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FT-NIR SPECTROSCOPY application note The Determination of OH Number in Polyols Using FT-NIR Spectroscopy Summary NIR spectroscopy is an invaluable tool for the quantitative analysis of a wide range of chemical compounds. In combination with a number of chemometric methods, the technique provides a fast, non-destructive route to the determination of physical and chemical properties. This note describes the use of FT-NIR spectroscopy in a typical polyol analysis with an outline of the development of the quantitative method used for the application. Introduction Polyols are long-chain polymers which contain alcohol functional groups and are produced via reactions involving organic oxides, acids and multi-functional alcohols. A wide range of products including, surfactants, foams, paint additives, and adhesives are manufactured using polyols. The production of polyurethanes, for instance, involves polyol intermediates. Polyol products are typically produced via batch reactor processes held at high temperatures (>250˚C). Current analysis of the polyols produced normally take the form of back titrations. The resultant OH number is an average value over a number of titrations. This analysis generally takes place in an off-site laboratory, often far removed from the conditions present at the production site. The sample is extracted from the bulk, and the production process halted while the analysis is completed; a process that can take several hours. Care must be taken in handling the chemicals involved, especially when analyzing samples at elevated temperatures. The possibility of moisture being absorbed from the air is highly detrimental to the analysis of the OH value. A system whereby the sample must be heated is applied if the sample is a solid or highly viscous liquid. Even when all these factors have been minimized or taken into account, the process still depends on the analyst. The current method for determining OH number is expensive, time consuming, and prone to human error. A typical NIR based application is faster, more precise and reliable than other methods. It also reduces the need to handle potentially hazardous substances. The bands in the NIR region (ca. 15000 – 3000 cm-1, 667 – 3333 nm) are primarily overtones and combination bands normally associated with C-H, N-H and O-H bonds. Since organic polymers are composed of carbon, hydrogen, nitrogen and oxygen atoms, the NIR spectra of polymers feature sharp, strong absorbance bands. Polymer manufacturers can use NIR spectroscopy to perform analyses on the production site. This has obvious advantages in the area of quality assurance. NIR analysis is also capable of process control as the analysis of intermediates occurs in real-time. Since many polyol reactions are irreversible, the analysis of such intermediates by NIR spectroscopy provides a commercially valuable tool. A typical NIR method development for OH determination consists of first assembling a set of preanalyzed samples which must span the OH range for which the calibration is intended. The number of samples used depends on the OH range and accuracy required, but for most applications in the range ca. 20 – 200 OH approximately 20 samples are adequate for calibration.
Once a set of reference samples is obtained, it is advisable to divide these into two sets; a set for calibration, and a set to test the calibration often referred to as the validation set. The suitability of the samples used in the calibration set is particularly important since it must closely match the product being manufactured at the production site. A calibration set must contain enough samples to enable the estimation of the calibration constants, and the range of the samples should be representative of the range of future samples. A calibration, based on ‘ideal’ samples prepared in a completely stable environment, may give very small errors when used for similar samples. However, when the calibration is used to produce a value for a real product, the accuracy is often degraded. Furthermore, a tradeoff exists between calibration sets covering a very wide range of OH number and the accuracy with which OH number can be predicted. Therefore, it has been found that the accuracy increases when using a method calibrated over a more restricted (ca. 50) OH range. The PLS (Partial Least Squares) method available in the Perkin-Elmer QUANT+ TM software uses the NIR spectra recorded to derive a calibration. This equation is then used to predict the value of the constituent of interest of future samples using their NIR spectra. An application that is analyzed by using spectroscopy is more precise than when measured by other methods. However, the accuracy of the method in predicting the true value of the future samples is dependant upon the accuracy of the reference method. It is therefore important to stress that the steps involved in setting up the calibration are crucial to the success of the NIR application. This is particularly true for the collection of suitable calibration standards, and the analysis of the calibration set by titration. This step should be carried out with an objective of producing the greatest possible accuracy. A suggestion for achieving this accuracy is to perform the titration in duplicate or triplicate. However, it is more important to have as many samples in the calibration set as possible. In comparison, it is better to collect twice as many calibration samples than it is to perform the reference analysis of the original calibration standards in duplicate. The most ideal, though costly and time consuming, situation would allow for the inclusion of a large set of calibration standards, with each sample analyzed in duplicate or triplicate. QUANT+ software was used to derive a calibration matrix between the calibration spectra and their Figure 1: Extremes of calibration set. reference OH values. The technique of PLS extracts those spectral contributions which are correlated with OH value, and eliminates the need to select specific wavelengths for the calibration. As previously stated, the near infrared region consists of many overlapping bands that contain overtone and combination bands involving the absorptions of mainly OH, NH and CH bonds. The bands of interest in the determination of OH value are due to the OH first overtone at 7150 – 6670 cm-1 (1399 – 1499 nm), the OH combination bands of 5260 – 4760 cm-1 (1900 – 2100 nm), and moisture content changes: ~5155 cm-1 (1940 nm). Experimental Fourteen standards of known OH value were supplied to serve as a calibration set. A further eight samples were also supplied to act as an independent validation set. These were used to compare the reference values with those values predicted by the calibration equation. All samples had been analyzed previously in triplicate by the reference ‘wet’ chemistry method (titration). The infrared spectra of all samples were recorded on a PerkinElmer FT-NIR Spectrometer. The samples were scanned in an NIR quartz transmission cell through a range of 10000 – 4000 cm -1 at a resolution of 8 cm -1 and strong apodization. All spectra were recorded in ratio mode using the shuttle accessory. The shuttle automatically collects alternate background and sample scans to eliminate the effects of water vapor in the air. A pathlength of 5 mm was considered to provide useful absorbance values. The cuvettes were cleaned with acetone and dried before refilling with the next sample. Two calibrated methods were built using the reference data and the spectra collected from the FT-NIR Spectrometer. These were setup using the QUANT+ software. Both methods used the range 9000 – 4528 cm -1 with an upper threshold of 1.5 A. The offset option was employed to minimize baseline drift
Page 1: A plot of Estimated value versus Sp

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