A single‐sample method to determine the hydroxyl values of polyols using mid‐FTIR spectroscopy

Abstract

A single‐sample Fourier transform infrared (FTIR) method suitable for automation was developed to determine the hydroxyl value (OHV) of lipid‐based polyols. It is a modification of the original split‐sample FTIR method using the reaction of p‐toluenesulfonyl isocyanate (TSI) with OH groups and measures the carbamate band formed in the differential spectrum before and after the reaction. In the new method, the TSI‐isocyanate absorption is measured before and after the reaction, the area change being proportional to the OHV of the sample, but is sensitive to moisture. Moisture compensation is achieved by monitoring the CO2 produced by the water‐TSI reaction and correcting for its additional contribution to isocyanate loss. The method was validated by assessing polyol mixtures containing added water, comparing the results to those from the moisture independent split‐sample FTIR method. The single‐sample FTIR procedure had similar accuracy and reproducibility as the split‐sample method when CO2 moisture compensation was implemented. The procedure was not affected by carboxylic acids but can be affected by the presence of amines. The advantage of the new OHV FTIR method is that it requires only a single sample to be prepared and analyzed, making it amenable to automation as well as reducing sample preparation time, solvent, and reagent use. Practical applications: This paper provides an automatable, rapid, and accurate FTIR‐based method for determination of hydroxyl value of polyols, which is a critical parameter of polyols and polyurethane production. The method can be employed by commercial laboratories and researchers working in this area for quality control and to facilitate improvement and development of new products. An FTIR method is described to measure hydroxyl values of polyols using the reaction with isocyanate employed in other methods. The new approach is suitable for automation and moisture compensation is achieved in the same experiment by measuring CO2 produced. Method validation indicates that the method is accurate compared to theoretical values.

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