Phosphorus based flame retardants have the characteristics of non toxicity, halogen-free, smoke suppression, good flame retardant effect, and environmental friendliness. Ammonium polyphosphate APP, as a common phosphorus based flame retardant, has received widespread attention from researchers. At present, ammonium polyphosphate has been widely used in polymer materials such as polypropylene (PP), epoxy resin (EP), and polyurethane (PU).
Introduction to ammonium polyphosphate
Ammonium polyphosphate, abbreviated as APP, also known as condensed ammonium phosphate or polyphosphate, was first synthesized in 1857 by reacting P2O5 and NH3 as raw materials. Its molecular formula is (NH4) n+2PnO3n+1, where n represents the degree of polymerization. When n is large enough, its chemical formula can be written as (NH4PO3) n.
Generally, the physicochemical properties of APPs with different degrees of aggregation are different. As n increases, the water solubility of APPs decreases. If n>20, it shows poor solubility in water, while if n<20, it shows easy solubility in water.
The APP has a high content of nitrogen and phosphorus, which can be applied in the fields of fertilizer and flame retardant. The degree of polymerization determines the purpose of the APP. Low molecular weight water-soluble APP is often used as an efficient fertilizer, while high molecular weight APP is a commonly used halogen-free and environmentally friendly flame retardant for polymers.
APP has two solid forms: crystalline and amorphous (glassy). Crystalline APP is a white powder that is relatively stable at room temperature and odorless. At present, there are six known crystalline APP types (Type I, II, III, IV, V, VI), with Type I and Type II being the most commonly used. Type I has a lower degree of polymerization, smaller molecular weight, and higher solubility, and is commonly used in the preparation of fireproof coatings and fertilizers. Type II has the characteristics of high degree of polymerization, low water solubility, stable structure, and high decomposition temperature, and is often used in the field of flame retardancy.
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