.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1high leverage samples and samples whose hydroxyl numbersare inconsistent w
16、ith 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 model with the refere
17、nce method.4.5 Statistical expressions 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 practi
18、ce 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 calibration procedure isstarted. Chemical structure, interferences, any nonlinearities,the effect of te
19、mperature, 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 which cannot be ad-equately controlled.5.2.2 Calibrations are generally considered valid only for
20、the 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 of the measured hydroxyl number.Procedures used for transferring calibrations between instru-m
21、ents are problematic and are to be 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 num
22、bers 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 composition as used for thecalibration set.Asignificant change in composition or contami-nants can also a
23、ffect the results. Outlier detection, as discussedin Practices E1655, 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 thisstanda
24、rd. 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 sources forNIR instruments. Most of the detectors used for NIR aresolid-state semiconductors. PbS,
25、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 Monochromator InstrumentGrating monochromatorinstruments, often called “dispersive” instruments, are com-monl
26、y 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 a given time.6.3.2 Filter-Wheel InstrumentIn this type of NIRinstrument, one or several narrow band filters are
27、 mounted ona turret wheel so that the individual wavelengths are presentedto a single 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 TeO2crys
28、tal is used in a noncollinear configura-tion in which acoustic and optical waves move through thecrystal at different angles. Variations in the acoustic frequencycause the crystal lattice spacing to change. That in turn causesthe crystal to act as a variable transmission diffraction gratingfor one w
29、avelength. 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 Diode (LED) InstrumentEach wave-length band is produced by a different diode. The majoradvantages of t
30、he 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-NIRinstruments, the light is divided into two beams whose relativepaths are varied by use of a moving optical element. The beamsare recomb
31、ined to produce an interference pattern that containsall of 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 alsoprovi
32、des 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 the labora-tory or for on-line instruments, or both.6.4.1 CuvetteQuartz or glass cuvettes with fixed o
33、r adjust-able pathlengths can be used in the laboratory.6.4.2 Flow-Through CellThis type of cell can be used forcontinuous or intermittent monitoring of liquid sample.6.4.3 Probes:FIG. 1 Schematic of a Near-IR SystemD6342 1226.4.3.1 Transmission ProbeTransmission probes com-bined with optic fibers a
34、re 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 fiber bundle and variable pathlength probefor sample measu
35、rements. 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 reflection occurs when an absorbing medium(the sample)
36、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 into the sample surface, where selectiveabsorption occurs. The resulting spectrum is very close to theconve
37、ntional 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 are available for the laboratory and, in the form offiber optic probes, can be used for on-line analysis. Th
38、is is anadvantage when handling viscous 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 a
39、nd 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 data, and toaverage spectra,6.5.5 The capability to perform transformations of log l/Roptical data into derivativ
40、es, 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 partialleast squares regression (PLS),6.5.7 The capability to store PCR or PLS loading, weights,scores or other de
41、sirable 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 standard error of validation(SEV), coefficient of regression, and the root mean squaredeviation (RMSD), and to display va
42、rious plots,6.5.9 The capability to perform cross-validationautomatically,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 so
43、ftware for collecting and modeling 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 containingan ab
44、sorbing 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 theTransmittance (T):A 5 log101/T! (1)where:T = the ratio of radiant power transmitted by the sample tothe radiant pow
45、er 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 the radiant power ismade without the sample being present in the light beam.7.1.2 A measurement is then conducted wi
46、th the samplepresent, and the ratio, 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”. Thesamp
47、le holder (cell) can be emptied, 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
48、sample and through an empty beam, 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 o
49、f calibrationsamples, validation 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 transmit 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.
