1 Characteristics of aluminum hydroxide flame retardant
Aluminum hydroxide used as flame retardant is mainly α-aluminum hydroxide, often expressed as α-A1(OH) 3 , which belongs to the monoclinic system and is a crystalline or amorphous white powder. Aluminum hydroxide decomposes into A1 2 O 3 and H 2 O when heated. The heat absorption measured in the range of 240~500℃ is 1967.2kJ/kg. The heat absorbed during the decomposition process is the main reason for its flame retardant effect.
Among all flame retardants, the usage of aluminum hydroxide accounts for more than 40%, mainly because aluminum hydroxide flame retardant has the following advantages: it has flame retardant, smoke suppression and filling properties, and does not produce secondary pollution; it has high whiteness and low covering power for colorants; it has moderate wear resistance and good molding and processing performance; it can produce flame retardant synergistic effects with a variety of other substances; it is abundant in source and low in price.
As a flame retardant, aluminum hydroxide has many excellent properties, but also has some shortcomings, mainly: (1) poor flame retardant efficiency, large addition amount; (2) strong polarity and hydrophilicity, poor compatibility with non-polar polymer materials; (3) low dehydration temperature, the main dehydration reaction is completed in the range of 200-320℃. The mixing and molding temperatures of many thermoplastics and high-temperature rubbers exceed 220℃. Therefore, if aluminum hydroxide is added during the processing, it will dehydrate to form bubbles, affecting the mechanical properties of the product. The poor thermal stability of aluminum hydroxide has always been the main reason for its limited use in high-temperature rubber and plastic materials. This is also one of the hot topics in the research field of flame retardant materials in recent years.
2 Methods to improve the thermal stability of aluminum hydroxide
The poor thermal stability of aluminum hydroxide is one of its shortcomings as a flame retardant. There are four main methods to improve the thermal stability of aluminum hydroxide.
2.1 Surface coating
After surface coating, a coating layer can be formed on the surface of aluminum hydroxide, which can improve the thermal stability of aluminum hydroxide to a certain extent. After aluminum hydroxide is coated, the water vapor diffusion rate decreases when it is thermally decomposed, which may delay the decomposition reaction. Due to the obstruction of mass transfer by the coating layer, the concentration of water molecules on the surface of the aluminum hydroxide core increases, that is, the equilibrium water vapor partial pressure of the aluminum hydroxide decomposition reaction increases, and the decomposition temperature increases accordingly.
Aluminum hydroxide surface coating refers to the use of certain substances containing elements such as P, N, Ti, Zr, Si, Mg, Ca, etc. as modifiers, which react with aluminum hydroxide in a certain system (physical reaction or chemical reaction), or react with certain substances in the system to form a coating layer on the surface of aluminum hydroxide.
Wang Jianli et al. used newly generated wet aluminum hydroxide as the core material and used a special process to evenly deposit magnesium hydroxide on the surface of aluminum hydroxide in a heterogeneous nucleation manner, forming a core of magnesium hydroxide coating aluminum oxyoxide. -Shell structure. The initial thermal decomposition temperature of the prepared aluminum-magnesium composite flame retardant can be increased to 260°C, and the flame retardant performance is further improved. After filling with organic polymers, its oxygen index can reach 33, reaching the flame retardant level. After the product is filled with organic polymers, all major mechanical performance indicators are better than the standard requirements. After Zhou Xiangyang et al. used composite modifier PNC to wet-coat aluminum hydroxide, the initial water loss temperature of aluminum hydroxide increased by 21°C, and the dispersion of the powder was improved at the same time. Inorganic-organic multi-layer coating modification or organic-organic multi-layer coating modification is a comprehensive surface modification treatment method that can not only improve the surface properties of aluminum hydroxide, but also significantly improve the thermal stability of aluminum hydroxide.
The aluminum hydroxide surface coating treatment process is simple and the treatment cost is low. The disadvantage is that the range of increasing the decomposition temperature of aluminum hydroxide is limited.
2.2 Chemical compound treatment
Chemical compounding of aluminum hydroxide refers to a method of chemically compounding with other substances under certain conditions to form new substances. This is an effective method to increase the decomposition temperature of aluminum hydroxide. The reaction between aluminum hydroxide and oxalic acid can generate basic aluminum oxalate. Basic aluminum oxalate has good thermal stability below 330°C and decomposes when it exceeds 450°C. Its flame retardant properties are similar to aluminum hydroxide. Due to improved thermal stability It has good properties and can be used for nylon, PBT, PET and other polymers. Phosphoric acid reacts with aluminum hydroxide to form pentavalent aluminum phosphate, which still has good thermal stability when the temperature exceeds 350°C. Aluminum phosphate has a very good flame retardant effect on oxygen-containing polymers such as thermoplastic polyester, PBT, PET and polyamide, and the addition amount is small. Only 15%~20% can make the oxygen index reach more than 40% . Chemical compounding of aluminum hydroxide can significantly improve the thermal stability of aluminum hydroxide, but the cost of the product after chemical compounding increases. At the same time, it is difficult to apply some chemical compounding methods on a large scale.
2.3 High purification treatment
High purification refers to improving the purity of aluminum hydroxide and reducing the content of ionic insoluble matter in aluminum hydroxide, especially Na 2 O, so that the mass fraction of Na 2 O is less than 0.2%. The product has low Na 2 O content, is ultra-fine, has a large specific surface area, and has excellent electrical properties. It can be used at 290°C. High purification of aluminum hydroxide can significantly increase the decomposition temperature of aluminum hydroxide and enhance some special properties of flame retardants. However, this method is difficult to prepare products, has high cost, and has limited application scale.
2.4 Partial dehydration treatment
After aluminum hydroxide is heated, part of the chemical water is removed, the apparent molecular number of chemical water is reduced, and part of the aluminum hydroxide is transformed from gibbsite type to boehmite type structure, which improves the thermal stability of aluminum hydroxide. After aluminum hydroxide is partially dehydrated and then modified with organic matter, the resulting product has high thermal stability, is non-hygroscopic, and has good compatibility with polymer materials. This method removes part of the crystal water in the aluminum hydroxide structure, which will reduce its flame retardant properties to a certain extent.
3 Conclusion
Aluminum hydroxide is a flame retardant with excellent comprehensive properties, and its poor thermal stability is one of its shortcomings. Methods to improve the thermal stability of aluminum hydroxide include surface coating, chemical composite treatment, high purification treatment and partial dehydration. Surface coating has been the most studied, and the surface coating process is flexible. It can be combined with the existing aluminum hydroxide flame retardant preparation process, and can also exist as a separate process. This method does not change the crystal structure of the aluminum hydroxide flame retardant, but only adds a coating layer on its surface, which can improve the stability of aluminum hydroxide to a certain extent. Chemical composite treatment completely changes the crystal structure of aluminum hydroxide, effectively improves the thermal stability of aluminum hydroxide, and the processing cost is relatively high. Low-cost chemical composite to improve the thermal stability of aluminum hydroxide is a direction that needs to be strengthened.
Your message must be between 20-3,000 characters!
