Ammonium Polyphosphate

Ammonium polyphosphate (APP) is an inorganic salt of polyphosphoric acid and ammonia. Because it contains phosphorus and nitrogen in its molecule, it has a good synergistic effect and has a good flame retardant effect. Alfa Chemistry offers various types of APP flame retardants to help you meet the flame retardant challenges of materials.

Classification

Classified by degree of polymerization

APP can be divided into three types classified by the degree of polymerization: low poly, medium poly and high poly. The higher the degree of polymerization, the lower the water solubility. The degree of polymerization of oligomeric APP is less than 20, which is water-soluble. The polymerization degree of high-polymer APP is greater than 50, which can be used as a flame retardant.

Classified by structure

APP can be divided into crystalline form and amorphous form classified by its structure. Crystalline APP is a long-chain water-insoluble polyphosphate. There are five variants of crystalline APP from I to V.

Among them, crystal phase II APP (APP II) has a significant advantage in the field of polymer materials due to its good water insolubility, high decomposition temperature, and good compatibility with polymer materials.

Crystalline Phase I APP (APP I) is characterized by a variable linear chain length. In APP I, n (the number of phosphate units) is usually below 100. Compared to APP II, APP I has a lower decomposition temperature (about 150°C) and higher water solubility.

Applications

• Considering factors such as price and cost, oligomeric water-soluble APP is mainly used for flame retardancy of cellulose materials such as paper and wood.
• APP has high thermal stability and can be formulated into solution or dry powder fire extinguishing agent, and can also be used in intumescent fire retardant coatings, intumescent flame retardant systems, etc.
• APP can be used as a flame retardant for polymer materials such as polyolefin, PVC, polyurethane, phenolic resin, and coatings.

Flame Retardant Mechanism

When phosphorus flame retardant (PFR) burns, it generates phosphoric acid, metaphosphoric acid, polymetaphosphoric acid, etc., which are covered on the surface of the resin to promote the carbonization of the plastic surface to form a carbon film. The polymetaphosphoric acid is covered in a viscous liquid state on the surface of the plastic. The solid or liquid membrane prevents the escape of free radicals and keeps out oxygen.

In addition, PFRs are also a free radical scavenger. Using mass spectrometry technology, it was found that any phosphorus-containing compounds have PO· formed when the polymer is burned. It can combine with the hydrogen atoms in the flame area to suppress the flame.

Research Information

Multifunctional APP

In addition to being a flame retardant, APP is a novel multifunctional adhesive. Guangming Zhou et al. designed a new inorganic binder ammonium polyphosphate and applied it in lithium-sulfur batteries. The results show that APP can reduce the flammability of the sulfur cathode as shown in the figure below. During the combustion process, APP decomposes to release ammonia/water vapor, and crosslinks to form an insulating polymer layer, which effectively protects the sulfur electrode. [1]

Schematic diagram of the flame retardant mechanism of APP-based sulfur electrodes [1]

Reference

1. Guangmin Zhou, et al. (2018). "An Aqueous Inorganic Polymer Binder for High Performance Lithium–Sulfur Batteries with Flame-Retardant Properties," ACS Cent. Sci. 4(2), 260–267.