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The burning process of plastics.

To understand the principle of plastic fire retardation, we must first understand the burning process of plastics.

The combustion process of plastics is an extremely complex thermal oxidation reaction, which leads to the combustion of heat, oxygen and combustible gases.

It is generally believed that the burning of plastics has gone through three stages.

The first stage, the thermal initiation process. The heat from external sources or sources of heat causes the plastic to change too much (that is, from solid to liquid) and chemical change.

The second stage is thermal degradation. This process is an endothermic reaction, and when the plastic absorbs enough heat to overcome some of the weak bonds within 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, and the result of the reaction produces gas phase combustible objects such as various monomers, flammable hydrocarbons, etc.

The third stage, the ignition process. When the second stage thermal degradation reaction produces a flammable concentration that reaches the ignition limit, it meets the oxygen in the atmosphere.

Ii. Flame retardant mechanism.

In plastic, the flame retardant can be added to a certain proportion to increase the oxygen index and the flame retardant effect is obvious. Of course, the oxygen index only indicates the flammability of the material and the flame retardant of the flame retardant, and a series of parameters should be adopted, such as the critical parameter of thermal self-ignition, the ignition energy of heat and the temperature of spontaneous combustion. Generally speaking, when the plastics containing flame retardant are burning, the flame retardant is used in various reaction areas (gas phase, * condensed phase). For different materials, the effect of flame retardant may be different.

The mechanism of flame retardant is more complex. But its purpose is always to cut off the combustion cycle in a physical and chemical way. The effect of flame retardant on combustion reaction is shown in the following aspects:

(1) in the condensed phase of flame retardant endothermic decomposition, so that the relative slow temperature rise inside the condensed phase, to delay the thermal decomposition temperature of the plastic, the non-combustible gases generated when using the thermal decomposition of flame retardant of gasification and heat to lower the temperature.

(2) the flame retardant is decomposed by pyrolysis, releasing the flame retardant of the OH (hydroxyl) radical in the combustion reaction, so as to terminate the chain reaction in the combustion process according to the free radical chain reaction.

(3) under the heat effect, the flame retardant has the heat absorption phase change, which prevents the temperature rise in the condensed phase and slows the combustion reaction until it stops.

(4) catalytic coagulation phase pyrolysis, producing solid phase products (coking layer) or foam layer, impeding the heat transfer function. This keeps the condensed phase temperature at a low level, resulting in a decrease in the formation of the gas as a reaction material (flammable gas decomposition product).