Quantification of Diisocyanates in Adhesives and Coatings with Transmission FTIR

Diisocyanates are used in various products, including precursors to adhesives and coatings. Unreacted isocyanate can pose health risks to consumers, so monitoring the polymerization reactions for the remaining product is necessary for many industries – especially where unreacted aromatic molecules can migrate through layers of packaging.

Authorities worldwide are monitoring the use of such chemicals and beginning to regulate them. The United States Environmental Protection Agency (EPA) is currently collecting data on the use of MDI. At the same time, since 2010, the European Union has banned selling products containing more than 0.1% MDI unless stringent conditions are met.² Most notably, from 24 August 2023, these restrictions were broadened to include all diisocyanates.

FTIR is often used in industry to test for unreacted aromatic NCO molecules, which can migrate through laminated packaging. Upon contact with the water molecules in foodstuffs, for instance, they can react to generate carcinogenic primary aromatic amines that are harmful to living organisms.

Lamination adhesives used in multi-layered flexible packaging are commonly a product of two-pot systems that undergo the following reaction:

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.

Isocyanide Calibration Plot

To build a calibration plot quantifying the amount of unreacted isocyanate in a sample, it is necessary to collect spectra of several test samples whose isocyanate concentrations are known and controlled.

The spectra were collected in transmission mode using the Pearl liquid transmission accessory from Specac. The accessory contains a horizontally mounted cell of 100 µm path length, a configuration that greatly simplifies the loading and cleaning of the calibration samples in the set.

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.

Figure 1. FTIR Spectra showing the Isocyanate region at various concentrations. Image Credit: Specac Ltd 

The peak height is proportional to the concentration of isocyanate in the sample, as expected from the Beer-Lambert law of absorbance versus concentration.

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 y=mx+c. An R² value of 0.998 indicated a close fit to the data.

The system must be constructed around the expected sample types when building a calibration dataset. Therefore, any calibration model should be made using the expected sample type (i.e. the expected type of isocyanate and the expected solvent). The cell pathlength is also a vital component. Hence, it is essential to construct a calibration model for each cell and regularly check the cell for changes in path length (for instance, by testing samples of known composition to check the calibration is still good).

Figure 2. Calibration plot showing Absorbance vs. Isocyanate wt%. Image Credit: Specac Ltd 

The Limit of Detection (LOD) & Quantification (LOQ)

The limit of detection at the lower bound can be determined using the following formula:

Y_LOD=3.3 σ/m

Where: Y_LOD = 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.

Therefore, the detection limit for the current work was determined to be 0.05 wt% isocyanate. The quantification limit (Y_LOQ=10 σ/m) was determined to be 0.17 wt% isocyanate.

Conclusions

The study effectively demonstrates the suitability of the Pearl for detecting isocyanates.

A 100 μm pathlength Oyster cell proves adequate for detecting isocyanate at the legal limit of 0.1 wt%. However, for precise quantification, an extended path length becomes imperative. Consequently, samples registering below the Limit of Detection (LOD) at 0.05 wt% can confidently be certified as falling beneath the legal threshold. Conversely, specimens within the 0.05-0.1 wt% range, while still compliant with legal limits, cannot be unequivocally affirmed as such.

Adopting a safety-oriented approach advocates stricter controls surpassing legal requisites. If necessitated, a lengthier pathlength Oyster cell further reduces the Limit of Quantification (LOQ) and quanties beneath the legal limit.

The Pearl system simplifies the intricacies associated with such measurements and expedites sample processing, empowering analytical chemists to screen a higher volume of samples daily.

Viscous samples, such as those encapsulated in isocyanate-laden greases, pose a challenge in traditional cells due to the inherent difficulty in consistently controlling path length. Introducing such dense samples into a conventional cell via syringes proves cumbersome. The deployment of the Oyster cell, however, ensures consistent path length reproducibility, unlocking the potential for routine Fourier-transform infrared (FTIR) screening of compounds usually considered impracticable.

Special Thanks To:

Dr. Erik Uhlein, Deputy Head of R&D at LUBRICANT CONSULT GmbH for providing the data.

This information has been sourced, reviewed and adapted from materials provided by Specac Ltd.

For more information on this source, please visit Specac Ltd.