Ammonium polyphosphate

Ammonium polyphosphate is an inorganic salt of polyphosphoric acid and ammonia that contains both chains and possibly branches. Its chemical formula [NH 4 PO 3 ] N (OH) 2 shows that each monomer consists of an orthophosphate radical of a phosphorus atom containing three oxygens and a negative charge neutralized by the ammonium cation causing the two bonds to polymerize. . In branched cases some monomers do not contain the ammonium ion and instead bind to three other monomers.

The properties of ammonium polyphosphate depend on the number of monomers in each molecule and on how often they branch. Short chains (n<100) are more water-sensitive and less thermally stable than longer chains (n>1000), [1] but shorter polymer chains ( such as pyro-, tripoly- and tetrapoly-) are more are soluble and show increasing solubility. With increasing chain length. [2]

Ammonium polyphosphate can be prepared by the reaction of concentrated phosphoric acid with ammonia. However, iron and aluminum impurities, soluble in concentrated phosphoric acid, form a gelatinous precipitate or “mud” in ammonium polyphosphate at a pH between 5 and 7. [3] Other metal impurities such as copper, chromium, magnesium and zinc form granular precipitates. [4] However, depending on the degree of polymerization, ammonium polyphosphate can act as a chelating agent to dissolve some metal ions in solution. Ammonium polyphosphate is used as a food additive, emulsifier , ( E number : E545) and 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 are polyolefins and especially polypropylene, where APP is part of intumescent systems. [6] Compounding with APP-based flame retardants in polypropylene has been described. [7] Further applications are thermosets, where APP is used in unsaturated polyesters and gel coats (APP mixed with synergists), epoxy and polyurethane castings (intumescent systems). APP is also applied to flame retardant polyurethane foam.

Ammonium polyphosphate, used as a flame retardant in polymers, has long chains and a specific crystallinity (Form II). They begin to decompose at 240 °C to form ammonia and phosphoric acid. Phosphoric acid acts as an acid catalyst in the dehydration of carbon-based poly-alcohols such as cellulose in wood. Phosphoric acid reacts with alcohol groups to form heat-stable phosphate esters . The esters decompose to release carbon dioxide and the phosphoric acid catalyst [ citation needed ]reproduce again. In the gas phase, the release of non-flammable carbon dioxide helps dilute the oxygen of the air and the flammable decomposition products of the burning material. In the condensed phase, the resulting carbonaceous char helps to protect the underlying polymer from attack by oxygen and radiant heat. [8] Use as an intumescent is achieved when starch-based ingredients such as pentaerythritol and melamine are added as expanding agents. The mechanism of intumescence and the mode of action of APP have been described in a series of publications.