Ammonium polyphosphate is an inorganic salt of
polyphosphoric acid and
ammonia containing both chains and possibly branching. Its chemical formula is
H4 PO3sub>n(OH)
2 showing that each
monomer consists of an
orthophosphate radical of a
phosphorus atom with three
oxygens and one negative charge neutralized by an
ammonium
The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
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), but short polymer chains (''e.g.'' pyro-, tripoly-, and tetrapoly-) are more soluble and show increasing solubility with increasing chain length.
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. Other metal impurities such as copper, chromium, magnesium, and zinc form granular precipitates. However, depending on the degree of polymerization, ammonium polyphosphate can act as a
chelating agent
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
to keep certain metal ions dissolved in solution.
Ammonium polyphosphate is used as a food additive,
emulsifier
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Altho ...
, (
E number
E numbers ("E" stands for "Europe") are codes for substances used as food additives, including those found naturally in many foods such as vitamin C, for use within the European Union (EU) and European Free Trade Association (EFTA). Commonly ...
: 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
polyolefin
A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More speciali ...
s, and particularly polypropylene, where APP is part of intumescent systems. Compounding with APP-based flame retardants in polypropylene is described in. 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 as 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 . 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. 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.
[Bourbigot, 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]
References
External links
*
*
Specialchem4polymers.com
{{Ammonium salts
Food additives
Polymers
Ammonium compounds
Inorganic fertilizers
Flame retardants