Flame retardant mechanism

Flame retardant mechanism

As shown in Fig. 2-1, when the material is heated, it decomposes, providing the gas “Fuel”for combustion. The combustible gas is ignited with enough oxygen or oxidant and an external ignition source, and the material begins to burn, the heat generated during combustion further promotes the decomposition of the matrix to produce combustible gas, which sustains the combustion process. Therefore, flame-retardant technology can achieve the goal of flame-retardant by controlling one or more conditions of maintaining combustion. Usually flame-retardant technology can complete the flame-retardant process by the action of gas phase, condensed phase or two phase.

This paper mainly describes the mechanism of gas phase flame retardant

Mechanism of gas phase flame retardant

Flame retardants in gas phase play a role in retarding or interrupting combustion, including quenching effect, dilution effect, taking away heat, isolating oxygen, blowing out effect and so on.

1. Quenching effect

The combustion of polymer materials is actually a chain reaction of free radicals, which can be blocked to slow down the combustion intensity. During the combustion of the flame-retardant composites, the thermal decomposition of the flame retardant will produce a free radical substance, which can capture the free radicals released by the combustion of the matrix, thus interrupting the chain reaction of the combustion of the polymer materials, has the effect of reducing or even interrupting combustion. This phenomenon is called quenching effect and the mechanism is shown in Fig. 2-2. For example, BR produced by the burning of brominated flame retardants can trap free radicals in the combustion reaction and block the chain reaction of combustion, for example, some flame retardants, such as aluminum diethylhypophosphite and phosphorus-phenanthrene compounds, can be decomposed to produce phosphorus compounds (such as PO·, PO2, etc.) , which can trap H·, OH·, R· and thus prevent or slow down the combustion chain reaction, the material is endowed with good flame retardancy and self-extinguishing.

As shown in Fig. 2-3, the TGD, a flame retardant derived from phosphaphenanthrene, first splits into two parts (m/z = 249) and fragments (m/z = 230 or 215) . As the molecular fragments further disintegrate: on the one hand, two kinds of phosphorus heterophy fragments will split into free radical fragments (pO m/z = 47; PO 2, m/z = 63) , phenoxy radical (m/z = 93) , phenyl methyl disubstituted phosphoryl radical (m/z = 139) , biphenyl radical (m/z = 152) and o-phenoxy radical (m/z = 169) On the other hand, tri-allyl-triazinone will split into a series of inert alkyl isocyanate free radical fragments (m/z = 208,125,83,70,56) and active alkyl free radical fragments (m/z = 41) . The free radical fragments such as PO·, PO2 and phenylmethyl disubstituted phosphoryl radical in the pyrolysis products of TGD can effectively quench the active free radicals such as HO·, H· and R· during the combustion process of the materials and play the role of gas phase flame retardancy.

2. Dilution effect

Dilution effect refers to the concentration dilution of the combustible gas from the decomposition of the combustible by the heat of the flame retardant, which retards or suppresses the burning of the material and dilutes the oxygen concentration in the combustion zone, to make it difficult to burn or reduce the intensity of combustion, thus achieving the goal of flame retardant.

For example, when ammonium polyphosphate is heated, it first decomposes to produce NH3, which dilutes the concentration of combustible gas in the mixture around the material and reduces the content of oxygen in the mixture Water vapor, NH3, N2 and other incombustible gases can be formed when zinc borate, melamine and cyanurate are heated, which can also dilute the concentration of combustible gases and oxygen.

3. Take away the heat

To take away heat is to take away part of the heat generated by the burning of the material, reduce the temperature of the burning area, restrain the thermal decomposition intensity of the material matrix, and slow down the release rate of combustible gases, it even prevents the material from maintaining its thermal decomposition temperature and producing combustible gases, which can slow the combustion reaction and even lead to spontaneous combustion. For example, in most cases, firefighters respond to fires by spraying water, one of the functions of water on fire is to extinguish fire by evaporation of water, which takes away a lot of heat and reduces the temperature of the fire area. Some inorganic flame retardants, such as Magnesium hydroxide, Aluminum hydroxide and brucite, which are rich in hydrogen and oxygen, produce a large amount of water vapor when heated, which takes away the heat of the system and prolongs the time for the materials to reach the ignition point, in addition, the droplet is also an important way to take away the heat of the combustion system, when the polymer is flame-retarded by chlorinated paraffin or its synergistic system with antimony oxide, because these flame retardants can promote the decomposition and melting of the polymer, the molten polymer drips away most of the heat, thus reducing the feedback to the matrix polymer fuel combustion delay or even stop combustion. However, hot droplets of fusible material can still ignite other substances, sometimes increasing the risk of fire.

4. Block out oxygen

The isolated oxygen in gas phase flame retardant refers to the dense gas released by the flame retardant, which covers the surface of the polymer and can prevent the contact between the polymer or combustible gas and oxygen, thus achieving the goal of flame retardant. For example, brominated flame retardants work in concert with antimony trioxide flame retardants to produce antimony tribromide gases that are denser than air and coat the material with oxygen to create flame retardants.

5. Blow out effect

The blowing-out effect is that in the course of burning of the flame-retardant material, the gas phase products produced by the decomposition of the flame-retardant agent in advance rapidly form larger inner bubbles in the substrate, and the bubbles contain higher concentration of flame-retardant components, when the pyrolytic gas in the bubble inside the carbon layer reaches a certain volume and pressure, it will break through the carbon layer and shoot out at a high speed in a very short time, the fast extinguishing effect on the flame is realized, and the mechanism is shown in Fig. 2-5. This effect was put forward by Yang Rongjie and Zhang Wenchao when they studied the flame retardant mechanism of epoxy.

The blowing-out effect currently occurs mainly in flame-retardant epoxy containing phosphene compounds as flame retardants. The structure of the curing agent and the flame retardant used in the epoxy has an important influence on the cross-linking of the epoxy and the release rate of the gas, even on whether the blow-out effect occurs or not.

For example, DO-PO-octa-phenyl cage-like oligomeric silsesquioxane (OPS) is used as flame retardant, and 4,4-diaminodiphenylsulfone (DDS) is used as curing agent to flame-retardant the epoxy, and the flame-retardant complex forms carbon layer during the combustion process, the internal matrix of the flame retardant epoxy is constantly broken down and replenished with combustible volatiles to the external surface. At the same time, the outside of the carbon layer began to cross-link, forming a carbon layer, play a certain role in gas phase barrier. During this process, the gases released by the flame retardant are aggregated in the molten matrix to form bubbles. When the pressure in the bubble exceeds the limit of the carbon layer, the gas accumulated in the inner part of the carbon layer is ejected from the inner part of the carbon layer.

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