Modification process of kaolin
Kaolin, also known as porcelain stone, is a kind of clay and clay rock dominated by kaolinite clay minerals. It belongs to non-metallic minerals and is an aggregate of water-containing aluminosilicates composed of multiple minerals. The pure kaolin is white, fine, soft and soft, with good plasticity and fire resistance and other physical and chemical properties.
The structural units of kaolin are stacked in layers, mainly in the form of plates, which are easy to crack along the direction parallel to the layer, and are processed into ultra-fine powder. Kaolin exists in the form of scales in nature.
Because kaolin is prone to agglomeration when its specific surface area and surface energy are too high, it cannot be uniformly dispersed when filled with organic polymer materials such as rubber and plastic, which is not conducive to the improvement of product performance. Therefore, surface modification of kaolin is needed to reduce its surface energy. After surface modification, kaolin powder can achieve the effects of being hydrophobic, reducing surface energy, improving its dispersion and compatibility with polymer-based composite materials.
The physical properties of kaolin before and after modification
| Kaolin | Pore volume/mL·g-1 | Specific surface area/m2.g-1 |
| Raw material | 0.08 | 9 |
| Acid modification | 0.30 | 111 |
| Alkali modification | 0.27 | 146 |
The surface modification of kaolin often uses surface chemical modification methods, so modifiers are added.
| Types of modifiers | Principle |
| A silane coupling agent | It is the most commonly used and most effective surface modifier for kaolin fillers. Generally, kaolin powder and a configured silane coupling agent are added to the modifier for surface coating treatment. |
| Silane coupling agent + silicone oil | In addition to the silane coupling agent, add 1-3% silicone oil for surface modification. Not only the mechanical and physical properties of the cable are improved, but also the electrical insulation and hydrophobic properties of the cable are improved or improved, and the electrical insulation in a humid or cold environment is significantly improved. |
| Unsaturated organic acid | The aminated kaolin treated with oxalic acid, sebacic acid, dicarboxyl acid, etc. can be used as a filler for nylon 66 and the like. |
| Cationic surfactant | Its polar groups act on the surface of kaolin particles through chemical adsorption and physical adsorption to improve the surface hydrophobicity of kaolin. |
| Inorganic modifier | Using the surface precipitation reaction of titanium dioxide and kaolin powder in an aqueous solution, washing, filtering and drying the precipitate can obtain kaolin coated with titanium dioxide on the surface. |
The surface modification process of kaolin generally has three methods: wet method, dry method and semi-dry method.
- Wet
The wet process requires pulping, dehydration, and drying processes, and the process is more complicated, especially dehydration filtration. If the particle size is less than 1250 mesh, it will be extremely difficult and complicated.
- Dry method
The dry modification process requires relatively high technology and equipment. The process completely eliminates the dehydration and drying link, and the process is simple.
- Semi-dry
While stirring the powder in the mixer, add an appropriate amount of water modifier and additives to mix in. After heating to a certain temperature and time, the product will be in a viscous state, and then slightly dried to obtain a modified product. The process omits the dehydration process and has higher production efficiency.
Commonly used modification methods of kaolin
| Method | Principle |
| Calcined modification | The calcination process kaolin removes structural water, crystal water, carbon and other volatile substances, and becomes metakaolinite. Calcined kaolin has the characteristics of high whiteness, small bulk density, large specific surface area and pore volume, good oil absorption, covering and abrasion resistance, and high insulation and thermal stability. |
| Coupling agent modification | It is suitable for the composite material system of various organic polymers and inorganic fillers. The surface of kaolin can interact with the coupling agent, and the compatibility of the kaolin modified by the coupling agent with the organic phase is improved. |
| Organic polymer modification | Utilizing modified surfactants, polymer separating agents, organic small molecule dispersants, etc., can be adsorbed on the surface of kaolin, thereby changing the charging status of the surface of kaolin. |
| Surface coating modification | Through physical adsorption or chemical adsorption, an organic or inorganic substance is coated on the surface of the kaolin to achieve the effect of surface modification. |
| Intercalation modification | Intercalation modification is to intercalate small polar molecules between kaolin layers to increase the interlayer spacing and change the hydrophilicity between the layers to lipophilic kaolin composite materials. |
- Calcined modification
| Temperature | Features | Application |
| Low temperature calcination (600℃-1000℃) | Its kaolin product is highly active | It is used to synthesize molecular sieve, aluminum salt chemical industry, plastic and rubber functional materials. |
| Middle temperature calcination (1000℃-1200℃) | Its kaolin product has high whiteness and good opacity | Used in papermaking and coating industry to replace titanium dioxide as structural pigment. |
| High temperature calcination (above 1200℃) | Used in the production of mullite-grained dense casting sand, high-grade refractory materials and special ceramics, etc. |
In the calcining modification process, the reaction is carried out at a certain temperature, and the surface activation degree is different for different reaction times.
| Time | 1 minute | 2 minutes | 3 minutes | 5 minutes |
| Activation degree | 83.6 | 90.2 | 95.8 | 98.6 |
- Surface coating modification
Through surface coating, the stability of the material structure can be enhanced, the activity of the catalyst can be increased, the agglomeration of the powder can be prevented, and the dispersion characteristics and fluidity of the powder can be improved.
The main factors affecting surface coating modification are as follows:
The properties of kaolin: The specific surface area of kaolin determines the amount of surface modification. The larger the specific surface area, the more dosage required.
The properties of the modifier: From the perspective of dispersion, the adsorption of non-ionic substances on the surface of kaolin is relatively large, but the effect is not ideal; although the adsorption of negatively charged substances is not much, the dispersion is good.
Reaction conditions: Under different temperatures and pH values, the adsorption amount of coating ions on the surface of the powder is different, which will also affect the results of surface modification.
- Intercalation modification
Kaolin intercalation methods include mechanochemical method, microwave intercalation method, liquid intercalation method, and ultrasonic intercalation method.
Kaolin intercalation nanomaterials have better plasticity, whiteness, easy dispersibility, and adsorption properties, and can give the materials optical, electrical and magnetic properties, and expand the application range of kaolin clay. The intercalation method is currently the most promising and effective technology for preparing nano-kaolin. Commonly used chemical additives include: potassium acetate, dimethyl sulfoxide, urea, formamide, hydrazine hydrate and its extensions, etc.
Kaolin itself is a very versatile and important non-metallic mineral, which is widely used in more than a dozen industries such as petroleum, plastics, coatings, refractory materials, ceramics, and papermaking.
- Application of Modified Kaolin in Coatings
Kaolin is added to white paint or paint in an appropriate amount to enhance gloss and improve its covering ability.
- Application of modified kaolin in plastics
The application of modified calcined kaolin in plastic products can make the surface smooth, improve its dimensional accuracy, deformation temperature, impact strength, chemical resistance, etc., and increase the amount of filling and reduce the cost.
- Application of modified kaolin in rubber
Adding modified kaolin powder to the rubber can improve the physical and chemical properties of the product, greatly reduce the cost, improve the product level, and increase economic benefits.
Article source: China Powder Network