Metal hydroxide flame retardants, aluminum hydroxide into the market early, consumption is much greater than magnesium hydroxide. Although the appropriate crystal form has been for many years, magnesium hydroxide crystal growth more difficult to control, the original production process of magnesium hydroxide is very complex, resulting in relatively high price of magnesium hydroxide, has been plaguing its practical application [1]. In recent years, a new process of magnesium hydroxide production has been developed on the basis of precipitation method and hydration method, so that magnesium hydroxide has a tendency to replace aluminum hydroxide.
1, magnesium hydroxide flame retardant mechanism
Magnesium hydroxide is a filler-type flame retardant, heat decomposition of the release of water vapor, while absorbing a lot of heat, can reduce the surface temperature of the material, making the degradation rate of polymer degradation, followed by the production of small molecules combustible material is also reduced . The release of water vapor dilutes the surface of the oxygen, so that combustion is difficult to carry out. Magnesium hydroxide forms a carbonized layer on the surface of the material to prevent the entry of oxygen and heat, and the magnesium oxide produced by the decomposition of magnesium hydroxide is a high-grade refractory material, so that when the burning source disappears, the fire stops automatically and serves as a flame retardant effect. Since the flame retardant of magnesium hydroxide occurs mainly in the degradation zone of polymer, the production of combustible is reduced, but little effect on the pre-combustion zone, the complete combustion of the combustible has little effect, the smoke is reduced, and the magnesium hydroxide can Dilute and absorb smoke, so magnesium hydroxide with smoke reduction effect.
2, the characteristics of magnesium hydroxide flame retardant
Magnesium hydroxide Mg (OH) 2, white solid powder, insoluble in alkaline substances, heat decomposition into magnesium oxide and water, heated to 340 ° C decomposition, 430 ° C decomposition the fastest, to 490 ° C completely decomposed. Magnesium hydroxide crystals belong to the group of divalent metal hydrate, the crystal structure is layered CdI2 type, forming a continuous hexagonal, Mg2 + and OH - layer overlap each other, each magnesium ion with 6 hydroxide ions with Forming an Mg (OH) 6 octahedron. As an inorganic flame retardant, magnesium hydroxide has many advantages compared with aluminum hydroxide: (1) Hydrogen and magnesium hydroxide have the following advantages: (1) Hydrogen (II) Alumina pyrolysis temperature of 245 ~ 320 ℃, and magnesium hydroxide decomposition temperature of 340 ~ 490 ℃ compared to the effective use of low temperature, suitable for processing temperature is relatively low resin such as ABS, acrylic resin and epoxy resin. Aluminum hydroxide as the decomposition temperature is low, part of the crystal water in the material processing has been decomposed, easy to make products more foam, porous, their flame-retardant effect is also reduced. The magnesium hydroxide can make the filled material withstand higher processing temperatures, help speed up the extrusion speed, shorten the molding time. And magnesium hydroxide decomposition of aluminum hydroxide than the large, high heat capacity, can absorb more heat, better flame retardant [2]. ② magnesium hydroxide particle size smaller than aluminum hydroxide, the material processing equipment wear is small, help to extend the service life of equipment. ③ magnesium hydroxide smoke reduction effect than aluminum hydroxide, and polymer can be toxic gases such as combustion of sulfur dioxide, carbon dioxide and so on. ④ rich in raw materials, easy to get, sea water resources contain a large number of magnesium salts, as well as magnesite, such as magnesite, dolomite and brucite.
3, the preparation of magnesium hydroxide
3.1 Magnesium hydroxide flame retardant production methods Industrial production of magnesium hydroxide flame retardants mainly have five methods: (1) with limestone and brine reaction production of magnesium hydroxide; (2) with sodium hydroxide and brine, halogen (4) the use of MgO hydrate generation of magnesium hydroxide, where the MgO must be light-burning products to the production of magnesium hydroxide; (3) the use of calcined magnesite, dolomite and brine, block reaction production of magnesium hydroxide; To ensure the hydration of the activity [7]; (5) directly to the natural brucite and surface modifier mixed grinding, processing after the magnesium hydroxide flame retardant.
3.2, magnesium hydroxide flame retardant characteristics of ordinary magnesium hydroxide is generally amorphous or hexagonal crystal, large specific surface area, the grain is easy to gather 2 times, the polarity is very strong, filled in resin, plastic, etc. Dispersion Is not good, and easy to make the material mechanical strength decreased significantly, especially the impact strength, so ordinary magnesium hydroxide is not suitable for filling with flame retardants. Magnesium hydroxide used as a flame retardant, it must undergo special treatment and surface modification, with a specific crystal. General requirements: ① crystal form of fiber-shaped, so can increase the elongation of materials and flexural strength. ② purity must be high, the higher the purity, flame retardant effect is better. ③ particles smaller the better The experiment proved that nano-magnesium hydroxide as a flame retardant filled into the material, the various aspects of performance, including flame-retardant effect, smoke and mechanical properties are better than micron magnesium hydroxide. ④ surface polarity is low, ordinary magnesium hydroxide is due to the high surface polarity, micro-internal stress, as a flame retardant filled into the material when the impact of the mechanical properties of materials. When the surface polarity is low, the particles accumulate into a group to reduce the dispersion and compatibility in the material increased, the mechanical properties of materials decreased [4]. So use the appropriate surface-active agent and the amount of surface treatment, to improve the compatibility with the polymer. ⑤ The specific surface area is less than 20m2 / g, the (101) azimuth distortion is less than 3.0 × 0.001, and the crystallite size of (101) orientation is more than 80nm, only the magnesium hydroxide with the above characteristics can be compatible with the material.
3.3, magnesium hydroxide surface modification of magnesium hydroxide surface treatment can be surface-active agent and magnesium hydroxide particles from the surface of the chemical reaction, in addition to the surface of magnesium hydroxide particles can be coated with surfactant. Both of which change the surface state of the particles, allowing the particles to produce new properties, and better material compatibility, to prevent the mechanical properties of materials [5,6]. Commonly used magnesium hydroxide surface modifier is a coupling agent, sodium stearate, sodium oleate and titanate, dosage 1% to 3%.
4, other uses of magnesium hydroxide
In addition to inorganic flame retardants, magnesium hydroxide can also be used in the field of wastewater treatment in the pollution-free neutralizer, paper industry, filler, flue gas desulfurization agent and heavy metal removal and other fields. Recently, Japanese scientists reported that the use of MgO / Mg (OH) 2 system repeated hydration and hydrolysis reaction to manufacture heat pump, the energy surplus in the case of energy storage, and then in the energy demand peak supply of energy, heat pump and engine Combined, and achieved good results.
5, magnesium hydroxide flame retardant development
(1), simplified halogenated production process, reducing brucite grinding energy consumption.
(2), Preparation of high purity ultrafine magnesium hydroxide, nano-magnesium hydroxide flame retardant smoke better.
(3), select the appropriate surface modifier and dosage, to improve the dispersion of magnesium hydroxide in the material, compatibility.
(4), the development of new composite flame retardants.
