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Near-infrared spectroscopy has been widely used in the polymer industry. Compared with traditional methods such as wet chemistry and chromatography, near-infrared spectroscopy has considerable advantages in process and quality control applications. Its basic advantages include the low to zero cost of consumables such as solvents, chromatography columns, and reagents; real-time analysis— -Generally less than 10 seconds of measurement time; each analysis has multiple components; eliminates sample preparation time; and eliminates many sources of system errors.
This article will introduce three FT-NIR polymer applications: 1) Online analysis of hydroxyl value of polyether polyol; 2) Real-time monitoring of isocyanate number during polyurethane reaction; 3) Offline quality control of styrene percentage in styrene copolymer .
Polyurethane polymers are most commonly formed by the reaction of diisocyanates or polyisocyanates with polyols. The isocyanates and polyols used in the manufacture of polyurethanes usually contain two or more functional groups per molecule. Different types of polyols and isocyanates can form different polyurethane products with different properties and end uses. The hydroxyl value of the polyol and the isocyanate number of the isocyanate are not only very important QC parameters for incoming materials, but also very important reaction control parameters. The traditional analysis method of hydroxyl value and isocyanate value is titration method, which is very time-consuming. Therefore they cannot be used for real-time process monitoring and adjustment. They also involve solvents and reagents that are not environmentally friendly. In contrast, near infrared (NIR) spectroscopy is fast, non-destructive, requires no sample preparation, and can be applied to offline, online, or online measurements.
Styrene butadiene copolymer (SBC) and its blends have been widely used in our daily lives, such as transparent disposable drinking cups, packaging films and toys. The styrene/butadiene ratio or styrene concentration in the SBC will affect its performance and end use. The styrene concentration in the copolymer can be measured by NMR or chromatography. Although these methods provide valuable information, they are time-consuming and costly. Compared with these methods, the NIR method is fast and easy to implement. Technicians without knowledge of spectroscopy can perform the analysis.
In this article, the focus is on the Fourier Transform Near Infrared (FT-NIR) analysis of the hydroxyl value in the polyether polyol, the number of isocyanates in the polyurethane reaction process, and the styrene concentration in the SBC pellets.
The polyether polyol samples came from the QC laboratory of the manufacturing plant and were analyzed using titration methods.
Polyurethane samples are regularly taken out of the reaction tank, and several batches of samples are collected and analyzed using traditional titration methods.
Styrene butadiene copolymer (SBC) particles with different styrene concentrations were obtained from customers, and the reference value of styrene concentration was obtained using the NMR method.
A QuasIR™ 4500 (Galaxy Scientific, Nashua, NH, USA) equipped with a temperature-controlled sample chamber (Figure 1.) was used to collect FT-NIR spectra of polyether polyol samples. The sample is measured in an 8 mm disposable glass bottle at 70°C with a resolution of 8 cm-1 and 32 scans.
The FT-NIR spectrum of the polyurethane sample was collected using QuasIR™ 2000 (Galaxy Scientific, Nashua, NH, USA) (Figure 2.) and a process transmission probe (5mm optical path) in the range of 4000-10000cm-1 resolution is 8cm-1 and 20 scans. The spectrum is recorded regularly and named after the recording time.
Use QuasIR™ 3000 (Galaxy Scientific, Nashua, NH, USA) equipped with a sample rotator attachment to collect FT-NIR spectra of SBC particles. Put the sample in a 98 mm cup with a low OH quartz window, and then load it on the sample rotator, which is eccentrically mounted on the 2 mm sample window of the integrating sphere. This increases the amount of sample to be measured and is suitable for uneven samples. Each sample was measured 3 times with a resolution of 8 cm-1 and 64 scans. Reload the sample between measurements.
Spectral Sage™ software is used for data collection, and Spectral Sage™ PLS software package is used for calibration development.
Figure 1. Galaxy Scientific QuasIR™4000 Fourier Transform Infrared Spectrometer with temperature control (optional) sample chamber and integrating sphere channel.
Figure 2. Galaxy Scientific QuasIR™2000 Fourier transform infrared spectrometer with standard SMA connector.
Figure 3. Galaxy Scientific QuasIR™3000 Fourier Transform Infrared Spectrometer with integrating sphere and optional sample rotator.
Collect samples of polyether polyol resin in a customer’s QC laboratory. The hydroxyl value is distributed around two levels, the low is around 38mg KOH/g, and the high is around 56mg KOH/g. To avoid any influence of temperature on the OH band, all sample spectra are measured at 70°C. Figure 4 shows the vector normalized spectrum of polyether polyol. A significant difference between the samples with high and low OH values can be observed. At 7000cm-1, the first overtone of OH stretch.
Figure 4. Normalized FT-NIR spectrum of a polyether polyol sample.
A total of 55 samples were used to develop partial least squares (PLS) OH calibration using the Spectral Sage™ PLS software package. The spectrum is preprocessed using the first derivative, and two spectral regions are used for calibration: 4575-5060cm-1 and 6100-9000cm-1. The PLS OH calibration was developed using 3 factors, and the accuracy of the calibration is expressed as an estimated root mean square error (RMSEE) of approximately 0.28 mg KOH/g (Figure 5).
Leave-one-out cross-validation is used to evaluate the calibration, and the accuracy is expressed as the root mean square error (RMSECV) of the cross-validation, which is approximately 0.32mg KOH/g (Figure 6).
Figure 5. PLS calibration curve for OH value. R2=99.91, RMSEE=0.28mg KOH/g.
Figure 6. Cross-validation results of polyether polyol samples. R2 = 99.88, RMSECV = 0.32 mg KOH/g.
Take out the polyurethane sample from the reaction tank and analyze it by titration. The five spectra collected around the sampling time are averaged and used to develop calibration. A total of 81 samples in 10 batches were used, and the remaining isocyanate number was in the range of 5.5% to 11.5%, which was used to develop a partial least squares (PLS) calibration model.
Figure 7 shows five normalized polyurethane spectra with different numbers of isocyanates (NCO). Vector normalize the spectrum to remove baseline changes. The obvious spectral change observed near 7000cm-1 is related to the first overtone of OH stretching. As the reaction progresses, more isocyanate and polyol are consumed.
Figure 7. Normalized FT-NIR spectra of polyurethane samples with various residual NCO concentrations.
Figure 8. Validation results of samples from 3 independent batches. RMSEP = 0.14%.
Figure 9. Normalized FT-NIR spectra of styrene butadiene copolymer samples with different styrene concentrations.
Figure 10. PLS calibration curve of styrene content in SBC pellets. R2 = 99.62, RMSEE = 0.25%.
Vector normalize the spectrum and use the spectral range of 4800cm-1 to 9000cm-1 for PLS calibration. Leave-one-out cross-validation is used to evaluate calibration. The NIR spectrum has a high correlation with the number of isocyanates (R2=98.36), and the accuracy expressed by the root mean square error (RMSECV) of cross-validation is about 0.19%.
Using 12 samples from the other 3 batches as an independent test set to verify the calibration, the predicted root mean square error is about 0.14% (Figure 8), which is consistent with the RMSECV result.
Obtained 30 bags of SBC pellets from the customer, with a styrene concentration between 65% and 82%. The vector normalized spectrum is shown in Figure 6. The shoulder strap around 8700cm-1 varies according to the styrene concentration. Since only 30 particle samples are available, all samples are used as calibration samples. The spectral range of 6090-9000cm-1 is used for calibration, and the first derivative and multiplicative scattering correction are used as data preprocessing methods. The obtained FT-NIR spectrum shows a high correlation with the styrene content (R2=99.62), and the accuracy expressed by the root mean square estimation error (RMSEE) is about 0.25% (Figure 10).
Leave-one-out cross-validation is used to evaluate the calibration, and the accuracy of the root-mean-square error (RMSECV) of the cross-validation is approximately 0.29% (Figure 11).
Figure 11. Cross-validation results of SBC particles. RMSECV = 0.29%.
The study shows that FT-NIR can be used as an online, online or offline quality control tool for polymer products. Results can be obtained in a few seconds, without the need for an experienced operator.
This information is derived from materials provided by Galaxy Scientific Inc. and has been reviewed and adapted.
For more information on this source, please visit Galaxy Scientific Inc.
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