Brucite manufacturers explain to you

Polymer materials have excellent performance, and have many characteristics that other materials do not have, such as light weight, good processability, high fluidity, easy molding, insulation, wear resistance and so on. However, most polymer materials are hydrocarbon organic structures, which are flammable and combustible materials. When burning, they have high heat release rate, high calorific value, fast flame propagation speed and are difficult to extinguish. When some materials burn, they also produce smoke and toxic gases, which pose a potential threat to human life safety and environmental protection. In recent years, the output value of the global flame retardant materials industry has increased year by year. At the same time, countries have successively promoted the laws and regulations on flame retardancy of materials, and put forward higher requirements for flame retardancy of polymer materials. 1. Flame retardant mechanism of polymer composites The combustion of polymer materials follows the law of three elements of combustion (combustible substances, combustion-supporting substances and ignition sources). Polymer materials are mainly of hydrocarbon structure and belong to combustible materials. Under normal circumstances, the combustion improver is the oxygen component in the air, including all kinds of oxidants. Polymer materials are usually used in the air and have sufficient contact with the oxygen in the air. Moreover, polymer materials sometimes add all kinds of oxidants, which will act as combustion improvers in the combustion process. The ignition source is open flame and various high-temperature places, and the ignition point of polymer materials is generally low. Some use occasions are easy to contact with high-temperature environment, which also makes polymer materials easy to catch fire and cause fire. The combustion of polymer materials can be divided into two processes: thermo-oxidative degradation and combustion, involving a series of links such as heat transfer, thermo-oxidative degradation of polymer materials in condensed phase, diffusion of decomposition products in solid phase and gas phase, oxidation reaction field formed by mixing with air and chain combustion reaction in gas phase. Therefore, when the polymer material is heated, it can be decomposed, and the combustible produced by decomposition reaches a certain concentration, and the system is heated to the ignition temperature, then combustion can occur. However, whether the ignited polymer material can continue to burn after the ignition source is stable depends on the heat balance of the combustion process. When the heat generated by combustion is greater than or equal to the total heat required in each stage of the combustion process, the combustion can continue, otherwise it will be terminated or extinguished. The combustion of a substance should meet the conditions of the three elements of combustion at the same time, so the flame retardant is to control the three elements from the opposite direction. As long as one of the conditions of the elements is destroyed, the combustion can be terminated. In order to achieve a good flame retardant effect, various flame retardant technologies are usually adopted, and the three elements of combustion are simultaneously controlled, namely, reducing the flammability of materials, reducing the concentration of combustion improver and reducing the temperature of combustion reaction to achieve the purpose of preventing materials from burning. The flame retardant mechanism of materials is as complex as the combustion mechanism, which often involves many influencing and restricting factors, and there are many classifications of flame retardant mechanisms, mainly including the following two mechanisms. 1.1 gas phase flame retardant mechanism When the combustion reaction is going on, the flame retardant added to the material is decomposed by heat, producing a large amount of inert gases such as water vapor, ammonia, carbon dioxide, etc., which can dilute the oxygen in the air and the combustible gas produced by the combustion of the material. At the same time, the thermal decomposition reaction of flame retardant needs to absorb a lot of heat, which also reduces the temperature of combustible gas. These factors work together to stop the combustion and achieve the purpose of flame retardant. The added inorganic flame retardants, such as magnesium hydroxide, aluminum hydroxide, brucite and some carbonates, belong to the gas phase flame retardant mechanism. During the combustion reaction, these flame retardants are decomposed by heat, absorb a lot of heat, and at the same time generate inert gases such as water vapor and carbon dioxide, which play the role of oxygen isolation and dilution, and the flame retardant effect is obvious. Another kind of gas phase flame retardant mechanism is free radical inhibition mechanism, and halogen-antimony flame retardant system is a typical free radical inhibition mechanism. The reaction generates a large number of active free radicals, which trigger a chain reaction, making the combustion proceed rapidly. Adding flame retardant materials, such as halogen flame retardant/antimony trioxide and other free radical terminators, can absorb the free radicals generated by the combustion reaction, thus interrupting the combustion chain reaction and playing the role of flame retardant. 1.2 condensed phase flame retardant mechanism Condensed phase flame retardant mainly refers to the formation of expanded carbon layer on the outer layer of condensed phase during the combustion reaction. This porous expanded carbon layer plays the role of heat insulation, fire prevention and flame retardant. All kinds of phosphorus-nitrogen synergistic flame retardant systems and new nano-layered flame retardant materials belong to this kind of flame retardant mechanism. This kind of flame retardant system needs to have efficient carbon-forming components, which are called carbon sources. The representative carbon source component is pentaerythritol, and the carbon-forming effect directly affects the flame retardant effect. 2. Classification of flame retardants There are many classification methods of flame retardants, mainly divided into halogen-free flame retardants and halogen-free flame retardants. 2.1 halogen flame retardant Halogen-based flame retardants mainly include bromine-based flame retardants and chlorine-containing flame retardants. Bromine-based flame retardants dominate organic flame retardants, accounting for more than 80% of the total organic flame retardants. Table 1 shows some commonly used bromine-based flame retardants. Chlorine flame retardants are not as many as bromine flame retardants, mainly including chlorinated paraffin, chlorinated polyethylene and tetrachlorophthalic anhydride used as unsaturated resin flame retardants. There is also a large variety of chlorine-containing polymer material-polyvinyl chloride, which contains a lot of halogen chlorine in its molecular structure and can play a very good role in flame retardant. Halogen-based flame retardants are widely used, but they are being questioned and challenged by more and more people at present. The harm of halogen-based flame retardants to the environment is gradually being recognized by people, not only because they produce a large amount of toxic smoke gas-hydrogen halide in the combustion process, but also because the use of halogen-based flame retardants still exists in environmental precipitation areas and ecological areas for a long time, and the harm to the environment and ecology is becoming more and more serious. Therefore, the European Union promulgated ＲOHS rules, which strictly restricted the use of some bromine-based flame retardants, and also forced manufacturers to continuously develop efficient, non-toxic and harmless flame retardant products. 2.2 Halogen-free flame retardants Halogen-free flame retardants are divided into organic and inorganic categories. 2.2.1 Inorganic halogen-free flame retardant Inorganic flame retardants mainly include aluminum hydroxide, magnesium hydroxide, brucite, phosphorus-nitrogen system, carbonate and some novel inorganic flame retardants. Among them, aluminum hydroxide, like bromine-containing flame retardant, occupies a high share in the market. Aluminum hydroxide flame retardant is characterized by flame retardancy, smoke suppression, low corrosion and low price, and its market growth rate is about 5%/year, which is higher than that of other flame retardants. Magnesium hydroxide, as a flame retardant, plays a similar role to aluminum hydroxide, but its decomposition temperature is 60℃ higher than that of aluminum hydroxide, its heat absorption is about 17% higher, and its smoke suppression ability is stronger. At the same time, its price is higher than that of aluminum hydroxide, so it is suitable for polymer systems with higher processing temperature. Usually, the two are used together, and the best synergy can be achieved by adjusting the ratio of the two. The flame retardant mechanism and effect of antimony trioxide play an obvious role in halogen-containing system, but it can also be used alone. Ultrafine antimony trioxide (below 0. 3 μ m) is widely used in flame retardant fibers. At present, this type of inorganic flame retardant is developing towards nanometer level, and nano magnesium hydroxide and nano aluminum hydroxide flame retardants have been studied. Some new inorganic flame retardant materials are also under research and development, such as nano-layered materials, nano-montmorillonite, nano-flake graphite, fullerene and graphene, which have certain flame retardant effects. Zinc borate has many functions, such as flame retardant, smoke suppression, char formation, combustion suppression and droplet prevention. Zinc borate is cheap, non-toxic and non-irritating. When the temperature is lower than 260℃, it still keeps crystal water and combines with hydroxyl groups by covalent bonds. Another characteristic of zinc borate is that it can keep the strength and elongation of many polymers well, and it will not reduce the aging strength of polymers. Zhou Yuxin and others made an experimental study on the preparation of ultrafine flame retardant-zinc borate by using a new impinging stream reactor. The results showed that the product had high purity and the average particle size was 20 ～40nm. Some carbonates which are easy to decompose when heated and burned are also being developed and used as new flame retardants, such as magnesium carbonate and basic carbonate. This kind of flame retardant has a higher decomposition temperature when heated, especially suitable for high-temperature flame retardant materials, and has a large decomposition heat absorption, and the water vapor and carbon dioxide produced by decomposition have the function of oxygen insulation and fire extinguishing. Feng Caimin and others studied the synergistic effect of nickel carbonate in phosphorus-nitrogen intumescent flame retardant system, and found that a small amount of nickel carbonate can greatly improve the flame retardant performance of the material, and 2% of nickel carbonate can increase the oxygen index from 27 to 37. Analysis shows that the addition of nickel carbonate can promote carbon formation, stabilize the carbon layer and increase the thickness of the carbon layer. The application research of inorganic phosphorus flame retardant has a long history, and phosphorus-ammonia flame retardant is very effective in flame retardant of cellulose. Among them, the mixture of ammonium phosphate, sodium ammonium phosphate, ammonium sulfate, ammonium stannate, ammonium phosphate and ammonium chloride is very suitable for fiber flame retardant. Recently, a new discovery has been made in the research on the blends of ammonium dihydrogen phosphate and diammonium hydrogen phosphate or low molecular weight ammonium polyphosphate with ammonium borate, ammonium sulfate, ammonium sulfamate and ammonium bromide, and their different combinations have obvious effects on the flame retardancy of synthetic fibers. Ammonium polyphosphate (APP) with high degree of polymerization is widely used in various fire retardant coatings. The higher the degree of polymerization, the better the flame retardant effect and the longer the flame retardant effect. The surface treatment, stabilization treatment and coating treatment of red phosphorus greatly improved the hygroscopicity, spontaneous combustion temperature, phosphine release, dust explosion concentration, falling spontaneous combustion and compatibility with polymers. Ammonium molybdate has a certain flame retardant performance and remarkable smoke suppression effect. It is used as a flame retardant and smoke suppression agent in the formulation system, which is common in all kinds of low-smoke halogen-free flame retardant formulations. 2.2.2 Organic halogen-free flame retardant Organic halogen-free flame retardants mainly include organophosphorus, nitrogen flame retardants and silicone flame retardant. Organophosphorus compounds are additive flame retardants. The metaphosphoric acid generated during combustion of this kind of flame retardants can form stable polymers, which cover the surface of composite materials to isolate oxygen and combustibles, and play a flame retardant role. Its flame retardant effect is better than bromide. To achieve the same flame retardant effect, the amount of bromide is 4 ~ 7 times that of phosphide. This kind of flame retardants mainly include phosphorus (phosphine) esters and so on, which are widely used in epoxy resin, phenolic resin, polyester, polycarbonate, polyurethane, polyvinyl chloride, polyethylene, polypropylene, ABS and so on. Wang Yanlin et al. prepared a polymeric sulfur-containing organophosphorus flame retardant PDPTP in the absence of solvent. By using the synergistic effect of phosphorus and sulfur contained in the flame retardant, the flame retardant performance of the composite can be improved. Organic nitrogen flame retardants are represented by triazine flame retardants, the main components of which are melamine and its derivatives. These flame retardants have multiple reaction functions, excellent thermal stability, char formation, compatibility and flame retardancy, so they are widely used, such as melamine, melamine cyanurate (MCA), MP, MPP and so on. The good environmental adaptability of silicone's flame retardancy makes it widely used in environment-friendly materials. Its characteristics are that the polymer flame retardant material prepared from silicone flame retardant has excellent mechanical properties, outstanding cold resistance and impact resistance, and the synergistic effect of silicone flame retardant and other flame retardants is good, and the silicon-containing layer generated by combustion participates in charring, which is beneficial to improve the charring and flame retardant effect of the material. 3 Classification of polymer flame retardant materials 3.1 Halogen flame retardant polymer materials Halogen-containing or halogen-containing flame-retardant polymer materials have been widely used in various electrical plastics and automotive plastics. Bromine-based flame retardants and antimony trioxide synergist are mainly added in the formula of this kind of materials, and the application of some new bromine-containing flame retardants to avoid various policies and regulations and reduce the amount of flame retardants has become the mainstream direction of the development of this kind of materials. 3.2 Halogen-free flame retardant polymer materials Contrary to the situation of halogen-containing flame retardant polymer materials, all kinds of halogen-free flame retardant, low-smoke halogen-free flame retardant polymer materials have promising development prospects. However, halogen-free flame retardant polymer materials have their own weaknesses. For example, the flame retardant performance is not as good as halogen-containing flame retardant materials, and the oxygen index of halogen-containing flame retardant polymer materials is generally less than 30, while the oxygen index of halogen-containing flame retardant polymer materials is generally more than 30. In some occasions with high flame retardant requirements, the oxygen index even reaches 40. At present, it is difficult to apply halogen-free flame retardant materials in such high flame retardant occasions. At present, halogen-free flame retardant polymer materials are mainly filled with aluminum hydroxide and magnesium hydroxide, and a small part of them use phosphorus-nitrogen intumescent flame retardant system. Halogen-free flame retardant materials filled with aluminum hydroxide and magnesium hydroxide are currently used in cable materials in the market, and phosphorus-nitrogen intumescent flame retardant materials are mainly used in engineering plastics industry. 3.2.1 Aluminum hydroxide and magnesium hydroxide filled flame retardant system This system is characterized by high filling. Because of the general flame retardant properties of aluminum hydroxide and magnesium hydroxide, a large amount of inorganic fillers need to be added, and the filler content must reach about 60% to have a certain flame retardant effect. To sum up, this kind of material has three major problems: flame retardancy, processing and molding and mechanical properties. As we all know, about 60% of inorganic filler has a great influence on the properties of polymer materials. If the surface of filler is not properly organized, the mechanical properties of the prepared composites will be seriously lost and the flow molding performance will be greatly reduced. 3.2.2 brucite filled flame retardant system At present, brucite, as an upgraded product of aluminum hydroxide and magnesium hydroxide, is popular in research and application. The flame retardant performance of brucite is not high. The oxygen index of domestic brucite flame retardant polymer materials alone is 25 ~ 28, which rarely reaches 30. 3.2.3 Phosphorus-nitrogen intumescent flame retardant polymer material Flame retardant polymer materials with phosphorus-nitrogen expansion system are mainly used for flame retardant modification of engineering plastics, such as flame retardant PBT, flame retardant ABS, flame retardant PC, etc. Intumescent flame retardant is a flame retardant with phosphorus and nitrogen as main flame retardant elements, which consists of acid source (dehydrating agent), carbon source (carbonizing agent) and gas source (foaming agent). When the polymer containing this kind of flame retardant is decomposed or burned by heat, a uniform porous carbon foam layer can be formed on the surface. This layer is heat-insulating and oxygen-insulating, which can prevent volatile combustible materials generated by polymer decomposition from entering the gas-phase combustion zone from condensed phase, and also has the functions of suppressing smoke and preventing molten droplets. Intumescent flame retardant meets the requirements of smoke suppression and toxicity reduction of materials, and is one of the research hotspots in the field of flame retardant. Phosphorus intumescent flame retardant polymer plastic products are used in household appliances, automobiles and other fields, which require higher flame retardant performance, mechanical properties and environmental friendliness, and the price is also higher. This kind of intumescent flame retardant system has a high technical level, which depends on the flame retardant performance of the flame retardant itself and the effect of the synergistic system. Xia Ying and others adopted the flame retardant system of magnesium hydroxide and coated red phosphorus, and the total content of flame retardant was about 43%, which could make the oxygen index of the composite reach 35, while only using magnesium hydroxide flame retardant, it was necessary to add more than 60% flame retardant to make the oxygen index reach 30. It can be seen that compounding phosphorus and nitrogen flame retardants in magnesium hydroxide and aluminum hydroxide flame retardant system is of great benefit to the improvement of flame retardant performance. However, this system has requirements in compounding, such as acid-base matching and product color requirements, because the addition of red phosphorus will make the product red. 4. Conclusion Flame-retardant polymer materials are widely used, especially halogen-free flame-retardant materials, which are guided by policies and have a promising market. Further research on halogen-free flame retardant mechanism, synthesis and development of various high-efficiency flame retardants and compounding of material formulas are the development direction of this kind of materials.