Principle of flame retardant
Principles of flame retardants
Flame retardants are functional additives that make flammable polymers flame retardant. They are mainly designed for the flame retardancy of polymer materials. There are many types of flame retardants, which are divided into additive flame retardants and reactive flame retardants according to their usage methods. type flame retardant.
Additive flame retardants are added to polymers through mechanical mixing methods to make the polymer flame retardant. Currently, additive flame retardants mainly include organic flame retardants and inorganic flame retardants, and halogen flame retardants (organic flame retardants). chlorides and organic bromides) and non-halogenated. Organic flame retardants are represented by bromine series, phosphorus nitrogen series, nitrogen series, red phosphorus and compounds, and inorganic ones are mainly antimony trioxide, magnesium hydroxide, aluminum hydroxide, silicon series and other flame retardant systems.
It is a substance that can increase the flame resistance of polymer materials. It is mainly used in polymer materials such as plastics, rubber, fibers, etc., and most of these materials can be burned. Especially for plastics, if they are to be used in transportation, construction, electrical equipment, aviation, space flight, etc., there is an urgent need to solve their combustion resistance problem. The use of flame retardants should generally meet the following conditions: not reducing the physical properties of polymer materials, such as heat resistance, mechanical strength, and electrical properties; the decomposition temperature should not be too high, but it cannot decompose at processing temperatures; durability Good; good weather resistance; cheap.
Generally speaking, organic flame retardants have good affinity. In plastics, brominated flame retardants have an absolute advantage in organic flame retardant systems. Although there are many “criticisms” on environmental issues, it has been difficult to be replaced by other flame retardant systems. .
Among non-halogen flame retardants, red phosphorus is a better flame retardant, with the advantages of less addition, high flame retardant efficiency, low smoke, low toxicity, and wide range of uses; red phosphorus, aluminum hydroxide, and expanded graphite Inorganic flame retardants such as these are used in combination to produce composite phosphorus/magnesium; phosphorus/aluminum; phosphorus/graphite and other non-halogen flame retardants. The amount of flame retardants can be greatly reduced, thereby improving the processing performance and physical and mechanical properties of plastic products. . However, ordinary red phosphorus is easily oxidized in the air, absorbs moisture, easily causes dust explosions, is difficult to transport, and has poor compatibility with polymer materials. The scope of application is limited. In order to make up for this shortcoming, a microcapsule coating process can be used to make it microencapsulated red phosphorus. In addition to overcoming the inherent disadvantages of red phosphorus, microencapsulated red phosphorus is highly efficient, has low smoke, does not produce toxic gases during processing, and its dispersion, physical, mechanical properties, thermal stability and flame retardant properties have been improved and improve.
Reactive flame retardants participate in the polymerization reaction as a monomer, so the polymer itself contains flame retardant components. Its advantage is that it has less impact on the performance of the polymer material and has long-lasting flame retardancy.
Generates free radicals that can react with plastics and act as a flame retardant. Their reaction products with plastics have extremely poor combustion properties.
Produces a gas that smothers flames. Such as antimony trioxide, when it encounters HCL emitted due to combustion in PVC, it reacts to form a suffocating gas, namely antimony nitrogen oxide.
Provides a coating that is insulating from oxygen. For example, the phosphide produced when phosphate ester flame retardants burn is an oxygen-blocking coating.
Absorbs the heat generated during combustion and plays the role of cooling and slowing down the burning rate. Such as aluminum hydroxide, the proportion of chemically associated water contained in its molecules is as high as 34%. This associated water remains stable at the processing temperature of most plastics, but begins to decompose when it exceeds 200°C, releasing water vapor. And for every molecule of aluminum hydroxide decomposed, 36 kcal of heat is absorbed.
Flame retardant mechanism
The mechanism of action of flame retardants is complex and not yet fully understood. It is generally believed that halogen compounds decompose when heated by fire, and the decomposed halogen ions react with polymer compounds to produce hydrogen halide. The latter reacts with active hydroxyl radicals (HO·) that proliferate in large quantities during the burning process of polymer compounds, reducing its concentration and slowing down the burning speed until the flame is extinguished. Among halogens, bromine has greater flame retardant effect than chlorine. The function of phosphorus-containing flame retardants is that they form metaphosphoric acid when they burn. Metaphosphoric acid polymerizes into a very stable polymer state and becomes a protective layer for plastics to isolate oxygen.
Flame retardants exert their flame retardant effects through several mechanisms, such as endothermic effect, covering effect, chain reaction inhibition, suffocation effect of non-flammable gas, etc. Most flame retardants achieve flame retardant purposes through several mechanisms working together.
1. Endothermic effect
The heat released by any combustion in a short time is limited. If part of the heat released by the fire source can be absorbed in a short time, the flame temperature will decrease, radiate to the burning surface and act on the gasified gas. The heat required to break down combustible molecules into free radicals will be reduced, and the combustion reaction will be inhibited to a certain extent. Under high temperature conditions, the flame retardant undergoes a strong endothermic reaction, absorbs part of the heat released by combustion, lowers the temperature of the surface of the combustible material, effectively inhibits the generation of flammable gases, and prevents the spread of combustion. The flame retardant mechanism of Al(OH)3 flame retardant is to increase the heat capacity of the polymer, allowing it to absorb more heat before reaching the thermal decomposition temperature, thereby improving its flame retardant performance. This type of flame retardant gives full play to its ability to absorb a large amount of heat when combined with water vapor to improve its own flame retardant ability.
2. Covering effect
After adding flame retardants to combustible materials, the flame retardants can form a glassy or stable foam covering layer at high temperatures to isolate oxygen.It has the functions of heat insulation, oxygen insulation and preventing flammable gas from escaping outward, thereby achieving the purpose of flame retardancy. For example, when organophosphorus flame retardants are heated, they can produce a cross-linked solid material or carbonized layer with a more stable structure. On the one hand, the formation of the carbonized layer can prevent further pyrolysis of the polymer, and on the other hand, it can prevent the thermal decomposition products inside it from entering the gas phase to participate in the combustion process.
3. Inhibit chain reaction
According to the chain reaction theory of combustion, free radicals are needed to maintain combustion. Flame retardants can act on the gas-phase combustion zone to capture free radicals in the combustion reaction, thereby preventing the spread of flames, reducing the flame density in the combustion zone, and ultimately reducing the combustion reaction speed until it is terminated. For example, the evaporation temperature of halogen-containing flame retardants is the same as or similar to the decomposition temperature of the polymer. When the polymer decomposes by heat, the flame retardant also volatilizes at the same time. At this time, the halogen-containing flame retardant and the thermal decomposition products are in the gas phase combustion zone at the same time, and the halogen can capture the free radicals in the combustion reaction and interfere with the combustion chain reaction.
4. Suffocation effect of non-flammable gas
Flame retardants decompose into non-combustible gases when heated, diluting the concentration of combustible gases decomposed from combustible materials below the lower combustion limit. At the same time, it also has a diluting effect on the oxygen concentration in the combustion zone, preventing the continuation of combustion and achieving a flame retardant effect. szGnue9c7
Disclaimer:
Disclaimer: Some of the texts, pictures, audios, and videos of some articles published on this site are from the Internet and do not represent the views of this site. The copyrights belong to the original authors. If you find that the information reproduced on this website infringes upon your rights, please contact us and we will change or delete it as soon as possible.
AA
