An Improved Technique for the Determination of Isocyanurate and Isocyanate Conversion by Photoacoustic FTIR

Conventional rigid polyisocyanurate (PUIR) foams blown with HCFC-141b often suffer from poorer compressive strengths, dimensional stability and inferior flammability properties when compared to foams blown with CFC-11. It is often hypothesized that these properties can be improved by increasing the isocyanurate (or "trimer") conversion by means of catalyst optimization and increased isocyanate index. A convenient, yet reliable method for the determination of the amount of isocyanurate in a PUIR foam has been missing in this industry for a long time, though several attempts have been documented in the literature. The purpose of this paper is to introduce an improved isocyanurate conversion test by photoacoustic Fourier Transform Infra Red (FTIR) technique. This involves creating a baseline through three anchor points at approximately 1637 cm⁻¹, 1469 cm⁻¹, and 1349 cm⁻¹. The absorbance of the isocyanurate peak at 1410 cm⁻¹ is taken relative to the absorbance of the phenyl peak at 1602 cm⁻¹. The phenyl peak is preferred to the urethane peak because the absorbance of phenyl groups in a foam is inherent to the amounts of polyols and isocyanates used in the foaming reaction, whereas the urethane linkages in the foam are created by a reaction between the two and are, therefore, variable depending on the extent of the reaction in the presence of catalysts and possibly water. In addition, the relative ratios of the absorbances of isocyanate end groups (at 2277 cm⁻¹) and carbodiimide groups (at 2136 cm⁻¹) to the phenyl groups can also be determined by generating a baseline through the anchor points at approximately 2470 cm⁻¹, 2207 cm⁻¹ and 2000 cm⁻¹. This allows one to gain a better assessment of the overall kinetics of the isocyanate reactions and creates opportunities to improve the isocyanurate conversion through formulation optimization. The method is not limited to polyisocyanurate foams, as isocyanate conversion is an important parameter to follow in polyurethane foams as well, especially in all carbon dioxide blown foams. The method was found to be quite reproducible, and further statistical analysis to ensure the validity of this technique is under way.