When PURF char layer is formed during the burning process, the char will be constantly exposed to air and oxidized at high temperature. If the char layer is rapidly oxidized, protection effect can be hardly achieved. So it is essential to improve the char stability for achieving high flame resistivity of PURF. It was reported that inorganic nanomaterials are helpful to enhance the anti-oxidation property of char layer. During the char-forming process, inorganic nanomaterials will form a compact barrier on the surface of PURF, which can increase PURF stability by restricting the permeation of oxygen. So far, quite a few inorganic nanomaterials have been utilized as flame retardants, such as montmorillonite (MMT) , carbon nanotubes , SiO2 nanoparticles and ] and carbonates . When CaCO3 and phosphate-containing compound are added together to PURF, it was proposed that stable calcium phosphate and calcium pyrophosphate will be formed at high temperature due to the reaction between them, resulting in a barrier on the surface of PU foam, which inhibit the permeation of oxygen and thus prevent the oxidation of char layer.