The application of ammonium polyphosphate in unsaturated polyester resins: A mini review

Ammonium polyphosphate (APP) is a common and commercially available flame retardant for various polymeric materials due to its low cost, ease of processibility, low toxicity, and environmental friendliness. This mini review focuses on the application of APP in unsaturated polyester resin (UPR) and outlines the flame retardancy, thermal stability, and mechanical properties of the APP-containing UPR composites. As an effective acid source, APP can facilitate the dehydration and carbonization of the UPR matrix during combustion, forming a graphite char layer on the matrix surface to slow down heat exchange and volatile release, thus suppressing the burning reaction. Due to the inorganic properties of APP, microencapsulation and surface decoration are usually conducted on it to enhance its compatibility with UPR and its flame-retardant efficiency. Moreover, different flame-retardant synergists are also applied to strengthen the flame-retardant performance of APP in UPR. In this review, we briefly summarized the influences of microencapsulated APP, surface-decorated APP, and APP/synergist on the properties of UPR, respectively. Then, the advancement and challenges of these APP systems are highlighted, and future research opportunities are also proposed.