Synergistic effect of expandable graphite and phenylphosphonic-aniline salt on flame retardancy of rigid polyurethane foam

In this work, rigid polyurethane foams (RPUF) were flame retarded by adding 8 wt % expandable graphite (EG) and EG/Phenylphosphonic-aniline salt (FR1) flame retardant compounds. The adding amount of EG/FR1 compound is 8 wt % as well, and different ratios (8:1 and 12:1) of EG to FR1 were designed. Detailed characterizations of the density, cellular structure, and thermal conductivity of RPUF were carried out. An increase in thermal conductivity was observed as the FR1 content increased. The thermogravimetric analysis (TGA) indicated that EG and FR1 accelerated the degradation of the foam but increased the char residue. The results from the fire tests showed that RPU3 improved the fire resistance of the RPUF on a higher grade than RPU1. In detail, RPU3 increased the limited oxygen index (LOI) value from 19.2% to 29.8% and reached V-0 rating on the vertical burning test. Results from the cone calorimeter test showed that the RPU3 effectively reduced peak of heat released rate (pHRR) in comparison to PU and RPU1 reference samples. A combined synergistic flame-retardant mechanism by the barrier effect of EG in condense phase and radical capture mechanism of FR1 in gas phase was proposed. Moreover, the compression performance of the foams increased by the combined addition of EG and FR1 at the same 12:1 ratio in the RPUF. A good interfacial adhesion between FR1 and PU matrix besides an increasing density of the foam, could be the responsible for this result.