Simple and green synthesis of calcium alginate/hydroxyapatite hybrid material without high temperature treatment and its flame retardancy

Aiming to improve the thermal stability and flame retardant properties of calcium alginate, calcium alginate (CaAlg)/hydroxyapatite (HAP) hybrid material was synthesized in situ by the sol–gel method at room temperature. It was found that the HAP particles were generated from nanospheres to urchin-like microspheres, finally to wrinkled sponge microspheres with the two-dimensional nanostructured surface. The results showed that the thermal stability and flame retardancy of the hybrid material were significantly improved, and its mechanism was proposed as HAP promoted the decarboxylation of alginate resulting in carbonization of CaAlg/HAP, and it was the function of solid phase before 350 °C, while the synergistic effects of gas phase and solid phase after 350 °C that led to the high flame retardancy of CaAlg/HAP.