When we talk about halogen-free flame retardant, there is a word that appears very frequently - APP. But it's not the mobile app, it's ammonium polyphosphate.
In fact, as the "soul figure" of the flame retardant system, the role of APP is far more than simply providing phosphorus and nitrogen.
01 What exactly is an APP?
Ammonium polyphosphate, is a halogen-free and environmentally friendly phosphorus nitrogen 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 usually divided into two crystal forms: Type I and Type II:
In the expansion flame retardant system, APP plays the role of an acid source - when heated, it decomposes into strong acids such as phosphoric acid, metaphoric acid, and polyphosphoric acid. These acids can catalyze the dehydration and carbonization of carbon agents, forming a carbon layer skeleton.
02 How is the APP made?
There are many synthesis methods for APP, and the commonly used ones in industry are ammonium dihydrogen phosphate urea method and phosphorus pentoxide ammonium phosphate urea method.
Ammonium dihydrogen phosphate urea method: Mix ammonium dihydrogen phosphate and urea in a certain proportion, polymerize at around 220 ℃, and obtain APP. But this method mostly produces Type I apps with low aggregation.
Phosphorus pentoxide ammonium phosphate urea method: Mix phosphorus pentoxide, ammonium phosphate, and urea, and heat them in an ammonia atmosphere (280~300 ℃) to obtain type II APP with high polymerization degree. This method requires high equipment requirements, but the product quality is good.
In recent years, the synthesis technology of piperazine modified ammonium polyphosphate has also emerged. By reacting piperazine with APP, piperazine structure is introduced to improve thermal stability and carbonization.
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