The combustion of polymers is a very intense and complex thermal oxidation reaction, characterised by the emission of smoke or a blazing flame. Flame retardants are a class of additives that prevent the ignition of plastics or inhibit the propagation of flame. The flame retardant effect of flame retardants is to prevent or inhibit the rate of physical or chemical change in the combustion process of polymeric materials. Specifically, the flame retardant effect is mainly reflected in the following aspects.
Flame retardant in the polymer combustion process of flame retardant effect:
Flame retardant flame retardant – heat absorption effect
The role of the heat absorption effect is to make the polymer material temperature rise occurs with difficulty.
For example, borax has 10 molecules of crystalline water, as the release of crystalline water to capture 141.8kJ/mol of heat, so heat absorption so that the temperature rise of the material is inhibited, resulting in flame retardant effect.
Flame retardant flame retardant – covering effect
The effect is to produce a stable covering layer at higher temperatures or to decompose to produce a foam-like substance that covers the surface of the polymeric material. This makes it difficult for the heat generated by combustion to enter the interior of the material and for the combustible gases generated by the thermal decomposition of the polymer material to escape. It acts as an air barrier to the material, thus inhibiting material cracking and achieving a flame retardant effect.
Flame retardant flame retardant – dilution effect
These substances can produce a large amount of non-flammable gas when decomposed by heat, so that the flammable gas produced by the polymer material and the oxygen in the air are diluted and do not reach the flammable concentration range.
Flame retardant flame retardant – transfer effect
The effect is to change the mode of thermal decomposition of the polymer material, thus inhibiting the production of combustible gases.
Flame retardant flame retardant – inhibition effect
Trapping the active intermediates of combustion reactions, such as HO- and H-, inhibits free radical chain reactions and reduces the rate of combustion until the flame is extinguished.
Flame retardant flame retardant – enhancement effect
Some materials do not have a flame retardant effect if used alone or have little flame retardant effect, but a variety of materials can be used to enhance the flame retardant effect, such as antimony trioxide and halogen compounds used together.
Flame retardant mechanism in polymer combustion process:
Flame retardant mechanism – gas phase flame retardant mechanism
Vapour phase flame retardant systems refer to the flame retardant effect of interrupting or retarding chain combustion reactions in the gas phase. Vapour phase flame retardancy mainly consists of:
- Free radical inhibitors are produced when the flame retardant material is heated or burned, thus interrupting the combustion chain reaction.
- The production of fine particles when the flame retardant material is heated or burned, which promote the binding of free radicals to terminate the chain combustion reaction.
- The release of large amounts of inert gas or high density vapour when the flame retardant material is heated or burned.
Flame retardant mechanisms – condensed phase flame retardant mechanisms
Condensed phase flame retardancy refers to the flame retardant effect produced by delaying or interrupting the thermal decomposition of flame retardant materials in the condensed phase. Condensed phase flame retardancy mainly consists of:
- Flame retardants that retard or interrupt the thermal decomposition of flammable gases and radicals that can be produced during the coalescence process
- Inorganic fillers with a large specific heat capacity in flame retardant materials, which make the material less likely to reach the thermal decomposition temperature through heat storage and thermal conductivity.
- The thermal decomposition of the flame retardant absorbs heat, so that the temperature rise of the flame retardant material is slowed or suspended.
- When the flame retardant material burns, it generates a porous carbon layer on its surface, which is difficult to burn, insulates the heat and oxygen, and prevents the combustible gas from entering the gas phase.
Flame retardant mechanism – interruption of heat exchange flame retardant mechanism
The interrupted heat exchange flame retardant mechanism means that part of the heat generated by the combustion of the flame retardant material is taken away, so that the material cannot maintain the thermal decomposition temperature and therefore cannot maintain the production of combustible gases, and so the combustion is self-extinguishing.
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