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Why do polymer materials have flame retardant properties? Flame retardant mechanism of polymer materials
Generally speaking, flame retardant polymer materials can be achieved by several types of flame retardant mechanisms, such as gas phase flame retardant, condensed phase flame retardant, and interrupted heat exchange flame retardant. By inhibiting the free radicals that promote the growth of the combustion reaction chain in the gas phase, it is a gas phase flame retardant; The flame retardant that takes part of the heat generated by the combustion of the polymer away to achieve flame retardant belongs to the type of interrupted heat exchange mechanism. However, since both combustion and flame retardant are very complex processes involving many influencing factors and constraints, it is often one-sided and difficult to strictly classify the flame retardant mechanism of a flame retardant system into a certain one in isolation. In fact, many flame retardant systems work simultaneously with multiple flame retardant mechanisms. 1. Mechanism of gas phase flame retardant The so-called gas phase flame retardant is to improve the flame retardant performance of the material by preventing the combustion of the combustible gas product decomposed by the polymer or preventing the flame reaction. Usually can be achieved by the following means. ①Use a flame retardant that can release reactive gas compounds under the action of heat. Such reactive compounds typically act on free radicals that affect flame formation or growth. The halogen-antimony flame retardant system commonly used in industry is one of the typical examples that act in this way. ②Adopt additives that can form fine smoke particles during the combustion process of the polymer. Such smoke particles can act in the growth of a heterogeneous flame, mainly catalyzing the binding and termination of free radicals in combustion. ③Select additives that can release a large amount of inert gas during decomposition. The presence of a large amount of inert gas can dilute the concentration of flammable gas produced by the decomposition of the polymer, and at the same time can reduce the temperature of the decomposition product of the polymer, so the obtained gas mixture will not make the flame grow when it encounters the action of the surrounding oxidant. . ④ In some cases, the added additives do not undergo chemical changes when heated, but only release heavy vapors. This vapor covers the surface of the combustible gas decomposed by the high polymer, affecting its normal exchange with air and oxygen, thus suffocating the flame. 2. Condensed phase flame retardant mechanism Condensed phase flame retardant mainly refers to preventing the thermal decomposition of high polymers and the release of flammable gases. It can be achieved by the following methods. ①Add additives that can prevent the thermal decomposition of the polymer from generating free radical chains in the solid phase. ②Add various inorganic fillers. Since inorganic fillers have a large specific heat capacity, they can play the role of heat storage; at the same time, because they are non-thermal insulators, they can also play a role in heat conduction. It is because of their existence that these effects prevent the rapid rise of the surface temperature of the polymer, so that it cannot reach the decomposition temperature at which thermal decomposition occurs and flammable gas occurs. ③ Add flame retardants that can decompose after endothermic, such as aluminum hydroxide, etc. Since this flame retardant has the characteristics of endothermic decomposition, it can effectively keep the polymer at a lower temperature without reaching the level of thermal decomposition. ④ Apply a non-flammable protective coating on the surface of the high polymer material, which can insulate the high polymer from heat and oxygen, and prevent the flammable gas generated by the decomposition of the high polymer from escaping into the combustion gas phase. Among the application examples, intumescent fire retardant coatings are typical. After being exposed to fire, it can form a carbonized protective layer with a thickness that is dozens or even hundreds of times greater than itself, thus playing the role of oxygen insulation, heat insulation and protection of the substrate. The flame retardant mechanism of the new intumescent flame retardant is similar to this. 3. Interruption of heat exchange flame retardant mechanism As mentioned above, an important condition for maintaining continuous combustion is that part of the combustion heat must be fed back to the polymer, so that the polymer can be continuously decomposed by heat, thereby providing the necessary conditions for maintaining combustion. fuel source. Therefore, if a certain flame retardant is added, the burning heat can be removed so that it does not return to the polymer, and the burning can be interrupted. In actual production, this flame retardant method has been applied. For example, in liquid or low molecular weight chlorinated paraffin or high polymer flame retardant with antimony trioxide in combination, because the flame retardant can promote the depolymerization or decomposition of the high polymer by heat, it is conducive to the occurrence of high polymer Heating and melting; when the burning polymer droplets drip from the body, a large amount of heat is taken away, reducing the heat fed back to the base material, thereby delaying the combustion and eventually interrupting the combustion.
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