Overview of Magnesium Hydroxide
Hebei Messi Biology Co., Ltd. stated that magnesium hydroxide is a high-filling and additive inorganic flame retardant and smoke suppressant. When heated, it decomposes and absorbs the heat on the surface of the combustible material to achieve a flame retardant effect; at the same time, it releases a large amount of water to dilute the oxygen on the surface of the combustible material, and the active magnesium oxide generated by decomposition will adhere to the surface of the combustible material, further preventing the progress of combustion. During the entire flame retardant process, magnesium hydroxide does not produce any harmful substances, and the decomposed products are flame retardant and can also absorb a large amount of harmful gases and smoke generated by the burning of polymers such as plastics and rubber. It is a Emerging environmentally friendly inorganic flame retardants.
Chemical properties
Magnesium hydroxide is a medium-strong base (the solubility of magnesium hydroxide is very small, and the solution is very weakly alkaline, and is sometimes treated as a weak base). When heated to 623K (350℃), it dehydrates and decomposes: Mg(OH)2 →MgO+H2O, which is easy to Soluble in acid or ammonium salt solution. Like magnesium oxide, it easily absorbs carbon dioxide in the air and gradually forms a basic carbonate composed of 5MgO·4CO2·xH2O. It decomposes into magnesium oxide and water above 350℃, but can only be completely dehydrated above 1800℃.
Magnesium hydroxide flame retardant has a high decomposition temperature (340°C ~ 450°C), and the thermal decomposition products are MgO and H2O. It does not release any toxic and harmful substances and does not cause any harm to the environment and human health. Therefore, magnesium hydroxide flame retardant has become One of the most popular inorganic flame retardants at present, it has broad application prospects.
Combustion-supporting mechanism of magnesium hydroxide
Magnesium hydroxide has a special layered structure, which makes it exhibit excellent thixotropy and low surface energy, and plays a good role in flame retardancy and smoke elimination for plastics. Magnesium hydroxide begins to decompose into magnesium oxide and water when heated at 340°C. When it is completely decomposed, the temperature can reach as high as 490°C. It absorbs a large amount of heat energy during decomposition. The specific flame retardant mechanism is:
(1) Magnesium hydroxide has a large heat capacity, absorbs a large amount of heat when it is thermally decomposed, and releases a large amount of water vapor at the same time, which not only reduces the temperature of the material surface, but also reduces the generation of flammable small molecular substances.
(2) A large amount of water vapor generated by thermal decomposition can also cover the surface of the material, reducing the oxygen concentration in the air at the combustion surface, thus hindering the combustion of the material.
(3) Magnesium oxide generated by the thermal decomposition of magnesium hydroxide is a good refractory material. It can not only cover the surface of the material, but also promote the carbonization of the polymer material, forming a carbonized layer to block the entry of heat and air, thereby effectively preventing combustion.
(4) Magnesium hydroxide acts as a redox reaction catalyst and can promote the conversion of CO into CO2 during the combustion process; the magnesium oxide produced by decomposition can neutralize SO2, CO2 and NO2 produced during the combustion process, thereby reducing the release of toxic and harmful gases.
Preparation of magnesium hydroxide flame retardant
① Physical crushing method
The physical crushing method is a method that uses mechanical or ultrasonic methods to crush and ultrafinely crush natural minerals (mostly brucite) to obtain magnesium hydroxide within the required particle size range. Although the physical grinding method is used to prepare magnesium hydroxide with a simple process and low cost, the prepared magnesium hydroxide has low purity and uneven particle size distribution. It usually requires the use of special grinding methods or the addition of grinding aids (or dispersants) during the grinding process. ) to obtain higher quality magnesium hydroxide. Therefore, its industrial application and development are greatly restricted.
② Chemical solid phase method
The preparation of magnesium hydroxide by the solid-phase method is a process in which solid metal salts and metal hydroxides are mixed in a certain ratio, ground and calcined, and a solid-phase reaction occurs to obtain the magnesium hydroxide product. This method has the characteristics of simple process and low cost, but it also has shortcomings such as low product purity, easy agglomeration, and poor dispersion performance, and is rarely used in actual large-scale industrial production.
③ Chemical vapor phase method
The gas phase method for preparing magnesium hydroxide is to use ammonia gas as a precipitant, and directly pass ammonia gas into a solution containing Mg2+ to prepare magnesium hydroxide. Magnesium hydroxide is prepared by gas phase method, and its quality is affected by factors such as ammonia gas flow rate, stirring intensity and reaction temperature. In the process of preparing magnesium hydroxide flame retardant through the gas phase method, due to the stable ammonia concentration, the product has the advantages of high purity, uniform particle size and good dispersion performance; at the same time, no moisture is introduced during the introduction of ammonia gas, and the obtained hydrogen The concentration of magnesium oxide slurry is high, the production process requires a small area, and the unit equipment yield is high. However, it requires high equipment and technology, and it is also prone to the problem of ammonia diffusion and environmental pollution.
④ Chemical liquid phase method
The preparation of magnesium hydroxide by the liquid phase method uses magnesium salt as the main raw material, and reacts it with an alkaline substance containing hydroxide ions (OH-) to form a magnesium hydroxide precipitate, which is then washed and dried to obtain the product. Liquid phase methods can be divided into direct precipitation methods, solvothermal and hydrothermal methods, precipitation-azeotropic distillation methods, ultrasonic chemical methods and microwave-assisted methods.
Application requirements for magnesium hydroxide flame retardant materials
Magnesium hydroxide as a flame retardant has the following requirements:
(1) It must have extremely high purity (Mg(OH)2>93%). High-purity magnesium hydroxide not only has high flame retardant properties, but also can reduce the amount added to the material.
(2) The particle size is small. The performance of composite materials prepared with micron-sized magnesium hydroxide in all aspects (including flame retardant effect, smoke elimination and mechanical properties, etc.) is far superior to that of micron-sized magnesium hydroxide.
(3) The surface polarity is low. When the surface polarity of magnesium hydroxide decreases, the degree of agglomeration will decrease, and the dispersion and compatibility will increase. Only when it is added to the polymer as a flame retardant material can it have better compatibility with the polymer. properties and reduce the impact on the mechanical properties of the material.
Reasons for modification of magnesium hydroxide flame retardant materials
Hebei Messi Biology Co., Ltd. said that currently, most of the magnesium hydroxide flame retardants produced on the market are micron-grade (d>5μm), with a wide particle size distribution, and require a large amount of filling in applications; in addition, the prepared magnesium hydroxide products are easy to Agglomeration, strong hydrophobicity, and incompatibility with high molecular polymers. In practical applications, magnesium hydroxide causes serious damage to the mechanical properties of polymer materials, which greatly limits the application of magnesium hydroxide flame retardants.
Obtaining magnesium hydroxide flame retardants with low surface polarity, strong hydrophilicity, small particle size and narrow distribution, and good compatibility with high molecular polymers through certain physical and chemical methods has become a hot research topic among current scientific and technological workers.
On the one hand, using organic functional groups to modify the surface of magnesium hydroxide can reduce the polarity of the surface of magnesium hydroxide and improve its compatibility with polymers; micro-nanoscale ultrafine magnesium hydroxide has a low filling amount and is prepared using it The composite material has good performance.
On the other hand, the flame retardant performance of micron-level magnesium hydroxide of the same mass is several times higher than that of micron-level magnesium hydroxide, and its impact on the properties of polymer polymer materials is also lower. Micro-nano-scale magnesium hydroxide with low polarity can be evenly dispersed in polymer materials, making the flame retardant and mechanical properties of the entire material consistent. Therefore, surface modification and ultrafineness of magnesium hydroxide flame retardants can solve the shortcomings in the application of magnesium hydroxide flame retardants.
Application of magnesium hydroxide
Magnesium hydroxide is an excellent flame retardant for plastic and rubber products. In terms of environmental protection, as a flue gas desulfurizer, it can replace caustic soda and lime as a neutralizing agent for acidic wastewater. It is also used as an oil additive for anti-corrosion and desulfurization. In addition, it can also be used in the electronic industry, medicine, sugar refining, insulation materials and the manufacture of other magnesium salt products.
(1) Flue gas desulfurization
The earliest countries in the world to use magnesium hydroxide for flue gas desulfurization were Japan and the United States, both of which began in the 1970s. The magnesium hydroxide used in this field is a low-end product in terms of purity. Its purity requirements are not very high, generally above 85%, and they are all in the form of slurry (containing 35% to 50% magnesium hydroxide). ) form application. Magnesium hydroxide desulfurization method uses magnesium hydroxide as an alkaline desulfurizer to absorb and remove sulfur dioxide in flue gas. This agent can be directly added to boiler fuel and has the advantages of simple operation, low cost, high desulfurization efficiency, no solid waste emissions, and not easy to scale. The reaction product magnesium sulfate is discharged as harmless discharge water without secondary pollution.
(2) Industrial wastewater treatment agent
Magnesium hydroxide is a weak base with buffering properties. No matter which acid is neutralized, and whether it is excessive or not, its highest pH value will not exceed 9, which happens to be the pH of the allowable discharge in the US Environmental Protection Agency’s “Clean Water Regulations” The highest value limit. Magnesium hydroxide has a large specific surface area and strong adsorption force. It is easy to adsorb and remove heavy metal ions such as Ni2+, Cd2+, Pb2+, Cr3+ and other harmful to the environment from industrial waste liquid. It can be used alone or in combination with lime, bentonite, etc. When magnesium hydroxide treats printing and dyeing wastewater, Mg(OH)2 is mainly used to be positively charged, and it is easy to adsorb negatively charged anionic dyes to decolor the dye wastewater. The magnesium hydroxide products used in this field are mainly in the form of paste or slurry, and the largest amount of magnesium hydroxide is slurry.
(3) Magnesium oxide series product precursors
Magnesium hydroxide is an important inorganic chemical product and an intermediate product for the preparation of high-purity magnesium oxide. A considerable part of magnesium hydroxide is used to produce various magnesium oxide products, especially high-purity series magnesium oxide.
(4) flame retardant
The industrialization and application of flame retardants began in the United States in the 1960s, and the early literature on magnesium hydroxide as a flame retardant was published in 1963, and the first literature on brucite as a flame retardant was published in 1980. Magnesium hydroxide has the characteristics of flame retardant, smoke suppression, high thermal stability, and non-toxicity. It is especially suitable for filling polymers whose processing temperature is higher than 180°C. It can not only save resin and reduce costs, but also mainly impart flame retardancy to composite materials. Therefore, magnesium hydroxide is growing rapidly in the field of flame retardants. Inorganic flame retardants are widely used abroad, but the application and development of inorganic flame retardants in our country is still in its infancy.
Conclusion
Hebei Meixi Biological Co., Ltd. stated that magnesium hydroxide has excellent properties in all aspects as a flame retardant, and its application scope will become more and more extensive. Moreover, magnesium hydroxide has the advantage of being cheap, saving costs for factory production. Preparing magnesium hydroxide by lightly burning magnesium powder increases the added value of the product, achieves effective utilization of resources, and can effectively enhance the competitiveness of enterprises in the market economy.