Polymer flame retardancy

Thread Content

When polymers are heated, they can decompose to produce volatile flammable substances; when the concentration of these flammable substances and the temperature of the system are high enough, combustion can occur. Therefore, the combustion of polymers can be divided into two processes: thermo-oxidative degradation and normal combustion. Obviously, the heat from the heat source used to heat the polymer must be sufficient to cause the polymer to decompose; the concentration of the combustible substances resulting from this decomposition must reach the ignition limit, and the system must be heated to the ignition temperature for combustion to occur. Whether the ignited polymer can continue to burn after the ignition source is removed depends on the heat balance during the burning process; when the heat generated by combustion exceeds the total heat required at each stage of combustion, the burning of the polymer will continue, otherwise it will go out. It is generally believed that the combustion of polymers consists of four elements: a heat source, oxygen, a combustible material, and radical reactions. Essentially, the flame-retardant effect of polymers is achieved by preventing or slowing down one or several of these factors. It mainly includes 6 aspects: improving the thermal stability of polymers, capturing free radicals to terminate chain reactions, forming a non-flammable protective film, absorbing heat, generating a high-density gas isolation layer, and diluting oxygen and flammable gases. The flame-retardant mechanisms that have been confirmed to date include: the solid-phase flame-retardant mechanism, the gas-phase flame-retardant mechanism, and the synergistic flame-retardant mechanism. The solid-phase flame-retardant mechanism, also known as the condensed-phase flame-retardant mechanism, is based on the following principles: (1) The added flame retardant is capable of slowing down or preventing the formation of flammable gases and free radicals resulting from the thermal decomposition of polymers in the solid phase; in other words, a chemical reaction occurs between these substances, and this reaction takes place at temperatures lower than those at which the polymer decomposes. (2) Filler-type flame retardants function as heat storage due to the high specific heat capacity of these inorganic substances; moreover, since they are not insulators, they can also conduct heat. As a result, polymers do not easily reach the thermal decomposition temperature. (3) The addition of endothermic, decomposable flame retardants can effectively prevent the temperature of the polymer from rising, keeping it below the thermal decomposition temperature. (4) After combustion, the flame retardant can form a porous protective carbon layer on the surface of the polymer. This layer provides flame resistance, thermal insulation, and oxygen barrier properties, and it also prevents flammable gases from entering the combustion gas phase, thereby interrupting the combustion process. Gas-phase flame retardancy mechanism: Gas-phase flame retardancy refers to the inhibition of the combustion of gases produced by the thermal decomposition of polymers, or the suppression of reactions associated with flames. The key aspects are as follows: (1) During heating or combustion, flame retardants can generate fine particles, which facilitate interactions between free radicals formed during combustion and thereby terminate the chain reaction. (2) When flame retardants decompose upon heating, they release large amounts of inert gases, which dilute oxygen and gaseous combustibles and lower the temperature of these combustible gases, thereby terminating the chain reaction. (3) When heated, the flame retardant produces only high-density steam, which can cover the flammable gases released by the polymer, preventing them from coming into contact with air and oxygen and thus stopping combustion. Synergistic flame retardant mechanism: To improve the efficiency of flame retardancy, it is common to use one flame retardant in combination with another substance known as a synergist; such a system composed of two or more components is referred to as a synergistic flame retardant system. Synergists themselves are not necessarily flame retardants. The flame-retardant effect of a synergistic system is often greater than the sum of the flame-retardant effects produced by individual components, which is why it is also commonly referred to as the synergistic effect (SE). Characterization of flame-retardant performance: The flame-retardant performance is mainly characterized by two indicators: self-extinguishing property and oxygen index. Self-extinguishing property: The shorter the self-extinguishing time, the better the flame-retardant effect, and it is generally expressed in seconds. Oxygen index: It refers to the percentage of oxygen required for a substance to burn. There are the limiting oxygen index (LOI) and the critical oxygen index (COI), with LOI being the one commonly used. When halogen-containing rubbers (such as fluororubbers, neoprene, CSM, and CM) burn, they decompose to release hydrogen halides (such as HCl) and flammable hydrocarbons. The temperature at which hydrogen halides are released is lower than the temperature at which the polymer backbone breaks down into smaller molecular fragments; therefore, hydrogen chloride is generally released earlier than flammable hydrocarbons. However, hydrogen halides are non-flammable, and they can prevent oxygen from reaching the material while diluting the flammable substances, thereby serving a flame-retardant function. Therefore, this substance is flame-retardant and has self-extinguishing properties; it is an inherent flame-retardant material. The table below lists the thermal decomposition temperatures and combustion properties of several halogenated rubbers: Within a certain range, the higher the halogen content in a halogenated rubber, the higher its oxygen index as well. Flame-retardant systems: They can be divided into two main categories: reactive and additive types. Reactive flame retardants are stable, provide long-lasting flame-retardant effects, and can prevent issues such as frosting and precipitation. They have little impact on the properties of polymers and are low in toxicity; however, their handling and processing procedures are relatively complex, which limits their widespread use in practice. Additive flame retardants are mixed with polymers during the polymer processing stage. Its thermal decomposition temperature must be at least 30°C higher than the processing temperature of the polymer, and it can be divided into two main categories: organic and inorganic. Organic flame retardants offer good flame-retardant effects, but they are highly toxic and do not meet the requirements for being smoke-free and low in toxicity. Inorganic flame retardants have good thermal stability; they do not produce harmful gases during combustion, offering high safety, and can also be used as fillers to reduce costs. Common inorganic flame retardants include aluminum hydroxide, magnesium hydroxide, and antimony oxide, among others. Halogen-antimony synergistic system: It is generally believed that its flame-retardant mechanism involves antimony trioxide (Sb2O3) reacting with HCl produced by the decomposition of halogen-based flame retardants at high temperatures to form antimony trichloride. This compound decomposes within the combustion zone, capturing the reactive free radicals present in the gas phase and thereby reducing the heat released during the combustion reaction ； Antimony trichloride vapor can remain in the combustion zone for an extended period, diluting the flammable gases; moreover, its high vapor density enables it to provide insulation and prevent the entry of oxygen ； Moreover, the decomposition of antimony haloxide that occurs during this process is an endothermic reaction, which can lower the temperature of the polymer and slow down its decomposition rate. Furthermore, at a higher temperature of 660°C, Sb2O3 can be vaporized. Metal hydroxides: Inorganic flame retardants are highly safe, featuring low smoke production and low toxicity. The commonly used inorganic flame retardants are mainly aluminum hydroxide and magnesium hydroxide. Aluminum hydroxide (ATH): also known as aluminum oxide trihydrate, its flame-retardant mechanism involves dehydration and heat absorption. The water released during separation serves not only as a coolant but also as a diluent. Furthermore, the alumina layer formed through dehydration has an extremely high surface area, which allows it to absorb smoke and combustible materials, thereby reducing the amount of carbon dioxide released during the combustion of the material. Magnesium hydroxide: The temperature for its dehydration reaction is 340–490°C; when heated, magnesium hydroxide releases its crystalline water while absorbing a large amount of heat ； The magnesium oxide produced by decomposition covers the surface of combustible materials, serving as an insulating layer ； Water vapor reduces the concentration of combustibles in the gas phase. Magnesium hydroxide provides flame-retardant, smoke-suppressing, and filling functions in polymers. Compared to aluminum hydroxide, magnesium hydroxide has a higher thermal decomposition temperature by 60°C, a higher heat absorption rate by 17%, and better flame-retardant properties; therefore, it is more suitable for polymers that are processed at high temperatures.