Introduction to the Flame Retardant Mechanism of Insulating Terminal Plastic Components

The flame retardant effect of plastic parts is not singular, but plays a role in different stages of combustion (heating, decomposition, ignition, spreading) through physical and chemical pathways. The main mechanisms can be divided into the following types:
1. Gas-phase flame retardant mechanism (trapping free radicals)
This is one of the most important and efficient mechanisms.
Principle: When flame retardants are decomposed by heat, they can release active substances (such as halogen radicals X·), which will "trap" the H· and HO· (hydroxyl group) radicals that maintain the flame chain reaction in the combustion reaction, so that their concentration drops sharply, so as to effectively interrupt the chemical reaction of combustion.

Typical flame retardants:
Halogen flame retardants (bromine, chlorine): high efficiency, but large smoke during combustion, may produce toxic corrosive gases (dioxins, etc.), currently limited in some fields with high environmental protection requirements.
Halogen-free flame retardants (such as nitrogen, phosphorus-nitrogen intumescent flame retardants): Non-combustible gases such as ammonia, nitrogen, and water vapor are produced by decomposition, diluting oxygen and combustible gas concentrations, while also trapping free radicals.

2. Condensed phase flame retardant mechanism (formation of carbon layer)
Principle: Flame retardants can promote the dehydration and carbonization of polymers on the surface of the flame, forming a dense and porous carbon layer. This carbon layer plays multiple roles:
Insulation: Blocks the transfer of external heat to the internal polymer.
Oxygen barrier: prevent internal combustible decomposition products from escaping, and also isolate external oxygen from entering.
Barrier layer: Protects the underlying material from melt droplets.
Typical flame retardants:
Phosphorus flame retardants: such as phosphate esters, red phosphorus. phosphoric acid and polymetaphosphic acid are formed by heat, which promotes the dehydration of oxygenated polymers (such as PC and PET) into carbon.
Intumescent flame retardant system (IFR): It is composed of acid source, carbon source and gas source. When heated, it foams and expands, forming a porous foam carbon layer dozens of times larger than the original thickness, which has excellent thermal insulation and oxygen insulation effect, and is the mainstream technology of high-end halogen-free flame retardant.

3. Cooling mechanism (endothermic cooling)
Principle: Some flame retardants absorb a lot of heat (high heat absorption) when decomposing, thereby reducing the temperature of the material surface and delaying the thermal decomposition process of polymers.
Typical flame retardants:
Metal hydroxides: The most commonly used are aluminum hydroxide (ATH) and magnesium hydroxide (MH). They absorb a lot of heat as they decompose and release water vapor, diluting combustible gases and oxygen. This is the most environmentally friendly, smoke-free flame retardant method, but requires a high amount of addition (usually > 50%), which can affect the mechanical properties and processing fluidity of the material.

4. Droplets and dilution effects
Droplet effect: Some materials (such as non-flame retardant PA6/PA66) melt and drip quickly when heated, carrying away heat and thus protecting the material above. But this is uncontrollable and dangerous because the molten droplets can carry flames to ignite other components. A good flame retardant system should suppress droplets, or make them non-combustible (limited droplets are allowed in grade V-2, and strictly limited in grade V-0).
Dilution effect: The decomposition of flame retardants produces non-combustible gases (such as water vapor, CO₂, N₂, NH₃, etc.), diluting the concentration of combustible gases and oxygen around the material.