Monitoring the Content of Free Isocyanate in Polyurethanes

Polyurethanes (PUs) are a class of synthetic polymers that are typically used to create flexible or rigid structures that are composed of either solid or foam material. Examples of these materials include mattresses, shoe soles, safety helmets, insulation, packing material, surfboards, wind turbine blades, as well as a number of safety features in cars.

PUs exhibit a wide variety of densities, hardness and durability as a result of the immense amount of additives and recipes that are available for the production of these materials. The formation of PUs involve the reaction between liquid di/polyisocyanates and polyols with a catalyst and various additives. The step additions of these chemicals occur in a reactor prior to the final mixture that is spread on rollers or injected into molds to be further processed according to customer specifications.

The reaction between polyisocyanates and polyols is quick and exothermic as it is initiated by the addition of a catalyst into the reactor until the reaction reaches its initial equilibrium. Three stepwise chemical additions are then conducted within the reactor to modify the PU properties. The final reagent quenches the isocyanate (NCO) functional groups from unreacted isocyanates. It is therefore extremely important for industries working with PU materials to know the precise concentrations of NCO during these manufacturing processes.

Stylization of suggested placement for NIR probe in an industrial PU reactor with real data correlation between primary titration method and NIR model showing how different mixing steps can be shortened.

Stylization of suggested placement for NIR probe in an industrial PU reactor with real data correlation between primary titration method and NIR model showing how different mixing steps can be shortened.

Determining the NCO Concentration

Determining the % concentration of NCO during in the manufacturing process of PU materials is a critical parameter that is used to determine the proper mixing ratio between the different reagents to ensure the optimized production of various PU characteristics. In a laboratory setting, this process is generally performed by titration methods by use of spot samples that are taken from the reactor at various points in the mixing process.

These titration methods are often slow and require the use of various chemicals that require proper disposal. An additional disadvantage to these procedures involves the transportation of these materials to the laboratory, as their exposure to ambient conditions limit the accuracy of the analysis.

Alternative Analysis of PUs

A safer way to increase PU production, decrease waste, as well as save time and money involves the consistent inline monitoring of the concentration of NCO. Reagent- free near infrared (NIR) spectroscopic analyzers enable the comparison of real-time spectral data from the process to the primary method (titration) to create a simple, yet indispensable model the industry’s specified needs.

To gain an improved control over the production of PU materials, a Metrohm Process Analytics NIR XDS system is ideal, as this equipment is capable of monitoring up to 9 process points with the multiplexer option.

Other Process NIRS applications related to PU:

  • Moisture determination in drying step
  • Hydroxyl number determination
  • Monitoring the curing of polyurethane elastomer
  • Determining percent linear expansion in polyurethane resins
  • Water content of polyols (%)
  • Determination of Acid and Alkalinity Numbers of Polyols
  • Determination of Acidity as Acid Number (AN) for Polyether Polyols

Related ASTM methods:

  • ASTM D2572: Standard Test Method for Isocyanate Groups in Urethane Materials or Prepolymers
  • ASTM D6342 (ISO 15063): Determination of hydroxyl number
  • ASTM E1655: Standar Practices for Infrared Multivariate Quantitative Analysis

Related Application Bulletin:

  • AB-414 Polymer analyses using near-infrared spectroscopy

NIRS XDS Process Analyzer

NIRS XDS Process Analyzer

Dedicated solutions for your sampling needs

Probe Type Applications Processes Installation
Micro interactance reflectance probe
  • Solids (powders, granules)
  • Slurries with > 15% solids
  • Bulk polymerization
  • Hot melt extrusion
  • Direct into process line
  • Compression fitting or welded flange
Micro interactance immersion probe
  • Clear to scattering liquids
  • Slurries with < 15% solids
  • Solution phase
  • Temperature- & pressure-controlled extrusion
  • Direct into process line
  • Compression fitting or welded flange
Micro transmission probe pair
  • Clear to scattering liquids
  • Slurries with < 15% solids
  • Solution phase
  • Temperature- & pressure-controlled extrusion
  • Direct into process line or reactor
  • Into a side-stream loop
  • Compression fitting or welded flange
Micro interactance reflectance probe with purge on collection tip
  • Solids (powders, granules)
  • Environments where sample amount is variable (e.g., fluid bed dryer)
  • Drying of granules and powders
  • Direct into the fluid bed dryer, reactor, or process line
  • Compression fitting or welded flange

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.

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