A Changing Regulatory Environment
Diisocyanates are used in a variety of products, for instance as a precursor for adhesives and coatings. In theory the isocyanate should be fully reacted and pose little risk to the end consumer. However, if the reaction is incomplete, exposure to unreacted isocyanate can lead to serious health risks, including asthma, lung damage and even death.
As a result, authorities around the world are increasing the regulations around the use of such chemicals, including Methylene Diphenyl Diisocyanate (MDI). The United States Environmental Protection Agency (EPA) is currently collecting data on the use of MDI, within products manufactured and sold within the USA and is considering implementing a requirement to routinely monitor for the presence of unreacted material. Since 2010 the European Union has banned the sale of products containing greater than 0.1% MDI unless stringent conditions are met [2]. From 24th August 2023, these restrictions will be broadened to include all diisocyanates with mandatory training required for anyone who handles such products at concentrations above 0.1 wt%, and a ban on their sale to anyone who has not completed such training.
It is therefore critical that companies handling such chemicals develop methods for ensuring that any products produced with diisocyanates do not contain greater than 0.1 wt% when they leave the factory. FTIR is an ideal tool for the accurate quantification of isocyanate compounds. Isocyanates exhibit a peak in a part of the spectrum that is usually empty, making FTIR ideally suited to the analysis of these compounds. We have previously published a method for the analysis of lamination adhesives using the Arrow consumable ATR technology [3]. Here we will investigate the use of the Pearl Liquid transmission accessory to analyse low concentrations of isocyanates.
Isocyanide Calibration plot
In order to build a calibration plot, samples of known concentrations of isocyanate in oil were measured in a 100 μm pathlength Oyster cell, within the Pearl accessory and the FTIR spectrum obtained. The Pearl enables fast sample turnaround times, with consistent reproducible pathlengths [4], making it perfect for quantification measurements. The spectra for samples between 0 and 1% isocyanate are shown in Figure 1. A characteristic peak at 2268 cm-1 is observed in the isocyanate region (2250-2275 cm-1), assigned to the fundamental stretching mode of -N=C=O.
As predicted by the Beer-Lambert Law, this peak increases with increasing concentration of isocyanate. Sample concentrations above ~1.5 wt% were totally absorbing and were therefore excluded from further analysis. For samples above 1.5 wt% the use of an oyster cell with shorter pathlength would enable quantification above this limit.
A simple calibration plot was constructed using a linear baseline, as shown in Figure 2. A simple regression analysis was performed using a fit to a linear line of the form y=mx+c. An R2 value of 0.998 was obtained indicating a close fit to the data. When building a calibration dataset it is important that the system is constructed around the expected sample types. Therefore, any calibration model should be made using the exact sample type expected (i.e. the expected type of isocyanate and the expected solvent). The cell pathlength is also a vital component so it is important to construct a calibration model for each cell, and for best practice to have regular checks on the cell for changes in pathlength (for instance by testing samples of known composition to check the calibration is still good).
The Limit of Detection (LOD) & Quantification (LOQ)
The limit of detection at the lower bound can be determined using the formula:
YLOD=3.3σ/m
Where: YLOD = Absorbance at the limit of detection, σ = standard deviation & m = gradient of the line of best fit. This can then be converted into the isocyanate wt% LOD using the parameters obtained from the regression analysis above.
The limit of detection for the current work was therefore determined to be: 0.05 wt% isocyanate. The limit of quantification (defined as YLOQ=10σ/m) was determined to be 0.17 wt% isocyanate.
Conclusions
In conclusion we have successfully demonstrated that the pearl is ideally suited for detection of isocyanates. A 100 μm pathlength Oyster cell is sufficient for detection of isocyanate at the legal limit of 0.1 wt%, although for accurate quantification a longer pathlength would be required. This means that any sample measured below the LOD (0.05 wt%) can be safely certified as below the legal limit. However, samples in the range 0.05-0.1 wt% which are below the legal limits could not be safely confirmed as such. From a safety perspective having a more stringent control that legally required is better. If required to do so, a longer pathlength Oyster cell would reduce the LOQ further and allow for quantification below the legal limit.
The Pearl system reduces the complexity of such measurements, and reduces sample turnaround times, enabling the analytical chemist to screen more samples per day. More viscous samples, such as greases containing isocyanates, cannot be studied in a traditional cell, since the pathlength of the cell cannot be consistently controlled. It is also difficult to introduce viscous samples into a traditional cell using syringes. The use of the Oyster cell enables consistent pathlength reproducibility, opening the door to routine FTIR screening of compounds that was not previously possible.
Special Thanks To:
Dr. Erik Uhlein, Deputy Head of R&D at LUBRICANT CONSULT GmbH for providing the data.
References
[1] https://www.epa.gov/assessing-and-managing-chemicals-under-tsca/risk-management-methylene-diphenyl-diisocyanate-mdi-and
[2] https://echa.europa.eu/documents/10162/46cb2ae0-ee6b-4319-8604-b5b1d4a41d53
[3] Lamination adhesives application note
[4] Pearl pathlength application note
Disclaimer: This article is based upon the authors understanding of the relevant regulations. Specac makes no legal claims for the use of any compounds mentioned above.
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