Surface modification is one of the most important deep processing modification methods for imported kaolin. It refers to the physical, chemical, or mechanical treatment of the surface of imported kaolin according to the needs of use, in order to improve the whiteness, brightness, surface activity, or compatibility with polymers of imported kaolin.

The basis for surface modification of imported kaolin is active groups such as Si-O bonds and Al - (O, OH) bonds. The characteristic is that even if chemical reactions are used, only the components at the interface level of the mineral deposit are modified, without altering the internal crystal structure and physicochemical properties. Currently, there are several main methods:

1. Calcination modification

Calcination modification refers to the high-temperature calcination of imported kaolin, with the intention of removing all hydroxyl groups from the surface of the imported kaolin and obtaining special surface properties.

The calcined imported kaolin has characteristics such as high whiteness, low density, increased surface area, oil absorption, high thermal stability, and insulation, making it suitable for use as filler in coatings, paints, cables, and other materials.

When calcining imported kaolin, attention should be paid to the selection of temperature. When calcined at a lower temperature, the activity of imported kaolin is relatively high; Calcined at higher temperatures, aluminum spinel can be formed, and mullite will be produced at a certain temperature. At this time, the activity of imported kaolin is relatively low, which cannot meet the needs of some polymer materials.

Therefore, when used in different products, different calcination temperatures should be selected. For example, when filling cable adhesive, the imported kaolin should have a large surface activity, so it is necessary to calcine the imported kaolin at low temperatures; When used as a filler for coatings, the calcination temperature can be too high to replace some pigments, but the temperature should not be too high to avoid mullitization.

2. Surface coating method modification

The surface coating method involves coating an organic substance on the surface of imported kaolin through physical adsorption or chemical adsorption to achieve modification.

The commonly used modifiers are stearic acid, glyceride trimethacrylate, trimethoxypropane tri Glycidol ether, low molecular polyethylene wax, etc.

The advantage of this method is that the modified imported kaolin can enhance the stability of the data structure and catalyst activity, weaken the aggregation degree of the powder, and improve the dispersibility and flowability.

3. Modification by external reflection method

The surface reaction method refers to the chemical reaction between the modifier and the active groups on the main surface of kaolin (- Al (Al) - OH, - Si O-Al -, and - Si (Al) - O), introducing hydrophobic or active hydrophobic groups, and then causing changes in the surface properties. This not only reduces the surface energy, but also improves the hydrophobicity and responsiveness of the imported kaolin.

Modifiers can directly modify imported kaolin, and can also react on its surface to generate ions. After ion exchange, the modification intention is ultimately achieved. Important methods of this method include esterification, halogenation, amination, etc.

Different modification conditions can be selected according to needs to achieve different modification intentions. This method has great advantages in manufacturing refined and specialized products, and is a primary aspect of deep processing of imported kaolin.

4. Coupling agent modification

The coupling agent modification method is a commonly used method for chemical modification of the surface of imported kaolin. The surface of imported kaolin is chemically grafted with organic coupling agents, and then the surface properties of imported kaolin are modified from hydrophilic to lipophilic, reducing the surface energy. When filled into polymer materials, compatibility with polymer materials is improved. Now common coupling agents mainly include silane, titanate, aluminate, zirconium Aluminate, etc.

Coupling agents are organic molecules with amphoteric structure. The primary functional groups include hydrophilic polar groups and lipophilic organic long chains. The hydrophilic polar groups can produce chemical reactions with the appearance of imported kaolin and connect to its appearance, while the lipophilic organic long chains can intertwine with the polymer chains. Perhaps the lipophilic long chains with active groups can form Chemical bond with the polymer reactions, improving the compatibility between imported kaolin and polymers, Improve the functionality of polymer materials.

The use of coupling agents to treat imported kaolin can effectively suppress its "phase" separation from polymers, increase the filling amount, save costs, and maintain good dispersibility together. This improves the inductive function of the polymer matrix, especially in terms of impact strength, tensile strength, flexibility, and flexural strength.

There are generally two treatment methods for coupling agent treatment: wet treatment and dry treatment:

Wet treatment involves immersing imported kaolin powder in a solution containing a coupling agent, acting at a certain temperature, separating the solvent from the imported kaolin powder, and then drying the powder. Compared to dry treatment, wet treatment has the advantage of evenly mixing powder and coupling agent, but the wet treatment process is more chaotic, solvent loss is large, and the cost is high; Dry processing involves placing imported kaolin micropowder into a high-speed mixing mixer, mixing and drying at a certain temperature, slowly adding solvents and additives dissolved in coupling agents, and after a certain amount of mixing treatment, a modified imported kaolin filler can be prepared. Due to the fact that the amount of modifier is generally only 1% to 5% of the powder amount, which is very small, in order to achieve good mixing and coating of modifier and powder during dry treatment, it is necessary to pay full attention to the equipment and operation process, and strive to achieve the wet treatment effect as much as possible.

5. Modification by intercalation method

The intercalation modification method is to use the weak bonding force between the layers of layered powder particles and crystals, or the presence of exchangeable cations between the layers, and to modify the interlayer and interface properties of the powder using methods such as chemical reaction or ion exchange.

Imported kaolin cannot undergo cation exchange, but there are - OH and Si-O bonds that are prone to forming hydrogen bonds between the imported kaolin soil layers. The interlayer spacing is small, and only some polar small molecules (such as HC-ONH2, CH3CONH2, etc.) are allowed to pass through. These polar small molecules can be inserted between the imported kaolin soil layers and damage their hydrogen bonds, supporting the interlayer spacing, making the hydrophilicity between the layers become hydrophobic, which is conducive to the entry of other organic macromolecules through the replacement process, This causes imported kaolin to disperse into various matrices in a nanoscale peeling state.

6. Mechanochemical modification

The mechanochemical modification method is essentially to use mechanical energy to activate particles and surface modifier to produce effect, so as to achieve the intention of transforming mechanical energy into Chemical energy, which can be completed by means of strong mechanical mixing, impact, grinding, etc.

This method has also made important contributions to the composite of powders, enabling the surface of the powder particles to be coated with a finer or more functional layer of powder particles through mechanical external forces. The above surface coating modification is chemical precipitation, while this method is mechanochemical action. The use of different machines and modification processes in the mechanochemical modification method results in different modification effects for the powder.

7. Agglutination co precipitation method

Wang Fang et al. used concentrated natural latex as the main material, modified imported kaolin as the filler, and prepared modified imported kaolin/NR composite materials through agglutination co precipitation method. They explored the factors affecting the physical and mechanical functions of the materials, and used scanning electron microscopy to observe the description of the tensile section and analyze it.

The results indicate that the prepared composite materials have good physical functions and there is no stress whitening phenomenon during the stretching process; When the mass fraction of potassium lactate solution is 5% and the dosage of modified imported kaolin prepared at a modification temperature of 80 ℃ is 40phr, the modified imported kaolin/NR composite material obtained has the best physical and mechanical functions.

8. Acid alkali treatment

Acid alkali treatment is also a surface auxiliary treatment method, which can improve the absorption and reaction activity of the powder surface (or interface) after acid alkali treatment.

Huo Zhihui et al. mixed Betaine modified imported kaolin suspension with natural latex, prepared Betaine modified imported kaolin/NR composite data by agglutination coprecipitation method, and discussed the influence of Betaine solution mass fraction, alkali metal ions, imported kaolin dosage and other factors on the physical function of composite data. The results showed that the imported kaolin modified by Betaine had a significant reinforcing effect on the vulcanizate. The scanning electron microscope of the section of the vulcanizate tensile sample shows a remarkable screen tracing, and the analysis of the transmission electron microscope shows that the Betaine modified imported kaolin particles are closely combined with the natural rubber matrix, and the interface is vague.

In addition to the above methods, the surface modification methods for imported kaolin include surface grafting, particle surface ion exchange, plasma modification, irradiation modification, chemical vapor deposition (CVD), and physical deposition (CVD).