Enhancement of fire performance for rigid polyurethane foam composites by incorporation of aluminum hypophosphite and expanded graphite

A series of rigid polyurethane foam/aluminum hypophosphite/expanded graphite (RPUF/AHP/EG) composites were prepared by one-step water-blown method. The effects of AHP/EG on the cell structure, thermal stability, flame retardant as well as combustion behavior of the composites were investigated by Scanning electron microscope (SEM), Thermogravimetric analysis (TG), Limiting oxygen index (LOI), UL-94 vertical burning test and Micro-combustion calorimetry (MCC). Flame-retardant tests and thermogravimetric analysis confirmed that the AHP/EG system enhanced high-temperature stability and flame-retardant performance of the composites. The char residue of RPUF/AHP15/EG15 at 700 degrees C was 22.4 wt%, the LOI value was 26.4 vol% with V-1 rating in UL-94 test. Thermogravimetric analysis-Fourier transform infrared spectrometer (TG-FTIR) indicated that AHP/EG system significantly inhibited the generation of hydrocarbons, CO2, CO and esters from the pyrolysis RPUF matrix. XPS analysis showed that the combination of AHP and EG could promote RPUF matrix to form aromatic and aromatic heterocyclic structure, which was benefit to the formation of dense char layer, thus achieving the purpose of flame retardancy. Based on the analysis, the flame-retardant mechanism of RPUF/AHP/EG composites was proposed.