Ammonium Polyphosphate Phase II (APP II, Microencapsulated)

Ammonium Polyphosphate Phase II (APP II, CAS 68333-79-9) is a high-molecular-weight, crystalline form of ammonium polyphosphate, the most widely used intumescent flame retardant (IFR) component for polyurethane, unsaturated polyester, and epoxy resin systems. Phase II APP (n > 1000 repeating units) has significantly lower water solubility than Phase I APP (n < 100), making it far more suitable for outdoor, marine, and humid-environment applications where water resistance of the fire protection is critical.

APP II functions as the acid source in the classic intumescent fire protection mechanism. Under fire exposure, APP decomposes to release polyphosphoric acid, which esterifies the carbon-hydrogen containing polymer matrix (or a separately added carbonific agent such as pentaerythritol) to form a carbon char. The blowing agent component (APP also releases ammonia and water on decomposition, or a separately added nitrogen-rich agent such as melamine) causes the char to expand into a thick, insulating foam that forms a barrier between the flame and the unburned material. This intumescent mechanism provides fire resistance ratings well beyond simple UL 94 V-0 testing — APP-based intumescent coatings provide 30–120 minutes cellulosic fire protection on steel structures.

Microencapsulated grades of APP II (polymer coating on APP particles) improve water resistance and compatibility with polar resin systems. Standard loading for UL 94 V-0 in UPR or epoxy composites is typically 25–40 phr APP combined with 5–10 phr pentaerythritol (carbon source) and 5–10 phr melamine (blowing agent) in the classic IFR ternary system.