In today’s society, polymer synthesis technology is developing rapidly, and materials such as plastics, rubber, and synthetic fibers are widely used in daily life. However, all high molecular polymers have defects.
Since they are composed of chains of carbon and hydrogen atoms, the compounds are flammable, which will cause the fire to spread rapidly.
When a certain amount of flame retardant is added to the substance, the flame retardant performance of the material will be improved accordingly, but the mechanical properties or other properties of the material will often be affected, and sometimes it will cause inconvenience to the processing.
Therefore, in order to maintain the comprehensive performance of flame-retardant composite materials, the comprehensive performance of flame retardants must be considered.
As a flame retardant material, magnesium hydroxide has many advantages:
(1) The decomposition temperature of magnesium hydroxide is high, and its ignition point is higher than that of aluminum hydroxide, which increases the heating temperature of polymer materials;
(2) MgO produced after combustion and dehydration is a kind of refractory material, and is a kind of basic oxide, which can absorb acid gas;
(3) Strong heat absorption capacity and high flame retardant efficiency;
(4) Low cost, easy to process in different polymer materials.
In recent years, magnesium hydroxide, as a green and environmentally friendly flame retardant, has been widely used.
Preparation of magnesium hydroxide
The two aspects that need to be controlled most in the preparation of magnesium hydroxide:
(1) Filtration performance. If the sedimentation performance of the magnesium hydroxide slurry is poor, it will affect the washing and separation operation process, resulting in a prolongation of the synthesis cycle of magnesium hydroxide and affecting the oxidation of hydroxide. Magnesium output;
(2) Control the shape, because magnesium hydroxide with a regular shape has a better flame retardant effect, so it is necessary to adopt suitable production methods and process conditions to meet the requirements.
The main production processes of magnesium hydroxide include direct precipitation method, magnesium-containing ore grinding method, magnesium oxide hydration method, etc.
direct precipitation method
At present, there are many methods for synthesizing magnesium hydroxide, among which the most used method is the precipitation method, which is easy to control the crystal morphology due to its low price, simple operation and easy control. In the precipitation method, it is mainly a salt solution precipitation, usually using a strong base, such as ammonia or sodium hydroxide, the most widely used magnesium salts are magnesium chloride, magnesium sulfate and magnesium nitrate, and the organic magnesium salt magnesium acetate is also occasionally used.
The ammonia method in the direct precipitation method is divided into a one-step method and a continuous precipitation method. The continuous precipitation method realizes the recycling of resources, reduces production costs, and ensures product quality.
Hydrothermal
The hydrothermal reaction can effectively control the shape and size of magnesium hydroxide. The properties of the product mainly depend on the type of precursor magnesium salt, the control of the solvent and the temperature of the reaction process; the shape of the product mainly depends on the pH of the solution and the reaction temperature. temperature. By adjusting the size of the pH, the morphology of magnesium hydroxide was synthesized as nano-flower, needle, flake and spherical.
The advantage of hydrothermal synthesis is that magnesium hydroxide with a specific surface area greater than 100m2/g can be obtained. The disadvantage is that high temperature and high pressure are used in industry, and the cost is high.
sonochemical synthesis
The sonochemical method uses ultrasonic waves with a frequency in the range of 20kHz-10MHz to induce the formation and collapse of microcells, and generate active sites under high temperature and pressure. Compared with the traditional method, this method occurs under extreme conditions, which can greatly increase the reaction rate and generate small crystals with more uniform morphology. The characteristic of sonochemical synthesis is that materials with different structure types can be produced by changing the reaction medium.
Sol-gel method
The sol-gel method uses highly active metal compounds as precursors, mixes them in liquid phase, undergoes hydrolysis and condensation, and generates metal hydroxides. It forms a stable transparent sol, which polymerizes slowly after aging to form a network structure, and loses the solvent during the reaction to form a gel. The gel is subsequently dried and sintered to prepare nanomaterials.
The reaction process of this method is that the precursor is dispersed and dissolved, hydrolyzed to form a monomer, polymerized to form a sol, and the nano-magnesium hydroxide material is prepared through processes such as drying and heat treatment.
Uses of Magnesium Hydroxide
Magnesium hydroxide has a wide range of uses, and the fields of flame retardancy and environmental protection are the main fields of application of magnesium hydroxide. In addition, because the magnesium hydroxide slurry has high activity, good adsorption performance, and is easy to adjust and controllable, magnesium hydroxide can also be used as a neutralizer to treat acidic waste liquid and rich sulfur oxides, remove heavy metals, and improve soil quality. Acidic, it can also be used as an antacid in pharmacy, fertilizer additives, etc.
Applications in Flame Retardancy
Magnesium hydroxide is an important halogen-free and environmentally friendly green flame retardant. The flame retardant mechanism is to generate a stable coating layer of magnesium oxide and water vapor. The flame retardant efficiency is low. To achieve a good flame retardant effect, the addition amount generally As high as 50%~60%, at this time, the mechanical properties and processing properties of the flame-retardant composite material are seriously reduced, and the usual method is to modify the surface of magnesium hydroxide and synergize the flame-retardant effect of magnesium hydroxide.
Magnesium hydroxide flame retardant can not only be used alone, but also can be used in combination with other synergists, such as magnesium hydroxide/red phosphorus, magnesium hydroxide/zinc borate, magnesium hydroxide/carbon nanotubes and magnesium hydroxide/carbon Black and so on. The synergistic flame retardancy of magnesium hydroxide will not only achieve a better flame retardancy effect, but also reduce the amount of flame retardant, thereby reducing costs.
Applications in the field of environmental protection
(1) Treatment of acid-containing wastewater
The neutralization reaction speed of magnesium hydroxide is slow, and the particle size produced after the neutralization reaction is larger and settles soon. In the treatment of acid-containing wastewater, magnesium hydroxide can simplify the operation process, reduce the operation time, have good controllability, and reduce the treatment cost.
(2) Heavy metal removal
Due to the large specific surface area of magnesium hydroxide particles and strong adsorption capacity, it can remove heavy metal ions such as Ni2+, Cd2+, Mn2+, which are harmful to the environment, and other heavy metal elements such as Mo, Co, Fe, W and V from industrial wastewater. etc. can also be removed with magnesium hydroxide, light-burned magnesium oxide or aluminum magnesium carbonate.
(3) Flue gas desulfurization
At present, there are nearly 20 kinds of relatively mature desulfurization technologies, among which the magnesium hydroxide desulfurization technology is economical and practical, and has a good development prospect in the future. The process advantages of magnesium hydroxide desulfurization mainly include high desulfurization efficiency, low investment cost, low operating cost, high comprehensive benefit, reliable operation and no secondary pollution.
As a flame retardant, magnesium hydroxide has excellent properties in various aspects, and its application range will become more and more extensive.
And magnesium hydroxide has the advantage of being cheap, which saves costs for factory production.
The preparation of magnesium hydroxide through lightly burned magnesium powder increases the added value of the product, realizes the effective utilization of resources, and can effectively enhance the competitiveness of enterprises in the market economy.