Correlation Between Elemental Composition and Performance
Halogen-based structures: Higher bromine content facilitates the cleavage of carbon-halogen bonds within the 200–300°C processing temperature range, thereby enhancing gas-phase flame retardancy through the scavenging of combustion free radicals; however, excessive bromine content leads to increased release of toxic hydrogen halide gases and significantly higher smoke density during combustion.
Phosphorus-based structures: The phosphorus content directly determines char-forming capability in the condensed phase; higher phosphorus levels increase the efficiency of generating dehydration products (such as metaphosphoric acid) upon heating, facilitating the formation of a dense char layer that insulates against heat and oxygen. Structures featuring phosphorus-nitrogen synergism simultaneously exert effects in both the gas phase (radical scavenging) and the condensed phase (char formation), resulting in flame retardancy efficiency far superior to that of structures containing phosphorus alone.
Inorganic hydroxide structures: Higher water-of-crystallization content in aluminum hydroxide or magnesium hydroxide results in greater heat absorption during thermal decomposition and improved cooling-based flame retardancy; however, high loading levels significantly reduce the mechanical strength and processing fluidity of the base material.
Correlation Between Molecular Chain Structure and Performance
Small-molecule structures: Examples include small-molecule phosphate esters; these possess low molecular weight and good fluidity, acting simultaneously as plasticizers, but they are prone to migration and exudation, resulting in poor long-term flame retardancy durability.
Polymeric/oligomeric structures: Examples include polymerized phosphate esters and brominated epoxy resins; the compatibility of their molecular chain segments with the matrix resin is greatly enhanced, minimizing negative impacts on the material's glass transition temperature while completely eliminating the migration and volatilization issues associated with small molecules, thereby significantly extending the effective lifespan of the flame retardancy.
Microencapsulated structures: Encapsulating active components like red phosphorus preserves high flame retardancy efficiency while mitigating issues such as susceptibility to oxidation and moisture absorption, as well as the risk of dust explosions; this approach substantially improves thermal stability and compatibility with polymer materials.
