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This review analyses over 170 publications researching APP (ammonium polyphosphate) as a PIN flame retardant in different thermoplastic composites, and summarising data on fire performance and on mechanical performance tests. Polymers considered include polyethylene, polypropylene (PP), polystyrene, PMMA, PET, ABS, polyamides (nylon) and poly(vinyl alcohol) (PVOH). APP shows to be an effective intumescent flame retardant, reducing peak heat release rate. It is noted that this can reduce fire temperatures in confined spaces and may so prevent ‘flashover’ occurring.

Also, incandescent combustion is inhibited. APP is indicated to act by generating in fire both insulating carbon foam and hard, glassy char on the polymer surface, with protect from fire heat and prevent access to oxygen to feed the fire. APP also releases water and ammonia, which have a cooling effect and dilute fire gases. APP fire retardancy performance can be optimised by appropriate addition of carbon sources, which contribute to char formation and structure, for example, expandable graphite, pentaerythritol (PER). In different polymers, synergies between APP a wide range of PIN flame retardants and additives, including zinc salts, manganese salts, nickel salts, magnesium and aluminium hydroxides, phosphorus-based PIN FRs, melamine compounds, phosphonium salts and combined metal LDHs (layered double hydroxides). Impacts of APP addition on polymer mechanical properties are considered, based on the research testing publications assessed, noting that impacts can be minimised by use of pre-treated APP (e.g. intensive balling, coating, microencapsulation) or by adding coupling agents (e.g. inorganic substances such as talc, silanes).The authors note the potential of APP for PIN fire safety in rubbers and bio-based polymers, suggesting that further research and testing is needed.