Polyurethanes (PU) are synthetic polymers used to create a wide range of products, from flexible or rigid foam structures to safety features in cars. The physical properties for PU products can vary widely. PU is formed by reacting liquid di/polyisocyanates and polyols with a catalyst and additives. Therefore, determining the free isocyanate content (%NCO) in the manufacturing process is a critical parameter to optimize production. Generally, spot samples are taken and analyzed in a laboratory by titration methods which are slow and generate waste. Inline monitoring of %NCO in real-time is a safer and more efficient way to monitor and improve PU production and reduce waste.

This Process Application Note presents a method to accurately monitor the percentage of NCO in the PU manufacturing process in «real-time» using near-infrared (NIR) spectroscopy. This can be done safely, reliably, and optimally with a 2060 The NIR-Ex Analyzer from Metrohm Process Analytics.

Polyurethanes are a class of synthetic polymers used to create solid or foam structures – flexible or rigid – such as mattresses, shoe soles, safety helmets, insulation, packing material, surfboards, wind turbine blades, and several safety features in cars to name a few. The variety of densities, hardness, and durability found in PU products is vast due to the sheer number of recipes available. Polyurethanes are formed by reacting liquid di/polyisocyanates and polyols with a catalyst and various additives. The step additions of these chemicals occur in a reactor before the final mixture is spread on rollers or injected into molds to be further processed to customer specifications.

The reaction between polyisocyanates and polyols is quick and exothermic, beginning with the addition of a catalyst into the reactor. The process reaches an initial equilibrium, after which three stepwise chemical additions are made in the reactor to modify the PU properties (Figure 1a). The final reagent quenches isocyanate (NCO) functional groups from unreacted isocyanates. Therefore, knowing the precise NCO concentration is crucial here.

Determining the %NCO in the PU manufacturing process is a critical parameter to help determine the proper mixing ratio between the different reagents for optimized production of various PU characteristics. This is generally performed in the lab by titration methods after taking spot samples from the reactor at various points in the mixing process.

Laboratory titration is slow and uses chemicals that require proper disposal. Another issue is that during transportation to the laboratory, the sample properties change as it is exposed to ambient conditions not representative of the reactor. A safer way to optimize PU production, decrease waste, and save time and money is by monitoring %NCO inline in near real-time.

Reagent-free near-infrared spectroscopic analyzers enable comparison of spectral data gathered direct from the process to a primary method to create a simple, yet indispensable model for manufacturing process needs. The 2060 The NIR-Ex Analyzer (Figure 2) by Metrohm Process Analytics has IMPACT as the embedded software, which manages the transfer of results using well-known industry communication protocols to any plant control room.

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Using NIRS technology, the process can be quickly monitored inline to determine the proper mixing ratio between the different reagents for optimized PU production. Figure 1 shows a trend chart of the %NCO versus time as determined by NIRS. An immersion probe specifically designed for these applications is used with a gap located in the probe tip (Figure 1b). Additionally, it is possible to gain more control over PU production using a 2060 The NIR-Ex Analyzer as it can monitor up to five process points with each NIR cabinet.

Wavelength range used: 1950–2080 nm.

Inline analysis is possible using the properties of transflectance and the micro interactance immersion probe. The sample flows through the gap between the probe body and high-energy mirror tip and adjusting the mirror tip defines the pathlength (equal to two times the gap) for analysis.

A primary method (such as titration) must still exist as a reference method. An appropriate range of samples covering the process variability should be analyzed by both methods to build an accurate NIR model. Correlations are made to process specifications.

• Optimize product quality and increase profit with fast response times for process variations
• Greater and faster return on investment
• No manual sampling needed, thus less exposure of personnel to dangerous chemicals