The structure of halogen-free flame retardants of phosphoronitrile type is mainly divided into two categories: cyclic phosphoronitrile and linear phosphoronitrile. The following is a detailed introduction:
Cyclic phosphazonitrile
The structural feature of cyclic phosphoronitrile is that phosphorus and nitrogen atoms are alternately arranged to form a cyclic skeleton. For instance, hexachlorocyclotriphosphazene (HCCTP) is the most fundamental phosphazene compound. In its structural formula, phosphorus and chlorine atoms are alternately arranged to form a hexagonal planar ring structure, which has aromaticity and good thermal stability. The phosphorus-chlorine bond in this structure is highly reactive and prone to nucleophilic substitution reactions, thereby introducing different side chain groups to synthesize cyclophosphazene compounds with specific functions.
Linear phosphazonitrile
The main chain of linear phosphazonitrile is composed of alternating single and double bonds of phosphorus and nitrogen atoms, and the side chains are replaced by organic groups. Polyphosphazonitrile is a typical linear phosphazonitrile structure. Its main chain has high stability, and the linear arrangement of nitrogen and phosphorus gives it good tortonal flexibility. Therefore, its glass transition temperature is low, and most polyphosphazonitrile is a low-temperature elastomer. By altering the side groups of polyphosphoronitrile, various polyphosphoronitrile derivatives with different properties can be obtained to achieve various applications. For instance, phenoxy polyphosphazonitrile is a synthetic polymer material with a special structure. Its molecular structure features a repetitive phosphorus-nitrogen skeleton, and the side chain is a phenoxy group, which endows it with excellent thermal stability and moisture and aging resistance.
