Preparation and surface modification of ultra-fine alumina hydroxide powder

Preparation and surface modification of ultra-fine alumina hydroxide powder

Aluminum hydroxide (ATH) has multiple functions such as flame retardant, smoke elimination and filling. It will not produce secondary pollution and can produce synergistic flame retardant effect with a variety of substances. It is widely used as flame retardant additive for composite materials and has become the largest amount of environmental protection inorganic flame retardant

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When ATH is used as a flame retardant additive, its content and particle size have great influence on the flame retardant and mechanical properties of the composites. In order to reach a certain flame retardant level, ATH is usually added with a large amount. When the amount of added ATH is constant, the finer the particle size is, the better the flame retardant effect is.

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Therefore, in order to better play the flame retardant effect of aluminum hydroxide powder, and reduce the influence of powder on the mechanical properties of composites when the addition amount increases, ultrafine and nanometer flame retardant are the new trends of development of ATH flame retardant.

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However, the particle size of the ultrafine powder is very small, the surface energy is high, and it is easy to agglomerate, and it is difficult to disperse evenly into the polymer matrix. Moreover, aluminum hydroxide powder is a typical polar inorganic material, which has poor affinity with organic polymers, especially non-polar polyolefin, and low interface bonding force, which leads to poor fluidity during material mixing and molding, and deterioration of processing performance and mechanical properties.

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Therefore, how to reduce the agglomeration between the ultrafine aluminum hydroxide particles, improve the interface compatibility between the aluminum hydroxide powder and the polymer matrix, improve their dispersion in the polymer matrix, so as to obtain excellent performance of flame retardant composites, has become a key problem in the application of ultrafine ATH in the field of flame retardant filling materials.

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  1. Preparation of ultrafine aluminum hydroxide
    Ultra-fine aluminum hydroxide can be prepared by physical and chemical methods. Physical method usually refers to mechanical method; The chemical methods include the fractionation method, sol-gel method, precipitation method, hydrothermal synthesis method, carbon fractionation method, high gravity method, etc.

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1.1 Mechanical method
The mechanical method is to wash, dry non-industrial alumina hydroxide and grind into fine granularity ATH powder by agitating mill and airflow mill. ATH powders produced by this method have irregular shape, coarse particle size and wide distribution, generally in the range of 5 ~ 15μm, and poor mechanical performance. When used in the production of wires and cables, the alumina hydroxide produced by this method is inferior to those produced by chemical method in terms of processability, ductility and flame retardant properties.

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The preparation process of mechanical method is simple, the experimental cost is low, but the product impurities content is high and the particle size distribution is not uniform, so it can not be widely used.

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1.2 Seed separation method
The core of the commonly used seed separation method is to add ultra-fine ATH crystal seeds into the prepared sodium aluminate solution, so as to prepare purer and finer ATH powder. And the seed quality is an important factor affecting the particle size of ATH powder.

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1.3 Sol-gel method
This method is mainly hydrolyzing aluminum compounds under certain water bath temperature, stirring rate and pH value to generate aluminum hydroxide colloid, and then transformed into gel under certain conditions, and finally drying and grinding the gel to produce small particle size aluminum hydroxide.

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1.4 Precipitation method
There are two kinds of precipitation methods: direct precipitation and homogeneous precipitation. Direct precipitation refers to add precipitant in aluminate solution under certain conditions to prepare high purity ultrafine aluminum hydroxid, in the process of precipitation, the mixing degree of precipitator and solution is the key factor affecting the properties of the final product. Unlike direct precipitation, the speed of homogeneous precipitation is slow.

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1.5 Hydrothermal synthesis method
Hydrothermal method is a method of preparing ATH by heating the closed reaction vessel and making the raw material react at high temperature and high pressure in the organic solvent medium.

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1.6 Carbon separation method
Carbon separation method is to prepare aluminum hydroxide by passing CO2 into sodium aluminate solution and controlling the reaction conditions.

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