Aluminium hypophosphite and expandable graphite showed synergies as a PIN flame retardant solution for rigid polyurethane foams. Both products improved fire performance of the foam, but even better results were obtained using a combination. Total loading of 20% with a weight ratio of 3:1 expandable graphite to aluminium hypophosphite improved LOI (limiting oxygen index)
fire performance rating by about 50% and reduced total heat release by a factor of about five times. Also, the two products combined had less negative impact on foam physical characteristics (cell structure, physical properties) than expandable graphite alone at the same total loading levels. Analysis suggests that the synergy effects are the result of production of a strong and compact char (with a “worm like” structure) which acts as a physical barrier on the foam surface in case of fire, preventing heat and mass transfer and so reducing combustion.
“Synergistic effect of expandable graphite and aluminum hypophosphite on flame-retardant properties of rigid polyurethane foam”, W-Z. Xu et al. J. Applied Polymer Science, 42842, 2015 http://onlinelibrary.wiley.com/doi/10.1002/app.42842/abstract