The essential principle of flame retardancy of halogen-free flame retardants mainly inhibits the combustion process through the combined effect of physical and chemical actions. Its core mechanism can be summarized into the following four categories, and its mode of action is illustrated with specific examples:
Condensed phase flame retardant mechanism
Principle: By forming a dense protective layer on the surface or inside the material, it isolates the transfer of oxygen and heat, and inhibits the release of flammable gases.
Example:
Phosphorus-based flame retardants (such as phosphate esters) decompose at high temperatures to form phosphoric acid or polyphosphoric acid, which catalyzes the dehydration and carbonization of the material surface and forms a continuous carbon layer. For instance, after adding phosphorus-based flame retardants to polypropylene, the thickness of the carbon layer increases by more than three times during combustion, and the oxygen transmission rate decreases by 80%.
Silicon-based flame retardants (such as silicone oil) migrate to the material surface and oxidize to form a silicon dioxide ceramic layer, whose thermal conductivity is only 1/100 of that of metals, effectively blocking heat.
2. Gas-phase flame retardant mechanism
Principle: By releasing inert gases or free radical scavengers, the concentration of flammable gases is diluted or the combustion chain reaction is interrupted.
Example:
Nitrogen-based flame retardants (such as melamine) decompose to produce nitrogen gas, ammonia gas, etc., reducing the oxygen concentration in the flame area from 21% to below 12%, and the flame goes out.
Phosphorus-nitrogen synergistic systems (such as ammonium polyphosphate and triazine compounds) release PO· free radicals, which react with H· and OH· in the combustion chain to reduce the flame propagation speed.
3. Heat absorption and cooling mechanism
Principle: Through the endothermic decomposition reaction of the flame retardant itself, the surface temperature of the material is reduced, and the thermal decomposition process is delayed.
Example:
Aluminum hydroxide (ATH) begins to decompose at 220℃, absorbing up to 1.17 kJ/g of heat, which can reduce the surface temperature of the material by 100-150℃. For example, adding 60% ATH to EVA cable material can reduce the peak heat release rate (PHRR) by 45%.
Intumescent flame retardants (such as the pentaerythritol – phosphoric acid – melamine system) absorb heat and release water vapor during decomposition. The dual cooling effect extends the time for the material to reach the thermal decomposition temperature by three times.
4. Mechanism for diluting combustibles
Principle: By increasing the filling amount of flame retardants or their decomposition products, the concentration of flammable components in the material is reduced.
Example:
When inorganic flame retardants (such as calcium carbonate) are filled in polymers, the proportion of combustible polymer matrix drops from 100% to 60%, and the theoretical heat release decreases by 40%.
Intumescent flame retardants decompose to produce a large number of porous carbon layers, with a specific surface area of up to 200 m²/g. They physically adsorb flammable gases, keeping the concentration of combustibles in the combustion zone below the lower explosive limit.
Synergy effect and compound mechanism
In practical applications, halogen-free flame retardants often achieve efficient flame retardancy through the synergistic action of multiple mechanisms. For example:
Phosphorus-nitrogen-silicon composite system: The phosphorus-based system promotes carbonization, the nitrogen-based system releases inert gas, and the silicon-based system forms a ceramic layer. The triple protection enables the material to pass the UL 94 V-0 level test (self-extinguishing time <10 seconds at a thickness of 1.6 mm).
Nano-modification technology: By compounding nano-montmorillonite (layered silicate) with phosphorus-based flame retardants, the nano-sheets can physically block gas diffusion and simultaneously catalyze carbonization, increasing the flame retardant efficiency by more than 50%.
Key parameters are related to performance
Carbonization rate: A high carbonization rate (such as 40% in the polyphenylene ether system) can form a continuous protective layer, blocking oxygen and heat.
Decomposition temperature: The decomposition temperature of flame retardants should match the processing temperature of the material (for example, ATH is suitable for processing below 200℃, and MDH is suitable for processing above 250℃).
Oxygen Index (LOI) : The LOI of halogen-free flame-retardant materials usually increases from 18% (for non-flame-retardant PP) to 28%-35%, significantly enhancing self-extinguishing ability.
Summary
The essence of flame retardancy of halogen-free flame retardants lies in blocking the interaction of the three elements of combustion (combustible material, oxygen, and heat) through mechanisms such as condensed phase protection, gas phase dilution, heat absorption cooling, and combustible material dilution. Its design needs to take into account both flame retardant efficiency and material performance. Through technologies such as compounding and nanoscale, a synergistic effect should be achieved to meet the dual demands of modern industry for safety and environmental protection.
