Chemical gradients in PIR foams as probed by ATR-FTIR analysis and consequences on fire resistance.

Taking into account numerous results from the literature, an in-depth exploratory study on the chemical gradients in the rise (or depth) direction was performed on polyisocyanurate rigid (PIR) foam insulation panels using ATR-FTIR spectroscopy. In particular, it was found that the isocyanurate/phenyl ratio is a quick and effective technique for providing an indication of the level of trimer conversion within each sample. More importantly, a detailed analysis of the C–N stretching vibration of the isocyanurate ring as a function of depth revealed a linear decrease in the frequency peak maximum with the increase in the isocyanurate ratio for all investigated PIR foams. This result suggests for the first time that the position (frequency) of peak maximum (C–N stretching of isocyanurate ring) may be used to quantify the extent of isocyanurate formation without the issues linked to the use of absorbances. It is proposed that this reduction in the frequency of the C–N stretching vibration of isocyanurate ring reflects the decrease in hydrogen bonding between N–H of urethane group and C O within the isocyanurate ring when the isocyanurate content increases (lower mobility associated with higher level of cross-linking). The heterogeneity of the isocyanurate content was physically confirmed by investigating the flammability of the foam samples taken at different positions in the rise direction (z-axis). • Strong isocyanurate gradients in polyisocyanurate foam panels in the surface vicinity. • Flammability test improvement with the increase in isocyanurate content. • Linear decrease of C–N peak maximum frequency with increase in isocyanurate content. • C–N peak maximum frequency applicable to quantify extent of isocyanurate conversion. • Downwards frequency shift may reflect the decrease of hydrogen bonding. [ABSTRACT FROM AUTHOR]