Kaolin Surface Modification: Mechanisms, Methods and Industrial Applications

Kaolin is an important non-renewable non-metallic mineral resource, and large, high-quality kaolin deposits are currently scarce. With continuous research on kaolin, downstream industries are shifting toward mid-to-high-end product demand. The market demand for high-quality kaolin is growing faster than the overall market, intensifying supply-demand contradictions in China. Therefore, researchers need to apply various processing technologies to make kaolin ideal for diverse industrial applications, thereby targeting different markets. Kaolin surface modification is a crucial process that enhances the material’s properties, making it more suitable for various industrial applications.

Inorganic modification

One method to save titanium dioxide is by surface modification and ultrafine processing, along with coating titanium dioxide powder onto the surface of inert kaolin particles. Manufacturers can use this composite titanium white powder as a pigment in plastics, rubber, paper, and coatings.

Organic modification

The purpose of organic surface modification of kaolin fillers is to improve their application performance in rubber, cables, plastics, paints, coatings, and chemical carriers. Common surface modifiers include silane coupling agents, organosilicon (silicone oil), polymers, surfactants, and organic acids.

Silane coupling agent modification

Researchers commonly use silane coupling agents as the most effective surface modifiers for kaolin. The modification process is relatively simple, usually involving adding kaolin and the prepared silane coupling agent into a surface modification machine for treatment. Many factors affect the treatment results, such as the viscosity and surface characteristics of kaolin (surface functional groups and reactivity), the type, amount, and application of the silane coupling agent, and the treatment time and temperature.

Modified silicone oil

Kaolin used as a filler in electrical wires and cables (such as PVC) is commonly surface-modified with silicone oil. After calcination, kaolin removes structural water, and consequently, the resulting metakaolin surface easily reacts with the oxygen in the silicone oil molecules. As a result, this process allows the silicone oil to coat the outer layer of the calcined kaolin particles, thereby forming a hydrophobic film of molecular thickness. Furthermore, research shows that as the amount of silicone oil and treatment time increase, the hydrophobicity of calcined kaolin improves.

Organic acid modification

The production cost of silane-coated kaolin fillers is relatively high.
U.S. Patent 4,798,766 employs amination treatment, followed by surface treatment of aminated kaolin and other silicate mineral fillers with unsaturated organic acids, such as oxalic acid, decanoic acid, and dicarboxylic acids. The modified kaolin treated with this method can be used as a filler in Nylon 66.

Organic amine modification

Cationic surfactants, such as octadecylamine, can also be used for the surface modification of kaolin. Their polar groups interact with the surface of kaolin particles through chemical and physical adsorption.

Intercalation modification

The kaolin unit layers contain -OH bonds and Si-O bonds, which easily form hydrogen bonds. Additionally, the small interlayer spacing allows some small polar molecules to intercalate between the layers, expanding the interlayer distance. This process changes the hydrophilicity of the interlayer to oleophilicity, reducing the surface energy of the interlayer and facilitating the intercalation of other organic macromolecules through a substitution process.

Researchers achieve the intercalation modification of kaolin by breaking interlayer hydrogen bonds and forming new hydrogen bonds, following the electron transfer mechanism. In the electron transfer process, a proton donor gains an electron to form a hydrogen bond. These intercalators contain functional groups such as -NH, -OH, -COOH, etc. The proton acceptor loses an electron to form a hydrogen bond, and these intercalators generally contain functional groups like C=O, -NO, -SO, etc. Intercalators that contain both proton donor and acceptor functional groups possess the dual ability to gain and lose electrons, enabling them to form hydrogen bonds alone or simultaneously. These intercalators contain functional groups like -CO-NH2, -CO-NH-, etc. For different organic molecules, the hydrogen bonds formed vary.

Conclusion

Kaolin is an important non-renewable non-metallic mineral resource, and large, high-quality kaolin deposits are currently scarce. With continuous research on kaolin, downstream industries are shifting toward mid-to-high-end product demand. Therefore, researchers must apply various processing technologies to make kaolin ideal for diverse industrial applications, ultimately targeting different markets.

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