APP, or Ammonium Polyphosphate, is a widely - used flame - retardant additive. It is an inorganic, halogen - free compound with excellent thermal stability and high phosphorus content, which provides strong flame - retarding performance. APP acts as an intumescent agent, forming a char layer when exposed to heat or fire to prevent the spread of flames and reduce smoke generation. Its non - toxic nature makes it suitable for various applications, including plastics, coatings, and textiles, helping manufacturers meet strict fire - safety standards while maintaining product performance.