Synergistic effects of expandable graphite and ammonium pentaborate octahydrate on the flame-retardant, thermal insulation, and mechanical properties of rigid polyurethane foam

The use of flame retardants (FRs) to improve the flame retardancy but also having good insulation and mechanical properties of rigid polyurethane foam (RPUF) has become significant due to the increasing demand in both the industry and academia. In the present study, a series of RPUF composites containing expandable graphite (EG), ammonium pentaborate (APB) octahydrate, and their binary blends were prepared with one-shot and free-rise methods. The effects of FRs on the FR and physical-mechanical properties of RPUFs were investigated. The results show that both EG and APB could improve the flame retardancy of RPUFs and reduced the smoke production. The FR effect of EG was better than APB and more importantly, synergistic effect was found between EG and APB. The best results were obtained by the foam in the composition of 15E and 5A. The cone calorimeter test results showed that the peak heat release rate (pHRR) and total smoke release (THR) of 15E/5A foam were lower than the foams of 20E and 20A. The pHRR and THR values of 15E/5A foam decreased about 57.5% and 42.8% compared to the neat RPUF, respectively. Total smoke production (m(2)) also reduced about 77.0% by 20E and 83.6% by 15E/5A foams. Thermogravimetric analysis indicates that the char residue of 15E/5A foam increased to 39.5%, which provided better flame retardancy. The foam composites have high compressive strength (105-150 kPa) and low thermal conductivity values (19.9-21.3 mW/mK). While the thermal conductivity of 15E/5A foam increased by 0.5%, its compressive strength increased by 6.1%.