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Flame retardant mechanism and preparation of magnesium hydroxide

Hebei Messi Biology Co., Ltd. stated that in recent years, fire accidents caused by polymers have attracted great attention from all walks of life, and the research and development of flame-retardant polymers has become a hot topic. At present, the organic flame retardants used on the market have harmful effects such as large amounts of smoke and toxic smoke, which are restricted by laws in Europe, the United States, Japan and my country. Therefore, inorganic flame retardants have received great attention.

Flame Retardant Magnesium Hydroxide

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.

Flame retardant 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

01 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.

02 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.

03 Chemical vapor chromatography

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 obtained 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.

04 Chemical liquid chromatography 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.

(1) Direct precipitation method

The direct precipitation method, also known as the alkali method, is a method in which magnesium solution directly reacts with an alkaline precipitant or precipitant precursor to generate magnesium hydroxide. According to the type of precipitant, it can be divided into lime method, ammonia method, and sodium hydroxide method. method and calcium hydroxide method, etc. The direct precipitation method is simple and easy to implement, has low equipment and technical requirements and is less likely to produce impurities. However, its reaction conditions affect the performance of the final product. The concentration of raw materials for the preparation of magnesium hydroxide, reaction process, reaction time, temperature, stirring rate, etc. are all current research the key of.

(2) Solvothermal and hydrothermal methods

Solvothermal and hydrothermal methods are chemical synthesis methods that are easy to control the size and dispersion of magnesium hydroxide. This method changes the properties of magnesium hydroxide under high temperature and pressure, and the magnesium salt in the raw material fully reacts and crystallizes with alkaline substances to form magnesium hydroxide with more uniform particles and higher dispersion. Current research on solvothermal and hydrothermal methods mainly focuses on improving the performance of magnesium hydroxide products, such as adding different types of organic solvents or additives, rationally adjusting chemical reaction time and reaction temperature, etc.

(3) Precipitation-azeotropic distillation method

The precipitation-azeotropic distillation method can improve the agglomeration phenomenon in the conventional preparation of magnesium hydroxide. The principle is that generally the sediment particles are filled with water molecules, and direct drying can easily lead to hard agglomeration of the particles under the action of capillary pressure. The azeotropic distillation method uses organic matter such as alcohols and water to form an azeotrope at a certain temperature, thereby converting hydrogen into The water in the magnesium oxide colloid is removed, thereby improving its dispersion and obtaining a product with good dispersion properties.

(4) Ultrasonic chemical method and microwave-assisted method

Both the ultrasonic chemical method and the microwave-assisted method belong to the new magnesium hydroxide flame retardant preparation process. The ultrasonic chemical method is a chemical reaction triggered under extreme conditions. It mainly relies on ultrasonic waves to trigger the formation and collapse of microbubbles, making them Active sites are generated to enhance the chemical reaction rate and ensure a more uniform and uniform morphology of magnesium hydroxide particles. The research on this method mainly focuses on aspects such as ultrasonic power and product performance, which can strengthen the comprehensive advantages of ultrasonic chemical method for preparing magnesium hydroxide flame retardant. The ultrasonic chemical method does not require pressure control of the reaction process, the overall reaction speed is faster, the reaction temperature is relatively low, and process control has more advantages.

Using microwave technology to prepare magnesium hydroxide consumes relatively little energy and does not cause serious pollution to the environment. At the same time, the microwave-assisted method can effectively shorten the chemical reaction time of magnesium hydroxide in the form of heating, allowing a more uniform high-temperature state to be formed inside the sample solution. The microwave-assisted method can be integrated with the hydrothermal method to further explore new ways of preparing magnesium hydroxide flame retardants and their deep application value.

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-scale magnesium hydroxide in all aspects (including flame retardant effect, smoke elimination and mechanical properties, etc.) is far superior to that of micron-scale 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

At present, 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 during application; in addition, the prepared magnesium hydroxide products are easy to agglomerate, have strong hydrophobicity, and are highly compatible with Molecular polymers are not compatible. 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.

Research on surface modification of magnesium hydroxide

As a flame retardant for high molecular polymers, the most important thing for magnesium hydroxide is to be well compatible with the high molecular polymers, achieve uniform dispersion, and ultimately achieve the purpose of flame retardancy. Surface modification of magnesium hydroxide using specific compounds can reduce the surface polarity of magnesium hydroxide, make its surface hydrophobic, and improve the compatibility between magnesium hydroxide and polymers.

Chemical modification of magnesium hydroxide surface is to use chemical methods to selectively adsorb or specifically adsorb or react chemically with functional groups in organic molecules or inorganic gel molecules on the surface of magnesium hydroxide powder, thereby coating the surface of the particles. The particle surface is organically modified or the polarity is changed, ultimately achieving surface modification. Commonly used surface modifiers mainly include silane coupling agents, titanate esters, aluminate ester coupling agents, higher fatty acids and their derivatives, etc.

Physical modifications of magnesium hydroxide usually include surface high energy modification and surface coating modification. Surface high-energy modification mainly modifies Mg(OH)2 through radiation and other methods to change the surface activity of Mg(OH)2. The modification process does not involve chemical reactions. Surface coating modification mainly involves the modification of Mg(OH)2 through dispersants.

Research on ultrafine magnesium hydroxide

Ultrafineness is one of the effective methods to enhance the compatibility between magnesium hydroxide and polymers and reduce the filling amount. The preparation process has a decisive impact on the physical and chemical properties of magnesium hydroxide such as particle size, particle size distribution, and morphology. Changing the preparation process is the most effective way to obtain ultrafine magnesium hydroxide.

Solvothermal and hydrothermal methods, microwave-assisted methods and ultrasonic chemical methods have been widely verified in the preparation of micro-nano-scale materials, and there have also been some research reports on the preparation of ultra-fine magnesium hydroxide flame retardant materials.

Hebei Messi Biology Co., Ltd. stated that in the past two decades, great progress has been made in magnesium hydroxide modification and ultrafine preparation research. Magnesium hydroxide’s non-toxicity, wide range of sources, and low price are the biggest advantages of this material as a flame retardant. With the strengthening of environmental protection, the application of magnesium hydroxide in flame retardant materials will continue to increase in the future, and the surface modification and ultra-fine preparation of magnesium hydroxide flame retardants will be used in the flame retardant material industry. of great significance.

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