Ammonium Polyphosphate: Modification Strategies and Synergistic Flame-Retardant Applications

Ammonium polyphosphate (APP) is widely used in intumescent flame-retardant (IFR) systems because of its environmental friendliness, low cost, and high flame-retardant efficiency. However, its practical applications are limited by high hygroscopicity, poor compatibility with organic substrates, and the high loading required in single-component systems. To address these limitations, extensive studies have been conducted on APP modification and synergistic flame-retardant systems. This review systematically summarizes the modification strategies and flame-retardant mechanisms of APP. The synergistic flame-retardant effects and mechanisms of APP combined with silicon-, boron-, and metal-containing compounds are also discussed. In addition, the effects on the flame-retardant performance of different structural characteristics, such as nanostructures, layered structures, and ring structures, are reviewed. Finally, the current challenges and future perspectives of APP-based flame-retardant systems are highlighted. This review provides useful guidance for the design, optimization, and practical application of advanced APP-based intumescent flame-retardant materials.