The action of the flame retardant can comprehensively slow down the speed of the combustion reaction, or make the initiation of the reaction (thermal spontaneous combustion) difficult, thereby achieving the purpose of suppressing and reducing the fire hazard.
First, the burning process of plastic
To understand the principle of plastic flame retardant, we must first understand the burning process of plastics.
The burning process of plastics is an extremely complex thermal oxidation reaction, which leads to the basic elements of combustion being heat, oxygen and combustible gases.
It is generally believed that the burning of plastics has gone through the following three stages.
The first stage, the thermal initiation process. The heat from an external heat source or source of ignition causes the plastic to undergo too much phase change (ie, conversion from solid to liquid) and chemical changes.
The second stage, the thermal degradation process. This process is an endothermic reaction. When the heat absorbed by the plastic is sufficient to overcome some weak bond energy between the atoms in the molecule, the plastic begins to degrade. The essence of this reaction is a free radical chain reaction in the presence of oxygen in the air, resulting in a gas phase flammable object such as various monomer flammable hydrocarbons.
The third stage, the ignition process. When the concentration of combustibles in the second stage of thermal degradation reaches the ignition limit, it meets the oxygen in the atmosphere.
Second, the flame retardant mechanism
Adding a flame retardant to the plastic in a certain proportion can increase the oxygen index and have a significant flame retardant effect. Of course, the oxygen index only indicates the flammability of the material and the flame retardancy of the flame retardant. A series of parameters, such as thermal spontaneous combustion critical parameters, thermal ignition energy, and thermal auto-ignition temperature, should also be used.
In general, when a plastic containing a flame retardant is burned, the flame retardant functions in various reaction zones (gas phase, * coacervate phase). The effect of the flame retardant may also be different for different materials.
The mechanism of action of the flame retardant is relatively complicated. But its purpose is always to cut off the combustion cycle by physical and chemical means. The effect of flame retardant on the combustion reaction is as follows:
(1) The flame retardant located in the condensed phase absorbs heat and decomposes, so that the relative temperature in the condensed phase is slowed down to delay the thermal decomposition temperature of the plastic, and the gasification of the incombustible gas generated by thermal decomposition of the flame retardant is utilized. Heat to lower the temperature.
(2) The catalytic condensed phase is thermally decomposed to produce a solid phase product (coking layer) or a foam layer that hinders heat transfer. This keeps the temperature of the condensed phase at a low level, resulting in a decrease in the formation rate of the gas phase reaction raw material (flammable gas decomposition product).
(3) The flame retardant is thermally decomposed to release a flame retardant that captures Â·OH (hydroxyl) radicals in the combustion reaction, so that the combustion process by the radical chain reaction terminates the chain reaction.
(4) Under the action of heat, the flame retardant exhibits an endothermic phase change, preventing the temperature in the condensed phase from rising, and slowing down the combustion reaction until it stops.
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