Surface modifier

At present, the main modifiers used in the surface modification of ultrafine aluminum hydroxide are surfactants ，coupling agents, etc.

Common surfactants are: sodium dodecyl benzene sulfonate, sodium stearate and silicone oil.The mechanism of modification  is that the polar groups at one end of the molecule react with inorganic materials or adsorb on the surface, while the long chain alkyl at the other end of the molecule has strong compatibility due to its similar structure with the polymer.

The modification mechanism of the coupling agent is that some of the functional groups in its molecules combine with the ultrafine inorganic materials in a chemical way.In addition, some carbon chains combine with polymer materials in a physical or chemical way, so that inorganic materials are closely connected with organic polymer materials. The common coupling agents are as follows: silane coupling agent, titanate coupling agent, aluminate coupling agent.

The modification methods

At present, ATH surface modification is mainly carried out by dry modification and wet modification.The dry modification method is characterized by putting the powder raw material and modifier or dispersing agent into a specific equipment, adjusting the appropriate speed to stir and mix, and making the modifier coated on the surface of aluminum hydroxide powder. This method is suitable for mass production.Wet modification refers to the modification method by adding the modifier into the prepared aluminum hydroxide slurry with a certain liquid-solid ratio and stirring and dispersing it fully at a certain temperature.This method is characterized by complex operation, uniform surface coating and good modification effect.

Modification mechanism

Surface modification of aluminum hydroxide refers to the adsorption or coating of another or a variety of substances on its surface to form a core-shell complex.Its main surface modification of organic modification, modification methods can be divided into two categories, including physical method refers to the use of surfactants such as senior fatty acid, alcohol, amine, ester, such as surface coating treatment, to increase the particle spacing, hinder the reunion between particles, at the same time improve the affinity between aluminium hydroxide and organic polymer, increase the flame retardant performance, improve the processing technology,and further improve the impact resistance of organic polymers.

Chemical method refers to the use of coupling agent surface modification of aluminum hydroxide, through its molecular groups react with modified powder surface to form chemical bonds, and to achieve the effect of modification, due to the coupling agent molecules have affinity for organic matter,it  can react with organic polymer, it make ATH together with organic polymer, thus improve the performance of the composite materials.The modification mechanism of silane, titanate, aluminate coupling agent and stearic acid modifier is due to the presence of inorganic and organophilic groups in their molecules, which make the coupling agent can be used as a molecular bridge between aluminum hydroxide and organic materials, connecting them closely together.

Characterization of modification effect

At present, two methods can be used to evaluate the modification effect of aluminum hydroxide powder.Among them, the direct method refers to the evaluation of the modification effect by measuring the flame retardant and mechanical properties of the composite material formed after filling with modified aluminum hydroxide.The characteristic of this method is that the process is complicated, but the test result is reliable.The indirect method is to evaluate the modification effect by measuring the physical and chemical properties of the surface before and after modification of aluminum hydroxide powder.

Specific evaluation indicators include:

1) Activation index.

As a kind of inorganic polar material, aluminum hydroxide will naturally settle in water.The surface of the modified powder is non-polar, and its hydrophobicity is enhanced, so that it can not settle in water.The change of activation index reflects the degree of activation on the surface of the powder, that is the characterization of the modification effect of the powder.

2) Oil absorption value.

The oil absorption value is an important index to characterize the dispersion of aluminum hydroxide in the polymer, which reflects the size of the porosity and specific surface area of the powder.Surface modification can improve the dispersion of the powders in the polymer and reduce the gap caused by the agglomeration of the powders, so the oil absorption value of the powders can be reduced by surface modification.

3) Dispersion stability.

In this method, the effect of surface modification was characterized by comparing the dispersion of different modifiers and  modified alumina hydroxide powders in the dispersion medium. The morphology and dispersion of the powders could be observed by scanning electron microscopy.