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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 through several types of flame-retardant mechanisms such as gas-phase flame-retardant, condensed phase flame-retardant, and interrupted heat exchange flame-retardant. Gas phase flame retardant is a gas phase flame retardant by inhibiting free radicals that promote the growth of the combustion reaction chain in the gas phase; a condensed phase flame retardant is flame retardant by delaying or preventing the thermal decomposition of polymers in the solid phase; and The flame retardant that takes away part of the heat generated by the combustion of the polymer to achieve flame retardancy belongs to the type of flame retardant that interrupts the heat exchanger. However, because both combustion and flame retardancy are very complex processes, involving many influencing factors and restrictive conditions, it is often one-sided and difficult to strictly divide the flame retardant mechanism of a flame retardant system into a certain type in isolation. In fact, many flame-retardant systems work simultaneously with multiple flame-retardant mechanisms. 1. Gas phase flame retardant mechanism The so-called gas phase flame retardant is to improve the flame retardant properties of materials by preventing the combustion of flammable gas products from the decomposition of high polymers or preventing the flame reaction. Usually can be achieved by the following means. ① Use flame retardants that can release active gas compounds under the action of heat. This reactive compound usually has an effect on free radicals that affect the formation or growth of flames. The halogen-antimony flame retardant system commonly used in industry is one of the typical examples of this way. ②The use of additives that can form fine smoke particles during the combustion of polymers. This kind of smoke particles can act on the growth of the flame in different phases, mainly to catalyze the combination 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 the 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 increase the flame when it encounters the surrounding oxidant. . ④In some cases, the added additives do not undergo chemical changes after being heated, but only release heavy vapor. This vapor covers the surface of the combustible gas decomposed from the polymer, affecting its normal exchange with air and oxygen, and suffocating the flame. 2. Condensed phase flame retardant mechanism Condensed phase flame retardant mainly refers to the effect of preventing thermal decomposition of polymers and the release of flammable gases. It can be achieved by the following methods. ①Add additives that can prevent the thermal decomposition of high polymer in the solid phase to generate free radical chains. ②Add various inorganic fillers. Since inorganic fillers have a large specific heat capacity, they can play a role in heat storage; at the same time, because they are non-insulators, they can play a role in heat conduction. It is precisely because of these effects that they prevent the rapid rise of the surface temperature of the polymer, making it unable to reach the decomposition temperature at which flammable gas is generated by thermal decomposition. ③Add flame retardants that can decompose after heat absorption, such as aluminum hydroxide. Because 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 polymer material, which can insulate the polymer, insulate oxygen and prevent the flammable gas generated by the decomposition of the polymer from escaping into the combustion gas phase. In the application examples, the intumescent fire retardant coating has a typical representative. After it is exposed to fire, it can form a carbonized protective layer that is dozens or even hundreds of times thicker than itself, thereby playing the role of oxygen barrier, 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 earlier, 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 will continue to undergo thermal decomposition and provide the necessary maintenance for combustion. Fuel source. Therefore, if a certain flame retardant is added to remove the burning heat so that it no longer returns to the polymer, the combustion can be interrupted. In actual production, this flame retardant method has been applied. For example, in liquid or low-molecular-weight chlorinated paraffin or its flame-retardant polymer used in combination with antimony trioxide, the flame retardant can promote the thermal depolymerization or decomposition of the polymer, which is beneficial to the formation of the polymer. Heated melting; when the burning polymer droplets drop from the body, a large amount of heat is taken away, reducing the heat feedback to the base material, so that the combustion is delayed, and finally the combustion is interrupted.
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