Ammonium polyphosphate

Ammonium polyphosphate (CAS 68333-79-9), commonly abbreviated as APP, is an inorganic salt used primarily as a flame retardant and intumescent agent across a broad range of industrial materials. Approved as E545 for certain applications, it functions by releasing ammonia and phosphoric acid under heat, forming a protective char layer that limits flame spread and combustion.

In plastics and polymer compounding, APP is incorporated into polyolefins, polyurethanes, and epoxy systems to meet UL 94 and other fire safety standards without halogenated additives. Intumescent coatings and fireproofing paints for steel structures rely on APP as the acid source in the intumescent system, activating expansion under fire conditions.

Textile and paper manufacturers apply APP-based treatments to achieve flame-retardant certification for technical fabrics, upholstery, and packaging materials. In agriculture, it also functions as a concentrated phosphorus and nitrogen fertilizer source, applied in liquid fertilizer blends and foliar formulations.

APP is supplied as a white powder in two principal crystal forms, Phase I and Phase II, with Phase II preferred for most flame retardant applications due to its thermal stability. Standard grades include uncoated and microencapsulated variants, the latter offering improved compatibility in polymer matrices. Technical and industrial grades are the most commercially prevalent.