Preparation and Thermal Safety of Ammonium Polyphosphate Doping Silica Aerogels: Effects of Content and Polymerization

Hydrophobic silica aerogels (SAs) have attracted much attention because of their excellent thermal insulation performance and have potential applications in energy conservation and emission reduction. However, the organic groups on its surface are flammable, which brings security risks and limits its application scope. In this study, two kinds of ammonium polyphosphate (APP) with different polymerization degrees, namely low-polymerization-degree APP (LAPP) and high-polymerization-degree APP (HAPP), were introduced into SA to prepare APP/SA composites, to improve the thermal safety of the materials. The results showed that APP with two polymerization degrees significantly delayed the initial decomposition and peak temperature of heat flow, and HAPP reduced the gross calorific value by 31.01% at most, which is 29.04% greater than that of LAPP, indicating that the effect of HAPP was slightly better than that of LAPP. With the increase in APP with two polymerization degrees, the density increased and the porosity decreased: LAPP system was 0.095–0.196 g/cm3 and 96.0–91.0%. Both made the thermal conductivity increase only slightly (up to 26.8 mW/m/K), but the sample still maintained excellent thermal insulation and hydrophobicity, which indicated that the addition of APP improved the thermal safety performance of SA while maintaining its basic excellent performance. This strategy provides an effective and simple way to improve the flame retardancy of SA, which makes SA more widely used in fields with strict requirements on thermal safety.