Flame retardant is a kind of additives that can improve the flammability, self-extinguishing or smoke elimination of flammable or combustible materials. With the continuous expansion of the application field of polymer materials, flame retardant technology has increasingly attracted people's attention. In general, flame retardants can be divided into organic and inorganic categories, in the plastics industry, inorganic flame retardants because of their good thermal stability, non-volatile, non-precipitation, do not produce corrosive toxic gases and other advantages, rapid development in recent years. Among inorganic flame retardants, magnesium hydroxide has multiple functions such as flame retardation, smoke elimination, drip prevention and filling, and is a popular inorganic flame retardant developed in recent years.
Classification of magnesium hydroxide
According to the production method, magnesium hydroxide can be divided into mechanical grinding method (that is, we often say brucite) and chemical synthesis method, of which the production capacity of mechanical grinding method is 300,000-500,000 tons, mainly concentrated in Liaoning. Chemical synthesis can be divided into hydrothermal method, precipitation method, magnesium oxide hydration method and so on. Magnesium hydroxide flame retardants are mainly used in various rubber and plastic fields, of which the largest amount is ultrafine magnesium hydroxide prepared by mechanical grinding.
Ultrafine magnesium hydroxide is a kind of inorganic flame retardant made from natural brucite mineral by ultrafine grinding and certain surface treatment. Magnesium hydroxide flame retardant produced by brucite, the source of natural minerals, the whole process is basically zero emissions, it can be said that from the source to the product are in line with the green requirements, are non-toxic and harmless, environmentally friendly flame retardants, so known as "green environmental protection flame retardants", with huge market application prospects.
Introduction of Brucite
The main component of brucite is magnesium hydroxide, which is one of the minerals with high magnesium content in nature, and the theoretical composition is: MgO 69.12%, H2O 30.88%. Brucite belongs to trigonal crystal system, common flake aggregate, sometimes fibrous aggregate, its cleavage surface is pearl luster, hardness 2.5, specific gravity 2.3~2.6g/cm³.
China is a big country of magnesium resources, and magnesium hydroxide flame retardants have unique resource advantages and good market prospects. According to statistics, the national reserves of brucite are about 25 million tons, of which the reserves of brucite deposits in Fengcheng and Kuandian in Liaoning Province are 10 million tons, which is the largest brucite mine found in the world at present; Shaanxi black forest (reserves of about 7.8 million tons), Jilin Ji 'an (reserves of more than 2 million tons); In addition, brucite deposits have also been found in Xixia, Henan, Qilian Mountains, Qinghai and Jianshibao, Sichuan.
Flame retarding mechanism of magnesium hydroxide flame retardants
Inorganic flame retardants generally play a flame retardant role in the gas phase or condensed phase. For ultrafine magnesium hydroxide flame retardants, the flame retardant mechanism can be summarized as follows:
1) Cooling effect. When heated, magnesium hydroxide will decompose and absorb a large amount of latent heat, thus reducing the actual temperature of the polymer surface, slowing down the cracking rate of the polymer and reducing the production of combustibles;
2) Dilution effect. Magnesium hydroxide releases a lot of water vapor when heated combustion, water vapor is a non-flammable gas, can play a role in diluting flammable gas and surface oxygen, so that combustion can not continue, play a vaporous flame retardant effect. At the same time, the filling amount of magnesium hydroxide flame retardant in the polymer system is large, and the concentration of fuel in the solid phase can also be diluted to a certain extent, so as to improve the flame retardant of the product;
3) Isolation effect. Magnesium hydroxide is decomposed into a dense layer of magnesium oxide after heating. Magnesium oxide is a kind of excellent refractory material. Covering the surface of polymers can improve the ability of polymers to resist flame, and play the purpose of insulating air to prevent combustion. The decomposed magnesia is also conducive to the formation of carbon on the polymer surface, and the carbon layer is also an excellent insulation layer.
4) Smoke suppression effect. The flame retardant effect of magnesium hydroxide mainly occurs in the outer layer of the solid degradation zone, and the effect on the solid degradation zone and the pre-combustion zone is very small, which makes the combustion of combustible materials unaffected, so the smoke generated is relatively less. The smoke released by the combustion of the outer layer of magnesium hydroxide will also be diluted or absorbed by the water vapor generated during decomposition, so it has a better smoke elimination effect.
In summary, magnesium hydroxide is an added type of high-efficiency flame retardant, with flame retardant, smoke elimination, drip prevention, filling and other multiple functions, not only can neutralize the acid corrosion gas generated in the combustion process, but also does not emit harmful substances, is a green environment-friendly flame retardant.
Advantages of ultrafine magnesium hydroxide flame retardants
Due to the good thermal stability of magnesium hydroxide, it does not produce corrosive gas, non-volatilization, long-lasting effect, no toxicity, and low price. At the same time, compared with aluminum hydroxide, it has a high thermal decomposition temperature, and has obvious advantages in flame retardant mechanism, decomposition capacity, heat capacity, carbonization, smoke suppression ability, hardness, production cost and so on. To be specific:
a) The decomposition energy of magnesium hydroxide (1137 kJ/g) is higher than that of aluminum hydroxide (1117 kJ/g), and the heat capacity is 7% higher, which helps to improve the flame retardant efficiency;
b) The hardness of magnesium hydroxide is lower than that of aluminum hydroxide, and the wear of the equipment is less during processing, which can extend the service life of the equipment;
c) The thermal stability of magnesium hydroxide is better than that of aluminum hydroxide. The decomposition temperature of magnesium hydroxide is higher, about 340℃. The decomposition temperature of aluminum hydroxide is 220 ° C, so magnesium hydroxide can withstand higher processing temperatures, which is conducive to speeding up the extrusion speed, especially suitable for polymers with higher processing temperatures.
d) Magnesium hydroxide is an additive type of highly effective smoke suppressant flame retardant, which has better smoke suppressant effect than similar inorganic flame retardants such as aluminum hydroxide;
e) Experimental studies show that magnesium hydroxide is better than aluminum hydroxide in promoting carbon formation on the surface of the polymer, so the flame retardant effect is better than aluminum hydroxide.