FR Types‎ > ‎Phosphorous‎ > ‎


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

 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.

APPI Structure

Crystal phase II APP (APP II)

As shown  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.

APPII Structure

Coating with different additives will modify the properties of Phase II Ammonium Polyphosphate, making them suitable for use in many applications. 

Ammonium polyphosphate acts as a flame retardant by a chemical effect in the condensed phase called "Intumescence" .

When plastic or other materials which contain APP are exposed to an accidental fire or heat, the flame retardant starts to decompose, commonly into polymeric phosphoric acid and ammonia. The polyphosphoric acid reacts with hydroxyl or other groups of a synergist to a non stable phosphate ester. In the next step the dehydration of the phosphate ester follows. A carbon foam is built up on the surface against the heat source (charring). The carbon barrier acts as an insulation layer, preventing further decomposition of the material.

Addition of synergetic products like pentaerythritol derivatives, carbohydrates and spumific agents (melamine etc.) will significantly improve the flame retardant performance of APP. Budenheim offers pre-mixed synergistic systems, which contain an optimised mixture of APP II, charring and blowing agents in a well balanced formulation. 


Ammonium polyphosphate is a non-reactive, inorganic material, that is compatible with many polymers. However, as indicated under "thermal stability", the processing temperatures limit the application of APP. The maximum processing temperature is 230°C. Also the APP should not be processed under pH >7, because ammonia will be released. Depending on the coating, the APP can have different pH values between 5-8. For pH sensitive application our melamine coated products can be used. Although APP II (FR has a very low water solubility (< 0,5 g/100 ml), in certain applications an even lower solubility is demanded. Different Coatings of APP can achieve lower water solubilities and a better durability under humid conditions. The FR performance is not affected by the coating. Ammonium polyphosphate Phase II has a good thermal stability > 300°C However, in combination with other chemicals (e.g. char promoter, blowing agent in intumescent mixtures) and under the mechanical stress and shear during extrusion, APP is limited to processing temperatures between 190 and 210°C. Under too drastic conditions decomposition of the APP will occur with the smell of ammonia. However, temperatures for the extrusion of PP/PE are well suited to the processing window of Ammonium Polyphosphate. With a loading of 18-25% of an APP based intumescent mixture, a V0 (UL 94, 1,6 mm) and thicknesses of 0.8mm can be achieved.

 Implementing the following conditions will help you to achieve the best results:

  • Use co-rotating twin screw extruder
  • Use side feeder for the flame retardant
  • Reduce amount of kneading blocks and other high shear elements downstream of the side feeder
  • Use only low shear mixing element if necessary
  • Air knife and strand pelletizer is preferred
  • Dry fast after pelletizing

Subpages (1): Coated APP