The mechanism of halogen-free flame retardants mainly includes the following aspects:
First, the role of heat absorption
Any combustion in a relatively short period of time the heat released is limited, if the fire source can be absorbed in a relatively short period of time part of the heat released, then the flame temperature will be reduced, radiation to the combustion surface and the role of combustible molecules that have been vaporized to crack into free radicals will reduce the amount of heat, combustion reaction will be inhibited to a certain extent. Under high temperature conditions, the flame retardant undergoes a strong heat-absorbing reaction, absorbs part of the heat released by combustion, reduces the temperature of the surface of the combustible material, effectively inhibits the generation of combustible gases, and prevents the spread of combustion. For example, aluminum hydroxide (Al(OH)3) flame retardant by improving the heat capacity of the polymer, before reaching the thermal decomposition temperature to absorb more heat and improve its flame retardant properties.
Second, covering effect
After adding flame retardants in combustible materials, flame retardants can form glassy or stable foam cover layer at high temperature to isolate oxygen, which has the effect of heat insulation, oxygen insulation and preventing combustible gases from escaping outward, so as to achieve the purpose of flame retardancy. Such as organic phosphorus flame retardants can produce a more stable structure when heated cross-linked solid material or carbonized layer. The formation of this carbonized layer on the one hand can prevent further pyrolysis of polymers, on the other hand, can prevent its internal thermal decomposition of the product into the gas phase to participate in the combustion process.
Third, inhibit chain reaction
According to the chain reaction theory of combustion, free radicals are required to maintain combustion. Flame retardants can act in the gas-phase combustion zone, capture the free radicals in the combustion reaction, thus preventing the propagation of the flame, so that the flame density of the combustion zone decreases, and ultimately reduce the rate of combustion reaction until termination. Such as halogen-containing flame retardants, its evaporation temperature and the polymer decomposition temperature is the same or similar, when the polymer is thermally decomposed, the flame retardant is also volatilized at the same time. With the thermal decomposition products at the same time in the gas-phase combustion zone, halogen will be able to capture the free radicals in the combustion reaction, thus preventing the spread of flame, so that the flame density of the combustion zone down, and ultimately make the combustion reaction rate down until the end.
Fourth, non-flammable gas asphyxiation
Flame retardant heat decomposition of non-flammable gas, combustible decomposition of combustible gases will be diluted to the concentration of combustion below the lower limit. At the same time, it also has the effect of diluting the oxygen concentration in the combustion area, preventing the combustion from continuing and achieving the effect of flame retardant.
Fifth, cohesive phase flame retardant mechanism
Halogen-free flame retardants at high temperature to form a glassy or stabilized foam cover layer, isolation of oxygen and has the role of heat insulation, oxygen insulation, thus preventing the combustible gases from escaping to the outside, to achieve the purpose of flame retardant.
Sixth, free radical capture mechanism
The flame retardant can act in the gas phase combustion zone to capture the free radicals in the combustion reaction, thus preventing the flame from spreading, reducing the flame density in the combustion zone, and ultimately reducing the speed of the combustion reaction.
Seventh, cooling mechanism
Flame retardants at high temperatures under the conditions of a strong heat-absorbing reaction, absorb part of the heat released by combustion, reduce the temperature of the surface of the combustible material, effectively inhibit the generation of flammable gases, prevent the spread of combustion.
Eighth, non-combustible gas mechanism
Flame retardant decomposes and generates non-combustible gas when it is heated, which dilutes the concentration of combustible gas from the decomposition of combustible materials to below the lower limit of combustion. At the same time, noncombustible gases can also dilute the oxygen concentration in the combustion zone, preventing the continuation of combustion, and play a role in flame retardant.
In practice, these mechanisms often exist and interact simultaneously to achieve the best flame retardant effect. At the same time, different types of non-halogenated flame retardants may have different flame retardant mechanisms and properties, so the selection and use of the specific application scenarios and needs to be considered.
