采用近红外技术测定多元醇的羟值

Near-Infrared Spectroscopy Application Note NIR-6 Near-infrared analysis of polyols This Application Note describes a fast， nondestructive， and reliable NIRSmethod for the determination of the hydroxyl number in polyols. Resultsare available in real-time， for which reason NIRS is highly suited forin-process quality control. Second-derivative spectra and linear least-squares regression provided hydroxyl numbers that comply with thoseobtained by titration. Introduction Polyols are long chain， liquid polymers， which can beproduced from the polymerization of organic oxides(e.g.， ethylene oxide， propylene oxide). Polyols are usedin many manufacturing processes to form polymericmaterials such as polyurethanes， which as a foam areused for cushioning and packaging， automobile seats， inhousehold furniture， etc. The end use for the materialsproduced using polyols are dependent on the molecularweight of the polyols used. Molecular weight is inverselyrelated to the number of hydroxyl end groups present ina given amount of polyol. Hydroxyl number (number ofhydroxyl end groups per gram of sample) is the typicalmeasure for determination of molecular weight forpolyols. The laboratory analysis involves a titration of thehydroxyl end groups， which is relatively time consuming.Near-infrared spectroscopy offers quick，nondestructive method of determining the hydroxylnumber in polyols. The analysis is performed in a matterof minutes on the whole sample without any samplepreparation or modification. Experimental Samples of polypropylene glycol， ranging in hydroxylnumber from 26 to 280 were used in this analysis. TheNIR spectra were measured in transmission using a FOSSModel e6500)spectrophotometerfrom 1100 to2500 nm. Since this instrument is not available anymore，the NIRS XDS Transmission Optiprobe Analyzer withsample heater or the NIRS XDS RapidLiquid Analyzer isrecommended. The samples were placed into 4 mmquartz cuvettes for this analysis， and the temperaturewas held at 32 ± 0.1 ℃ since the hydroxyl band istemperature sensitive. Sample scans were generatedfrom 32 co-added scans for each sample and referencedagainst 32 scans of air. Results and discussion The absorbance (log 1/T) spectra for the polyol samplesare shown in Figure 1. The variation of the O-Hcombination band near 2070 nm and that of the firstO-H overtone near 1460 nm are clearly evident fromthis figure. Figure 2 shows the second derivative spectrafor these samples， which was calculated to compensatefor any baseline variations which often occur inanalyzing 《real world》 samples. Figure 1Polypropylene Glycol Figure 22nd Der. Polypropylene Glycol It is observed in Figure 2 that variation near 2070(hydroxyl combination band) and at 1460 nm (hydroxylovertone band) are increasing with increasing hydroxylnumber. To demonstrate the ability to (quantitate hydroxynumber by NIR， a linear least-squares regression wasperformed， using the second derivative spectra and theknown hydroxyl numbers for each sample. A model wasdeveloped at 2070 nm， yielding a correlation (R) of0.999 and a standard error of calibration (SEC) of 4.20OH#S. Figure 3 shows the scatterplot of NIR-calculated versuslaboratory-reported results. Method description Figure 3Lab vs. Calculated Percents Table 1： Typical measurements for polyols and polyolsderivatives Hydroxylnumber Primary hydroxyl number Secondary hydroxyl number EO/PO Ratio Residual oxides Moisture Ester-based polyols Acid number Hydroxyl number Moisture Substituted polyols Primary amines Secondary amines Polyurethanes Isocyanate levels Hydroxyl number The NIR results agree to within 5 hydroxyl numbers ofthe lab data . The achievable error by the NIR analysis islimited by the accuracy of the lab analysis， which forsamples with hydroxylyl numbers in the hundreds istypically about ±5. To increase the accuracy for those samples below 100hydroxyl numbers (which have better accuracy in the labanalyses) a separate calibration equation for thesesamples is developed. Conclusions NIR spectroscopy can be used to accurately and rapidlydetermine the hydroxyl number in polyols. Other NIRmeasurements of interest for polyol manufacturers arepolyols made from propylene and ethylene oxides，resulting in a《mixed polyols》. NIR has successfullyanalyzed the hydroxyl number in mixed polyols， as wellas the amount of primary hydroxyl (attached to anethylene unit on the polyol) and secondary hydroxyl (ona propylene unit) groups. The methyl substitution rating of the polymer backbonecan be determined using the ratio of methyl absorption(due to the propylene portion)， to that of methyleneabsorptions (due to both ethylene andpropyleneportions of the polymer). Other successful NIR analyses include moisture， residualoxide， and primary and secondary amine levels insubstituted polyols. NIR therefore appears to be uniquely suited foranalyzing the hydroxyl end groups of polyols (both formolecular weight information， as well as differentiationof the type of end group) and characterization of thebackbone of the polyol. The analysis requires no samplehandling， temperature correction or compensation. Anumber of analytical parameters can be monitoredresulting in better control of the process of producingthese materials. co