Flame Retardants Center >> Ammonium Polyphosphate Center

Description & Benefits

Ammonium polyphosphate is an inorganic salt of polyphosphoric acid and ammonia. The chain length (n) of this polymeric compound is both variable and branched, and can be greater than 1 000. Short and linear chain APPs (n < 100) are more water sensitive (hydrolysis) and less thermally stable than longer chain APPs (n >1000), which show a very low water solubility (< 0.1 g/ 100 ml).

Chemical Formula and structure:  [NH4 PO3]n

Chemical Structure of Ammonium polyphosphate (APP)

Figure 1: APP Structure

APP is a stable, non-volatile compound. In contact with water APP it slowly gets hydrolysed to monoammonium phosphate (orthophosphate). Higher temperatures and prolonged exposure to water will accelerate the hydrolysis. Long chain APP starts to decompose at temperatures above 300 °C to polyphosphoric acid and ammonia. Short chain APP will begin to decompose at temperatures above 150 °C.

There are two main families of ammonium polyphosphate : Crystal phase I APP (APP I) and Crystal phase II APP (APP II).

Crystal phase I APP (APP I) is characterized by a variable linear chain length, showing a lower decomposition temperature (aprox 150°C) and a higher water solubility than Crystal Phase II Ammonium Polyphosphate. The general structure of APP is given below.
In APP I, n (number of phosphate units) is generally lower than 100.

Chemical Structure of APPI

Figure 2 : APPI Structure

Examples of commercial grades are listed in table 1

g/100 ml
Chain length
P2O5 % pH D50 (micron)
FR CROS 485 3 60 69 5-6 16

Crystal phase II APP (APP II)

As shown in figure 2 below, APP II structure is cross linked/branched. The molecular weight is much higher than APP I with "n" value higher than 1000. APP II has a higher thermal stability (decomposition starts at approximately 300°C) and lower water solubility than APP I.

Chemical Structure of APPII
Figure 3: APPII Structure

Example of commercial non coated grades are listed on table 2:

g/100 ml
P2O5 % pH D50 (micron)
FR CROS 484 0.5 72 5-6 18
FR CROS S10 0.5 72 5-6 7

Coating with different additives will modify the properties of Phase II Ammonium Polyphosphate, making them suitable for use in many applications. (see benefits of coated grades)


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