Combustion and thermal degradation mechanism of rigid polyurethane foams modified with expandable graphite and ammonium polyphosphate

Expandable graphite (EG) and ammonium polyphosphate (APP) were used to improve flame retardancy of rigid polyurethane foams (RPUF). The combustion behaviour of RPUF/EG/APP was investigated by limiting oxygen index (LOI) and cone calorimeter test. The thermal degradation behaviour, the composition and micro-morphology of char were surveyed by TG, X ray photoelectron spectroscopy and scanning electron microscope, respectively.The results show that the optimum ratio of EG and APP is 7:3 and synergistic effect of EG and APP for RPUF is excellent when the addition level of EG/APP is 20%, the LOI reached to 36.0% and the heat release is minimum. In the combustion process of flame retardant RPUF, the smoke and CO suppressant effect of RPUF/EG is outstanding at prophase in spite of loose char layer, the char layer of RPUF/APP is continuous and dense but its thermal stability is insufficient and produced more smoke, and the char layer of RPUF/EG/APP has obviously effect on the insulation heat and has better effect on the inhibition of smoke and CO production. The mechanism of RPUF/EG/APP is related to the increase of thermal oxidation resistance and ratios of N/C and P/C on char surface when polyphosphoric acid permeated EG char layer.