ammonium polyphosphate

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| OtherNames = {{Unbulleted list|Exolit AP 422|FR CROS 484|CS FR APP 231}}

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| CASNo = 68333-79-9

| ChEBI = 147408

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| Formula = H(NH₄PO₃)_nOH

| MolarMass = 97.01 g/mol

| Appearance = white powder

| Density = 1,9 g/cm³; bulk density = 0,7 g/cm³

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Ammonium polyphosphate is an inorganic salt of polyphosphoric acid and ammonia containing both chains and possibly branching. Its chemical formula is H(NH₄PO₃)_nOH showing that each monomer consists of an orthophosphate radical of a phosphorus atom with three oxygens and one negative charge neutralized by an ammonium cation leaving two bonds free to polymerize. In the branched cases some monomers are missing the ammonium anion and instead link to three other monomers.

The properties of ammonium polyphosphate depend on the number of monomers in each molecule and to a degree on how often it branches. Shorter chains (n < 100) are more water sensitive and less thermally stable than longer chains (n > 1000),[http://www.sinoharvest.com/products/Ammonium-Polyphosphate.shtml] {{Webarchive|url=https://web.archive.org/web/20100522123855/http://www.sinoharvest.com/products/Ammonium-Polyphosphate.shtml|date=2010-05-22}} but short polymer chains (e.g. pyro-, tripoly-, and tetrapoly-) are more soluble and show increasing solubility with increasing chain length.{{cite patent|country=US|number=4041133|title=Ammonium polyphosphate production|pubdate=1977-08-09|assign=Union Oil Co. of California|inventor1-last=Young|inventor1-first=Donald C.}}

Ammonium polyphosphate can be prepared by reacting concentrated phosphoric acid with ammonia. However, iron and aluminum impurities, soluble in concentrated phosphoric acid, form gelatinous precipitates or "sludges" in ammonium polyphosphate at pH between 5 and 7.{{cite patent|country=US|number=4721519|title=Stable ammonium polyphosphate liquid fertilizer from merchant grade phosphoric acid|pubdate=1988-01-26|assign=American Petro Mart Inc.|inventor1-last=Thomas |inventor1-first=William P.|inventor2-last=Lawton |inventor2-first=William S.}} Other metal impurities such as copper, chromium, magnesium, and zinc form granular precipitates.{{cite patent|country=US|number=3044851|title=Production of ammonium phosphates and product thereof|pubdate=1962-07-17|assign=Collier Carbon & Chemical Co.|inventor1-last=Young|inventor1-first=Donald C.}} However, depending on the degree of polymerization, ammonium polyphosphate can act as a chelating agent to keep certain metal ions dissolved in solution.{{cite book|url=https://books.google.com/books?id=GP1caeWDUWkC&q=precipitation+ammonium+polyphosphate+solution&pg=PA51|title=The Fertilizer Encyclopedia|first1=Vasant|last1=Gowariker|first2=V. N.|last2=Krishnamurthy|first3=Sudha|last3=Gowariker|first4=Manik|last4=Dhanorkar|first5=Kalyani|last5=Paranjape|date=8 April 2009|publisher=John Wiley & Sons|isbn=9780470431764|accessdate=30 June 2018|via=Google Books}}

Ammonium polyphosphate is used as a food additive, emulsifier, (E number: E545) and as a fertilizer.

Ammonium polyphosphate (APP) is also used as a flame retardant in many applications such as paints and coatings, and in a variety of polymers: the most important ones are polyolefins, and particularly polypropylene, where APP is part of intumescent systems.Weil, E.D., Levchik, S.V. Flame retardants for plastics and textiles, p. 16. Hanser Publishers, Munich, Germany, 2009 Compounding with APP-based flame retardants in polypropylene is described in.{{cite web|url=http://www.mindfully.org/Plastic/Flame/Ammonium-Polyphosphate-FlameApr02.htm|title=As a flame retardant|website=Mindfully.org|accessdate=30 June 2018|archive-url=https://web.archive.org/web/20070913225629/http://www.mindfully.org/Plastic/Flame/Ammonium-Polyphosphate-FlameApr02.htm|archive-date=13 September 2007|url-status=dead}} Further applications are thermosets, where APP is used in unsaturated polyesters and gel coats (APP blends with synergists), epoxies and polyurethane castings (intumescent systems). APP is also applied to flame retard polyurethane foams.

Ammonium polyphosphates used as flame retardants in polymers have long chains and a specific crystallinity (Form II). They start to decompose at 240 °C to form ammonia and phosphoric acid. The phosphoric acid acts as an acid catalyst in the dehydration of carbon-based poly-alcohols, such as cellulose in wood. The phosphoric acid reacts with alcohol groups to form heat-unstable phosphate esters. The esters decompose to release carbon dioxide and regenerate the phosphoric acid catalyst {{Citation needed|reason=Literature is clear on release of water, but would need reference for CO2|date=March 2021}}. In the gas phase, the release of non-flammable carbon dioxide helps to dilute the oxygen of the air and flammable decomposition products of the material that is burning. In the condensed phase, the resultant carbonaceous char helps to shield the underlying polymer from attack by oxygen and radiant heat.{{Cite patent|country=US|number=4515632|title=Activated ammonium polyphosphate, a process for making it, and its use|pubdate=1985-05-07|assign=Hoechst AG|inventor1-last=Maurer |inventor1-first=Alexander|inventor2-last=Staendeke |inventor2-first=Horst}} Use as an intumescent is achieved when combined with starch-based materials such as pentaerythritol and melamine as expanding agents. The mechanisms of intumescence and the mode of action of APP are described in a series of publications.Camino, G.; Luda, M.P. Mechanistic study of intumescence, p. 48 f, in Le Bras, M.; Camino, G.; Bourbigot, S.; Delobel, R. Eds., Fire retardancy of polymers; The use of intumescence, The Royal Society of Chemistry, Cambridge, UK, 1998Bourbigot, S.; Le Bras, M. Intumescence flame retardants and char formation, p. 139 f, in Troitzsch, J. Ed. Plastics flammability handbook, 3rd Ed., Hanser Publishers, Munich, 2004