Abstract: The effect of molybdenum trioxide (MoO3) on smoke suppression of rigid polyurethane foam (RPUF) filled with expandable graphite and ammonium polyphosphate was studied by smoke density test, limiting oxygen index, and horizontal-vertical burning test. Meanwhile, the role of MoO3 in condensation phase was analyzed by thermal gravimetric analysis and X ray photoelectron spectroscopy. It was found that with a MoO3 loading of 4 parts per hundred polyol, the maximum smoke density of the RPUF was reduced by 22 %, the amount of residual char was increased from 8.0 % to 16.2 % at 835 ℃. The reason was that MoO3 can promote the cross linking of RPUF to form char and retain more phosphorus containing compounds in condensation phase.

Key words: rigid polyurethane, foam, molybdenum trioxide, smoke suppression, flame retardant