Flame retardancy and thermal degradation of rigid polyurethane foams composites based on aluminum hypophosphite

Rigid polyurethane foam/aluminum hypophosphite (RUPF/AHP) composites were fabricated by one-step water-blown method. Furthermore, thermal conductivity test and scanning electron microscope (SEM) confirmed RPUF/AHP composites presented well cell structure with thermal conductivity of 0.0434–0.0466 k W/m · k and density of 59.0–60.3 kg m−3. Thermogravimetric analysis test showed significantly enhanced Tmax value of RPUF/AHP in second stage, indicating enhanced thermal stability of the composites, which was as also confirmed by thermal degradation kinetics analysis. Flame retardant test revealed that AHP presented high flame retardancy efficiency to RPUF, only 5 php AHP incorporation can make RPUF/AHP5 reach V-1 rating in UL-94 test. Microscale combustion calorimetry (MCC) test showed that AHP could effectively decrease peak of heat release rate (PHRR) of RPUF/AHP composites. RPUF with 30 php AHP loading presented PHRR value of 164 W g−1, which was 20.3% decreased compared with that of virgin RPUF. Thermogravimetric analysis-Fourier transform infrared spectroscopy (TG-FTIR) was introduced to investigate gaseous products in degradation process of RPUF/AHP composites. It can be found that AHP enhanced the release of CO2, isocyanate compound while inhibit the release of hydrocarbons, aromatic compounds and esters. Furthermore, scanning electron microscope (SEM) and x-ray photoelectron spectroscopy (XPS) were applied to research char residue of the RPUF/AHP composites. It was confirmed that AHP could promote C and N elements in RPUF matrix to form aromatic and aromatic heterocyclic structure, which combined with aluminum phosphate to form compact char residue, thus significantly inhibit mass and heat transmission in combustion. Consequently, a possible gas-solid flame-retardant mechanism of RPUF/AHP composites was proposed.