Phosphorus-based flame retardants are a class of flame retardants that utilize phosphorus as the core flame-retardant component; based on their chemical structure, they are primarily categorized into inorganic and organic types. Inorganic phosphorus-based flame retardants include red phosphorus and ammonium polyphosphate (APP). Red phosphorus is a common inorganic variant known for high flame-retardant efficiency; during combustion, it converts into substances such as phosphoric acid and metaphosphoric acid, forming a protective layer that excludes oxygen. However, it is highly prone to moisture absorption and oxidation, is sensitive to heat, friction, and impact, and possesses an inherent color. Ammonium polyphosphate (APP) is an inorganic polymer with a variable degree of polymerization; it features good thermal stability and low water solubility, decomposing upon heating to release phosphoric acid and ammonia, which promote dehydration and char formation.
Organic phosphorus-based flame retardants include phosphates, phosphites, and phosphonates. Phosphates, such as tricresyl phosphate (TCP) and triphenyl phosphate (TPP), offer both flame-retardant properties and plasticizing effects. Phosphites, such as triphenyl phosphite (TPPi), provide flame retardancy alongside antioxidant capabilities. Phosphonates, such as dimethyl methylphosphonate (DMMP), are characterized by high phosphorus content, high flame-retardant efficiency, and relatively low volatility. However, some organic phosphorus-based flame retardants suffer from issues such as high volatility and migration, or adverse effects on material properties.
The flame-retardant mechanisms of phosphorus-based flame retardants primarily involve gas-phase and condensed-phase mechanisms. In the gas-phase mechanism, the flame retardant decomposes to generate phosphorus-containing radicals (such as PO•); these capture H• and OH• radicals to interrupt the chain reaction, while the decomposition gases dilute the combustible gases. In the condensed-phase mechanism, the flame retardant decomposes to produce phosphoric acid, metaphosphoric acid, and similar compounds; these promote the dehydration and carbonization of the material to form a thermal-insulating char layer or create a glass-like molten coating on the material's surface.
Flame Retardant Mechanisms
The flame retardant mechanisms of phosphorus-based flame retardants primarily involve two modes: gas-phase action and condensed-phase action.
Phosphorus-containing additives function mainly in the condensed phase. Their mechanisms are as follows:
The gas-phase mechanism involves the decomposition of the flame retardant to produce volatile phosphorus-containing radicals (such as PO•). These radicals scavenge hydrogen radicals (H•) and hydroxyl radicals (OH•) in the flame zone, thereby interrupting the combustion chain reaction. Simultaneously, non-combustible gases (such as ammonia and water vapor) generated during decomposition dilute the concentration of combustible gases.
The condensed-phase mechanism is the primary mode of action for phosphorus-based flame retardants. Upon heating during combustion, phosphorus-containing compounds decompose to form substances with strong dehydrating properties, such as phosphoric acid, metaphosphoric acid, and polymetaphosphoric acid. These substances promote the dehydration and charring of polymer materials containing functional groups like hydroxyls, creating a dense, non-flammable carbonaceous thermal insulation layer on the material's surface. The formation of char reduces heat transfer from the flame to the condensed phase. Phosphoric acid absorbs heat by inhibiting the oxidation of CO to CO2, thereby slowing the heating process. Additionally, certain phosphoric acid products can melt to form a high-viscosity, glassy liquid film or-through synergy with nitrogen sources-create an intumescent foamed char layer. This layer effectively blocks the transfer of external oxygen and heat into the material while preventing the escape of combustible volatile products from within.
To enhance flame retardancy, synergistic systems combining phosphorus-based flame retardants with other flame-retardant elements-such as phosphorus-nitrogen or phosphorus-silicon combinations-are frequently employed. For instance, a synergistic system comprising layered silicates and phosphorus-based flame retardants can exert flame-retardant effects in both the gas and condensed phases, significantly boosting overall efficiency.
These substances-including red phosphorus, ammonium polyphosphate, phosphoramides, tricresyl phosphate, and most other phosphorus-containing compounds-can raise the ignition temperature of the material.