Ammonium polyphosphates: Correlating structure to application

• Exist in six phases with unique but diverse physical and chemical properties. • Main applications: fire retardant, fertilizer, food and feed. • Acid-catalyst causing intumescence or inorganic barrier formation upon heating. • Dual nutrient with good hydrolytic stability and chelating properties. • High water-binding affinity and low toxicity for food and feed. Ammonium polyphosphates (APP) are widely used as nontoxic, biodegradable additives for food, fire retardancy and fertilisers. APP has been shown to exist in six distinct phases, APP–I to APP–VI. Commercial products identified as APP may contain one or more of these phases. Direct synthesis routes to APP involve condensation of monoammonium phosphate or diammonium phosphate with urea or melamine as condensing agents, while indirect synthesis can be obtained by interconversion of one APP phase to another. The most important chemical properties for APP as a fertiliser are nitrogen and phosphorus content, water solubility, hydrolytic stability, and chelating properties (sequestering trace metals). For fire protection, chemical properties such as low water solubility or high hydrolytic stability, compatibility with polymers in plastics or coating formulations, thermal stability and promotion of crosslinking are beneficial. As food additive, low toxicity and water–binding ability are vital. This work found that few studies report on degree of polymerisation, particle size distribution, chain branching, phase, crystallinity or purity of APP, despite APP being the main functionalising additive in specific systems. It was found that some phases of APP (especially APP–III, IV, and VI) remained relatively uncharacterised, but development of new synthetic routes, and improved characterisation, opens new possibilities for commercial exploitation. The unsystematic terminology used to describe these phosphates is addressed in a glossary.