How is Ammonium Polyphosphate Produced?

When discussing halogen-free flame retardants, one term appears frequently—APP. But it’s not the mobile app; it refers to ammonium polyphosphate.

As the “soul” of intumescent flame retardant systems, APP’s role goes far beyond simply providing phosphorus and nitrogen.

Firstly, let’s understand: What exactly is APP?

Ammonium polyphosphate, with the general formula (NH4)n+2PnO3n+1, is a halogen-free, environmentally friendly phosphorus-nitrogen-based flame retardant.

Structurally, APP can be seen as a condensation product of ammonium phosphate, and its properties vary greatly depending on the degree of polymerization (n). It is typically classified into two crystal forms: Type I and Type II.

In intumescent flame retardant systems, APP acts as an acid source—decomposing upon heating to generate strong acids such as phosphoric acid, metaphosphoric acid, and polyphosphoric acid. These acids catalyze the dehydration and carbonization of the charring agent, forming a char layer framework.

Secondly How is APP produced?

There are many methods for synthesizing polyphosphate (APP), with the most commonly used industrially being the ammonium dihydrogen phosphate-urea method and the phosphorus pentoxide-ammonium phosphate-urea method.

Ammonium dihydrogen phosphate-urea method: Ammonium dihydrogen phosphate and urea are mixed in a specific ratio and polymerized at around 220°C to obtain APP. However, this method mostly yields type I APP with a low degree of polymerization.

Phosphorus pentoxide-ammonium phosphate-urea method: Phosphorus pentoxide, ammonium phosphate, and urea are mixed and heated under an ammonia atmosphere (280~300°C) to obtain type II APP with a high degree of polymerization. This method requires sophisticated equipment but produces high-quality products.

In recent years, a synthesis technology for piperazine-modified ammonium polyphosphate has also emerged. By reacting piperazine with APP, a piperazine structure is introduced, improving thermal stability and char-forming properties.

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